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PREDATION BEHAVIOR OF OCTOPUS JOUBINI CUVIER BY BENJAMIN MERLINO A Thesis Submitted to the Division of Natural Sciences New College of Florida In partial fulfillment of the requirements for the degree Bachelor of Arts Under the sponsorship of Dr. Sandra Gilchrist Sarasota, Florida May 2013
ii Acknowledgements I would like to thank God, my Savior, for being the light in my life and helping me through this every step of the way. I would also like to thank Dr. Sandra Gilchrist for all the help she has given me throughout this thesis journey. Without her guidance I surely would have been utterly lost, Gabe and Tian for all their encouragement, my parents for always having faith, Allaire for tolerating me, and Lauren Brenzel for helping me deal with all the magical difficulties everyone faces.
iii Table of Contents .. ..ii List of Tables and Figures .. 1.2 Optimal Foraging Theory..................................... ............... .. ..... .......4 1.3 Foraging.............................................................. ............ .. ..... ..........6 1.4 Hunting and Feeding Behavior.................................. ..... ........ .. .......11 1.5 Prey Selection............................................................. ... ..... ...... ........21 1.6 Octopus joubini .......................................... ...................... ...... .. ........23 2. Methods............................................................................................. .... ... ... ...26 2.1 Collection and Maintenance of Octopuses....................... .... .......26 2.2 Initial Ob servations........................................................... ... ..... .......29 2.3 Shell Observations........................................................... ... .... ........29 3. Results.......................................................... ................................. ... ...... ........31 3.1 Basic Behavior............................................................... ... ... .... ........31 3.2 Object Recognition........ ..... ............... ............................. ..... ............32 3.3 Drilling and Pulling.............. ... ............. ............... ... ................... ................ ..34 3.3.1 Gastropods......................... ... ................................ .... ........34 3.3.2 Pagurus sp. ........................... ... ............................. .... .......35 3.3.3 Drill Hole Locations................ ... ............................ .... ......37 3.4 Prey Preference.......................................... .. ......................... .... ......44 4. Discussion......... .......................................................... .. .................. ...... ........47 4.1 Behavior................................................................. .. ........ ...... .........47 4.2 Feeding........................................... ........................ .. ....... ..... ...........48 4.3 Drilling.................................................................. .. ........ ..... ...........50 4.4 Prey Preference...................................................... .. ...... .... .............54 5. References.................................................... ..................... .. .. ........ ...............56
v PREDATION BEHAVIOR OF OCTOPUS JOUBINI CUVIER Benjamin Merlino New College of Florida, 2013 ABSTRACT Octopuses make decisions while foraging based on a range of factors, including prey size, prey species, habitat foraged and availability of prey. This thesis examines the overall predation behavior of Octopus joubini on gastropods and Pagurus sp. A total of 7 octopuses were studied in situ and observed for overall behavior. Shells of consumed prey were observed for the presence of drill holes, locations of holes on drilled shells, and overall shell size. Object interaction and p rey preference was also noted. Shells were collected and organized into groups of drilled and intact, then further arranged by size. It was found that the sutures of the body whorls were the most efficient drill site, and that prey size is directly relat ed to the decision to drill as well as overall handling time. Prey was found to be chosen based upon size and shortest amount of handling time to consume. Dr. Sandra Gilchrist Division of Natural Sciences
1 Introduction 1.1 Trophic Dynamics Trophic dynamics have been described as the flow of energy throughout an ecosystem and the transfer of this energy from one part to another (Lindeman 1942) The overall structure of an ecosystem i s established by the feeding relationships between various levels among these trophic units within the food web. Elton (1927) described how this intricate relationship between all organisms controls the balance of an eco system. He suggested that prey spe cies are generally specialized in what they can consume for energy, unlike predators that possess the ability to survive on numerous things. This connection is used to determine the various levels of a food web, known as the trophic levels. Each level ta kes energy from the previous level and gives it to the succeeding level showing how energy is transferred from the lowest organisms to top predators ( Figure 1 1 ) Figure 1. 1: A basic overview of a generalized food web of a model ecosystem Image from: Lindeman, 1942
2 This trophic level concept can be used to depict abundance of an organism within an ecosystem. Furthermore, it can show the nature of control and regulation over resources and production, revealing the regulating factor in an ecosyst em (Hairston 1993). This is used in ecological control mechanism s which find the controlling factor in a system. T wo main theories have a risen in establishing the influencing factor in a food web. First describe d is the top down hypothesis. This is whe n a higher level organism in the food web regulates the lower level organisms (Fretwell 1987). For example, w hen an overall abundance of vegetative resources are available to herbivores, predators become the controlling factor in the population of the org anis ms. I f predators feed on a certain lower level organism less, then the vegetation will become less abundant due to higher consumption by herbivores. If the same predator feeds on lower level organisms more, then the vegetation would in turn become mo re abundant due to less grazing by the herbivores. The second described hypothesis is called the bottom up hypothesis which involves lower level organisms not being as abundant or edible to the higher level organisms. This then makes the lower level or ganisms the controlling factor within an ecosystem (Schmitz 2008) These theories came from the idea of multi trophic interaction s called trophic cascade. This concept is defined as having the actions of a higher level organism consuming its energy in a food web creating a higher abundance of the lower organisms beneath the organisms being consumed ( Schmitz et al 2000). This concept has held true through various experimentation. Silliman and Bertness (2002) conducted an experiment that illustrated the e ffects of trophic cascade on a salt marsh ecosystem. A dominant grazer within a Georgia salt marsh (the periwinkle, Littorina irrorata) was known to be
3 able to eradicate a grassland ecosystem within months if not consumed by their natural predators In the marsh tested, Littorina irrorata populations were cont rolled by various crabs and turtle s. The researchers hypothesized that if the consumers were removed, the marsh would be destroyed. By caging off several transects and simulating different pop ulation densities of periwinkle, they found that when unconsumed by predators the snails were able to destroy almost all the vegetation the y fed upon as shown in F igure 1. 2 Figure 1. 2: Overall effect of the removal of consumers on a marsh grassland habitat over time. Image from: Silliman and Bertness 2002
4 1.2 Optimal foraging theory In nature, most animal s must feed to survive. This means they must actively search for f ood sources, also known as foraging. In optimal foraging theory, decisions and behaviors are predicted by comparing an organism to a theoretical optimal forager (MacArthur and Pianka 1966) Natural selection favors organisms t hat perform behaviors allowi ng for better overall fitness. This idea directly correlates to optimal foraging theory, showing that the most successful forager will maximize their fitness, allowing for the continuation of their genes. Fitness is measured by reproductive success. In turn, to be reproductively successful, an organism must be able to successfully forage Foraging success can be measured using things such as energy loss vs. energy gain, ease of prey capture, handling times, and overall rate of encounters (Pyke et al 1977) When an organism performs a profitable foraging behavior, they become more successful, maximum fitness can be achieved and passed onto offspring. Overall, there are four categories within optimal foraging theory. Pyke (1984) described them as op timal diet, patch choice, allocation of time to patches, and movement between patches When choosing which prey items will be considered an optimal diet, several factors must be taken into consideration. How easy is this organism captured, how much energ y will be gained from consumption, and how abundant is this prey? If an organism is very high in energy, but very difficult to both find and obtain, then it may not always be worth it to forage for that specific prey item. Instead, it has been seen that many organisms forage in a more generalist way; capturing an array of prey items that are easily obtained in order to gain the maximum amount of energy without expending nearly as much (Baum et al 1999) That is not to say that when presented with an
5 oppo rtunity to consume a rare energy rich prey item, the organism will fail to do so but rather will maximize overall benefit. This is learned through trial and error of various prey items found over time. When selecting patches, trial and error is again u s ed. Patches are ranked by distance to patch and resource abundance. Learning allows for the recognition of various patch types and their locations. This then lets an organism choose to which specific pat ches they want to go further maximizing the over all fitness of an organism A patch may be chosen due to an abundance of not highly desired, but easily obtain ed prey items (Pyke 1984) In contrast, a patch may also be chosen for consistently having a select few highly desired prey items. A close patch with many less desired prey items may or may not also be chosen over a farther patch with more desired prey items, depending on the preference and what it deems an optimal choice (MacArthur and Pianka 1966) When choosing the amount of time spent at any one patch, an organism must consider the abundance of the prey item the desirability of the prey item at a certain patch and if too much time is spent in one specific patch, will the patch abundance become far less in the futu re due to acts of the predator? Because of this, a predator must manage its resources and gain knowledge to obtain the maximum from any one patch without harming its abundance in the long run due to over foraging. Movement between patches must also be ta ken into account while foraging. This time spent traveling must be worth the energy spent to do so (Drossel et al 2001) Again, remember ing the location of all patches capable of being foraged is key. This learning of abundant locations allows a predato r to map out its foraging journey to be most efficient. Depending on its current
6 condition, an organism will likely take more risks as it becomes more desperate while foraging. It may travel farther, foraging for longer, or consume far more in one specif ic patch than it normally does (Yamamoto 2004) Two types of foragers have been described: energy maximizers and time minimizers When a predator has a limit on the amount of time it can spend foraging, it will attempt to consume as much as possible in that all otted time to increase its maximum increase proportionally to the energy gained while foraging. Alternatively, when an organism has a fixed energy requirement, they instead attempt to gain this certain amount of energy in the shortest amount of time possible (Leite et al 2009) Predation also plays a key role in maximum foraging theory. When presented with predation while foraging, an orga nism must decide if th e threat is worth the gain during foraging. This leads to an attempt to maximize efficiency and minimize predation risks. If the risk is too high, certain areas may be avoided all together. Once the risks begin to outweigh the gain, the abundance of a c ertain patch becomes pointless (Drossel et al 2001) 1.3 Foraging Foraging is of vital importance to the survival of an octopus By using effective foraging techniques, they are able to maximize their overall fitness and have a much higher rate of en ergy intake (McQuaid 1993). This idea is supported by optimal foragin g theory Unlike organisms possess ing a uniform methodology for foraging, octopuses are capable of learning and altering their foraging techniques based on the env ironment and prey availability (Leite et al 2009). F oraging patterns can vary greatly between species,
7 environmen t, and individuals (based upon differing experience). This is supported by several researchers finding variable foraging distances and time spent foraging over all It has been observed that octopuses may travel anywhere from 5 120m in search of food, and anywhere between 1 2 hours at a time foraging (Forsythe and Hanlon 1997). An sions can be seen in F igure 1.3 Figure 1.3: Hypothesized path of foraging of O. vulgaris based upon selective pressure of the octopus to consume and avoid predation. Image from: Mather, 1991
8 Due to their vulnerability while foraging for prey, many octopuses are considered time minimizing foragers By intensely searching for prey while performing many cryptic behavior s octopuses lower their overall time exposed to predators while maximizing the amount of food collected ( Scheel et al 2007 ) Du ring foraging, octopuses use one of two major types of movements: crawling or swimming as seen in Figure 1.4 While swimming, hops and jumps are performed to move short distances (Mather 1991). They also are capable of fast backward swimming, which is mainly observed when returning from foraging or escaping predators. Figure 1.4: Examples of several foraging movements performed by octopuses. Top left) crawling behavior. Top right) poking in search of prey. Bottom) two forms of web overs. The octopuse s depicted can be seen covering a rock or substrate while speculatively searching for prey. Image from: Rosebrock, 2000
9 Octopuses do not hunt for specific prey items (Mather et al 201 2) Instead, speculative searching is performed. They investigate all th e various habitats passed using an array of te chniques suited for each These methods u sed are predominate ly tactile and have been categorized into two main groups. The first, call ed pouncing, occurs when the octopus envelops a rock, coral small patch of substrate, or grass bed area with its arms and web ( Hanlon et al 2008) It then uses its arms and suckers to dig through wh at is caught under its webbing. The second method used is called groping or poking ( Mather 1991, Forsythe and Hanlon 1997). When an object is too large to engulf with its webbing, the octopus instead inserts its arms into holes and crevices, and simply removes the prey These techniques are illustrated in F i gure 1.4 Hanlon and colleagues (2008) found that Thaumoctopus mimicus spent 32% of its time actively foraging when away from its den. The other time was spent swimming or sitting. Figure 1.5: Locomotion performed by Abdopus aculeatus. A) Jetting; B) swimming; C) crawling; D) walking. Image from: Hufford, 2006
10 Rosebrock ( 2000) described both Octopus vulgaris and O. briareus performing a variety of tasks in several combinations to maximize foraging effectiveness. There w ere 9 total move ments described for these two species tuck hold, webover pause, jet, pull, crawl and poke, and tuck hold and poke. She described these octopuses obtaining food and tucking them away while continually searching with t heir other arms. Once collected, prey may either be brought back to the den or eaten while still searching for food. Food be ing tucked away can be seen in F igure 1.6. Thaumoctopus mimicus and O ctopus insularis were both observed capturing and consuming prey while still foraging, but also bring ing several back to their den for later consumption (Hanlon et al 2008, Leite et al 2009) This allows for a larger amount of food to be consumed throughout the day, wheth er foraging or not. Figure 1.6: An octopus that has tucked away prey items under its webbing. Image from: http://www.nwf.org/News and Magazines/National Wildlife/Animals/Archives/2003/Outsmarting the Competition.aspx
11 In laboratory studies, octopuses have been shown to be efficient visual predators capable of detecting and attack ing prey based on sight alone (Maldonado 1964 Hanlon and Wolterding 1989 ). This does not s eem to be the case in the wild. Although it may not be the source of prey detection, vision is still used for spatial recognition and orientation (Boal et al 2000, Forsythe & Hanlon 1997, Mather 1991). Octopuses which forage a certain area one day tend to not forage there the next, hinting that they have a visual memor y of certain areas visited (Boal et al 2000). The visitation of certain patches after several days has also been noted. Furthermore, w hen returning from foraging, octopuses generally use steady pace swimming in place of crawling. Forsythe and Hanlon (19 97) and Mather (1991) also noted that returning routes differ from the initial route taken to forage. This suggests that octopuses are capable of learning their surroundings visually. Vision is also used in predator recognition and cryptic behavior deter m ination. It allows them to match their surrounding s accurately and conceal themselves from potential threats (Leite et al 2009). In some cases, octopuses can switch from tactile to visual location. Hanlon and colleagues (2008) noted the discovery of a pr ey item by T. mimicus that soon escaped. This prey was followed visually until captured. 1.4 Hunting and Feeding Behavior Hanlon and Messenger (1998) described 7 overall mo des of hunting in cephalopods, 4 of which are applicable to octopuses: ambu shing, pursuit, hunting in disguise and speculative searching When ambushing, octopuses lie and wait for po tential prey items to pass by them. These octopuses hide in shells or bury themselves
12 in the substrate, such as O. joubini and O. burryi (Hanlon 1983 ). Once the prey is within arms distance, it is grabbed and pulled in for consumption as shown in figure 1.7 When pursuit hunting, then laun ches itself at it. While doing so, the octopus has a chance to miss if the prey item moves out of the determined position (McFarland 1981) O ctopus cyanea has occasionally been seen pursuing crabs it sees while foraging (Yarnall 1969). Figure 1.7: A frame by frame shot of sh owing the use of lie and wait or ambush tactics. Image from: Sumbre et al, 2005
13 While hunting in disguise, octopuses match their surroundings and mimic the world around them to trick both predator and prey, allowing for less energy spent fleeing from predators or pursuing prey (Hanlon and Messenger 1998). Hapaloclaena display body pa tterns similar to algae to hunt prey more effectively ( Borrelli et al 2006). Octopuses have bee n known to assume an array of patt erns to match their environment When hunting, prey are unable to recognize the octopus. They may also be unaware that what they are seeing is a potential threat to them. This cryptic behavior also allows for concealmen t fro m predation while hunting. By matching its environment while searching for food, an octopus is able to f ocus on locating prey instead of continually fleeing from potential predators. One spectacular example of this is presented by Hanlon and colle agues (2008). After observing 3 species of mimic octopuses ( Thaumoctopus mimicus, Wunderpus photogenicus in North Sulawesi, Indonesia, their ability to match not only substrate color and textures, but other organisms became apparent. This ability can clearly be seen in F igure 1.8. While stationary, they matched the substrate flawlessly. Furthermore, they were capable of resembling various invertebrates, including colonial tunicates, retracted polycha ete tube worms, and sponges. In fact, the mimic octopuses were so adept at mimicry that Hanlon (2008) noted they viewing within 1 m before an octopus could be distinguis While mobile, they swam along the substrate resembling a flat fish such as a flounder. swims, these octopuses created obscured and confusing body patterns, possibly to confuse predators into being unsure of exactly what thes e octopuses were.
14 Figure 1.8: Mimic octopus: A) sentinel state at den opening; B) normal foraging coloration; C) flatfish mimicry; D) flatfish model, banded sole; E) lion fish mimicry; F)lion fish model; G) sea snake mimicry; H) sea snake model. Image from: Norman et al 2001 Most benthic octopuses exhibit speculative hunting using either pouncing or grouping. When pouncing, an octopus engulfs a rock, coral, or patch of sediment with its webbing and feels under it with the tips of its a rms in hopes of finding food. Groping occurs when probing holes in sediment, rocks, or other places unable to be covered by the These methods allow for a wide range of species consumed. Due to their incredible ability to learn, these t actic s are well suited for octopuses (Hanlon and
15 Messenger 1998 ). These tactics also present two strategies that are beneficial to octopus hunting. They may use a win stay or win switch approach to hunting patches, as stated by Mather (1991) Win stay b rings the octopus to the same forage patch based on previous success. Win switch drives an octopus to search for new flourishing patches after another has been successfully visited. Table 1.1: A description of each of the nine foragi ng movements used by octopuses (Rosebrock 2000). Within these tactics, 9 movements have been described in successful prey capture: poke, crawl, tuck hold, webover, pause, jet, pull, crawl and poke, and tuck hold and poke (Rosebrock 2000). Poking may be considered the same thing as groping. Crawling is the movement from one patch to another. Tuck hold is the capture of prey and tucking it into the webbing or holding it in one of the many arms. Webover is again the same as pouncing. Pause is a period when no movement or foraging is taking place. Jet i s the rapid backwards swimming using jet propulsion via the siphon. This generally occurs during predator avoidance or pursuit capture of escaped prey. Pull is the act of pulling a prey item from a hole or crevice. Crawl and poke is the active movement o f the octopus w hile poking in search of prey. Finally, tuck hold and poke is the seizure of prey while Movements Description Poke Probing holes in sediment, rocks, or other places unable to be covered by the Crawl The movement from one patch to another Tuck hold Capture of prey and tucking it into the webbing or holding it in one of the many arms Webover Pause Period when no movement or foraging is taking place Jet Rapid backwards swimming using jet propulsion via the siphon Pull The act of pulling a prey item from a hole or crevice Crawl and poke Active movement of the octopus while poking in search of prey Tuck hold and poke The seizure of prey while still investigating the surrounding holes in hopes of finding more prey
16 still investigating the surrounding holes in hopes of finding more prey These are listed in T able 1.1 Once prey is captured, it is either consumed w hile foraging is continued or it is brought back to the den for later consumption. Octopuses generally feed on four main types of organisms; bivalves, gastropods, crustaceans and even fish (though this can vary). When feeding on a prey item, octopuses first attempt a method called pulling (Fiorito and Gherardi 1999). By simply using their dexterity and strength, they attempt to pull prey items out of their shells or pull them apart and eat them. Pulling is usually attempted first since it takes much l ess energy to perform. When pulling, the handling time is lowest and the overall energy use, though higher than average, does not last for that long. The process of physically pulling lasts only seconds at a time (McQuaid 1994). Bivalve prey are genera lly the only organisms pulled, though octopus es have been known to pull hermit cr abs from their shells as well (Fiorito and Gherardi 1999). Bivalve size also contributes to the decision to pull. The larger and stronger the bivalve, the less likely an octo pus will be able to pull it apart. Because of this, smaller octopuses tend to choose even smaller prey in order to conserve the energy exerted (Leite et al 2009). Overall, though this method is highly efficient when applicable, it does not allow for the consumption of many other prey items, such as gastropods or larger bivalves. When unable to pull, octopuses resort to drilling (Steer and Semmens 2003) Drilling occurs when either pulling of a bivalve shell has failed or organism cannot be pulled from its shell (gastropods, shelled crustaceans). Though this process takes much more time and energy than pulling, it is much more effective at extracting
17 prey Once unsuccessful at pulling, an octopus will rotate its prey item to its desired position and ready it for drilling (Steer and Semmens 2003). This rearrangement depends upon the prey species and size. It has also been noted that switching from pullin g to drilling may be considered a compromise between the overall time taken to handle and feed (which may be used for finding more prey or feed on easier prey items), and overall energetic cost of a specific prey item being drilled (Anderson et al 2007). It has been seen in gastropods that pulling may not even be attempted due to morphological features that makes pulling nearly impossible. The overall decision process of an octopus feeding can be seen in F igure 1.9. Though location of drill holes varie s greatly based on prey species and size, octopus size and age, as well as geographical location of an octopus, the process of drilling is the same throughout. It is done using both mechanical and chemical processes. Examples of pulling, drilling, and dr ill holes in shells can be seen in Figure 1.10. Many factors affect the decision made when drilling into the shell of a prey item. Generally, the hole location differs between bivalves and gastropods. This difference is mainly due to both internal and e xternal morphology. Anderson and colleagues (2007) showed that in bivalves, Octopus rubescens chose its drilling location on bivalves based on the location of the adductor muscle. They fed bivalves to 10 Octopus rubescens over a period of time. The vari ous thicknesses across the shells were noted and drill holes for each were examined. The data showed that although the shell near the adductor muscles was thicker, it was their preferred drilling location. This is because the neurotoxin injected by the o ctopus would have a direct effect on the nervous system via the adductor muscle (Cortez et al 1995).
18 Figure 1.9: Flow diagram showing the behavioral sequence performed by O. vulgaris to get bivalve prey items. Image from: Fiorito, 1999
19 Figure 1.10: a ) Octopus vulgaris going through the different stages of pulling a bivalve open. b ) Drill holes in a gastropod shell made by O. vulgaris. c ) Octopus vulgaris attempting to capture, pull, drill, then consume a prey item. d ) Octopus rubescens drilling a bivalve. Images from (in order): Fiorito, 1999; Arnold, 1969; Fi orito, 1999; http://www.oac.cdlib.org/ view?docId =kt2x0n9933&chunk.id=d0e4138 &brand=oac4&doc.view=entire_text a c b d Holes indicated by arrows. Holes indicated by arrows. Holes indicated by arrows.
20 In gastropods it has been found that hole boring locations are generally chosen Arnold 1969). This target area is usually considered the apical spire of gastropod she lls. This area is the thinnest and is very accessible when handling prey (Nixon and Macconanchie 1988). The grip of an octopus makes this area easy to drill. Specific envenomation of certain muscles is not as necessary in gastropods a s long as the gast ropod itself is injected somewhere on its body, it will relax. This higher location also ensures skin penetration of a retreated gastropod. These locations do vary based on several factor s and may not always be the same in both the lab and the wild. I n bivalves, the valve periphery was also drilled. This was seen by Cortez and colleagues (1995) in a small amount of prey items examined. Arnold and Arnold (1969) and Nixon and MacC onachie (1988) observed gastropod shells to have drill holes located in t he sutures of the body whorls In these studies, time, prey size, and prey species were factors. Due to certain shell morphology, the apical spire is not as easy to access when handling. efficiently dri ll in its preferred location. In some instances, several holes were drilled after the initial was unsuccessful at properly reaching the prey item inside (Nixon 1980). The octopuses themselves also influence the locations of hole borings. This includes individual experiences, observation of others, size, age, and geographical location. Hanlon and Messenger (1998) described Octopus vulgaris in both the Mediterranean and Caribbean using different techniques to drill holes even though they are the same s ize and species. These techniques were uniform throughout individuals in certain areas. Mather and colleagues (2012) noted that the feeding habits of several
21 species of octopus vary. Hatchlings uninfluenced in a lab setting tended to feed the same altho ugh their parents were from different locations. The researchers did note that differences in hole borings may be attributed to some prey being more encountered by a certain species than others. Mather (2006) also observed differences at various stages in ontogeny. As an octopus becomes older, it gains more experience. These trials allow for active learning, giving the octopus a greater chance of success as well as expending the least amount of energy while feeding. Overall general size of an octopus has an effect as well ( Steer and Semmens 2003 ). Overall, small octopuses drill more and choose varying locations based on the most efficient way to handle their prey. Their size limits their overall ability to drill as cost effectively as their larger co nspecifics. 1.5 Prey selection Octopuses are considered generalist s in terms of prey selection (Anderson and Mather 2007). This is supported by the wide range of prey items found within the middens outside their dens (Cortez et al 1995, Mather 1991, H anlon et al 2008). Although they are considered generalists, octopuses have learned to specialize on three main prey types: gastropods, bivalves, and crustaceans (Mather et al 2012). Furthermore, individual octopuses have been known to specialize and pre fer certain prey types when given the chance. Mather and colleagues (2012) observed midden contents over several years in five separate locations. Overall, the octopuses seemed to feed on an array of species, most individuals had several specialized pref erences. Even the same species of octopus in the same locations were consuming very different prey items.
22 These decisions in prey choices are shaped by several factors, including octopus size, location, experience, and prey species, size, and availabili ty (Leite et al 2009). Steer and Semmens (2003) found that smaller octopuses prefer smaller prey, especially in bivalves. Smaller prey makes it possible to expend less time and energy extracting the prey via pulling instead of drilling. As the prey size increases, pulling becomes less feasible. Octopuses do choose larger prey as well depending on their preference. It may be far more worth it. Prey species play a lar ge role in the selection of prey items. The literature does provide conflicting data on prey preferences by species. In the wild, gastropods seem to be the preferred organisms to consume (Steer and Semmens 2003, Cortez et al 1995, Hanlon and Forsythe 200 7); but in captivity, crustaceans are the most desired prey item among almost all octopuses (Hanlon and Messenger 1998, Hanlon and Wolterding 1989). These conflicting ideas can be explained when looking at the overall lifestyle of each prey species. Gast ropods require much less energy to capture and can easily be carried back to their middens. Crustaceans, in some situations, must be ca ught and consumed where found. This is dependent upon several factors including foraging area (i.e. grass beds, coral, and sand ), octopus size, crab size, and crab species. They are also easily eaten in a short amount of time All in all, the data may be inconsistent, but can be explained and supported. This ease of consumption as well as high nutritional value is also why crustaceans can be considered the most desired prey item (Hanlon and Messenger 1998).
23 Octopuses livin g in close proximity to each other may prefer completely different prey. T he age and size of the octopus can play a role in the preferred prey choice (Iribarne et al 1991). Experience is also a major influence on what each individual desires. Two closel y spread octopuses may forage through completely different patch types, meaning they will encounter a vastly different microhabitat of organisms. What may be a common prey item for one octopus could end up being a rare occurrence for another (Onthank and Cowles 2011, Iribarne et al 1991). Furthermore, same specie octopuses in vastly different locations experience this as well. Experiences also differ when encountering various prey items. What is simple to capture for one octopus may be a possible threat to another based on previous encounters. 1.6 Octopus joubini The Atlantic pygmy octopus, Octopus joubini is a small benthic species of octopus found within the tropical waters of the Caribbean and Gulf of Me xico near Florida at depths of < 10m. They grow anywhere from 2 4cm but reach sexual maturity at sizes as small as 1.6cm (Figure 1.11) These octopuses are nocturnal and spend a large portion of their time in gastropod and bivalve shells. Although they are mainly lie and wait predators, they occa sionally forage when the need arises due to lack of prey availability. Because of their small size, abundance in select areas, and overall benthic lifestyle, O. joubini has been extensively studied in terms of life history, spatial distribution, behavior, and culturing in laboratories (Forsythe and Toll 1991)
24 Figure 1.11: A young Octopus joubini (mantle 1.5cm). Image from: Ben Merlino Females lay broods of anywhere from 150 2500. After broods are laid, the female soon dies. These eggs are approximately 2.6 3.8mm depending on when they were laid. After 4 6 weeks the eggs hatch and begin their planktonic swimming stage of development. These hatchlings are on average 2.5mm and feed within the first 24 hours of hatching. After about 1 month the young octopuses become fully benthic, though they face a 90 95% mortality rate by the time this stage is reached. In 3 4 months, the octopuses become completely sexually mature. At hatching they have 6 8 suckers, but obtain hundreds more throughout ontogeny. As they mature, their reddish brown color becomes quite apparent and the skin is overall very smooth with ocular papillae across the skin Arm length is very short compared to other octopuses, measuring in at only 2 3 times the mantle length. In the wild, competition for shells to inhabit occurs often McLean (1983) noted
25 that shells are not only homes for the octopus, but fo r crustaceans and many fish such as the gobiids Prime living conditions are found in warm waters (25C) in grass beds with sandy bottoms. Octopus joubini is capable of learning what is considered its prime habitat (Mather 1972). Because of their living conditions, O. joubini individuals rarely forage far, or at all, from their home shell. Foraging too far or for too long can result in another organism inhabiting their home. Even though much is known about their life history and behavior, not much information has been recorded on their feeding behavior. In laboratory studies, it was seen that these octopuses were picky in their prey choice and not many researchers strayed from providing their favored food shrimp. Mather ( 2006 ) did find them feedin g on hermit crabs in the wild, but their method overall for obtaining their shelled prey items is unknown. Although pulling and drilling can occur, which is used? Further, when drilling, hole position on the prey item has not been described. It is not kn own whether prey species and/or size affect the decision to drill or the hole placement.
26 Methods 2.1 Collection and Maintenance of Octopuses The O. joubini used in this study were obtained from Gulf Specimen Marine Laboratory (http://www.gulfspecimen.org/catalog/specimens/index.html) Seven octopuses in total were used. Ten were obtained, but three died within 24 hours of arrival. Three recently hatched juveniles were received in August 2011. E ach had a mantle length of app roximately 1cm and arm length of about 2cm (Figure 1.11) Four wild caught adults were obtained in November 2011. Mantle length of the second octopuses ranged from 2 3.5cm with an arm length of 3 6cm. The exact age and weig ht of the specimens were not de termined At Pritzker Marine Biology R esearch lab, the octopuses were housed in separate aquaria in the upper level of the laboratory. Each octopus was kept in an identical 10 gallon aquarium placed in a sea table (Figure 2.1) The aquaria had screen l ids with water flowing from a hose into the aquarium through the lid. The water was filter ed by overfl owing out of the aquarium into the sea table. It would then be removed from the sea table through a protein skimmer. Aquaria were checked to ensure app ropriate salinity, pH, and te mperature. The first three aquaria were supplied with two plastic plants, a flat piece of coral, a large shell, and a gravel bottom The second four aquaria were supplied with a small pvc pipe, a shell, and two plastic plants It was determin ed that in the first three aquaria it was increasingly difficult to locate the octopuses, so t he setup was simplified The new setup allowed for quick finding of the octopuses while still maintaining their level of comfort with various hi ding places.
27 Figure 2.1: The overall set up of the aquaria. Image from: Ben Merlino The first three octopuses were initially given a diet of thread herring, but once experimenting began, they were fed small hermits crabs ( Pagurus sp ) These octopuses were fed several hermit crabs every other day during initial observations, though this fluctuated occasionally depending upon their feeding habits at any given time. The second four octopuses were fed a diet of Pagurus sp., Palaemonetes sp., and various gastropod species. These octopuses were alternately fed gastropods or hermit crabs every few days or until all prey items were consumed. Grass shrimp were used for feeding when other prey items were unavailable. All prey items were cau ght in Sarasota B ay F igure 2.2 near the Pritzker Marine Biology Research Center These prey items were
28 chosen because they occur naturally in the habitat where O. joubini are found and are most likely the common prey items for these octopuses. F igure 2.2 : The area in Sarasota bay where the prey items were collected, as indicated by the red box. The arrows show the location of Pritzker Marine Biology Research Center in reference to the collection areas. Image from: Google maps
29 2.2 Initial Observations Due to a lack of literature on some aspects of the subject, initial obse rvations were conducted to obtain a better idea of the general behavior of O. joubini Because they are active nocturnal ly observations took place anywhere between 2200 and 0300 between the months of July and August of 2012 During observations, any notable behaviors were recorded. This was also the time in which the octopuses were fed. This allowed for foraging behaviors to be viewed and recorded. Anything was consi dered a behavior if continually observed, especially in reaction to a certain stimuli Behaviors from previous studies of other species were used as a reference. While feeding, handling time, grip on the prey item, movements done while capturing and feed ing on prey, and method used for extracting the prey item were recorded. Various objects were also placed into the aquaria to see how these octopuses interac t with new objects. A rock, a L ego piece, a glass marble, and an unfamiliar artificial plant were used. The reaction to each new object was recorded. This included time taken to make contact, which arm(s) used to investigate the object, and what was done with each object that was presented. An ethogram was the generated based on the data collected. 2.3 Shell Observations The second four O. joubini were used in determining drilling behavior based on prey size, prey type, species, and shell size. The octopuses were fed every several days either gastropods or Pagarus sp. Hermit crabs and gastropo ds were never given to the octopus simultaneously. Before each pr ey item was placed into the aquarium the species were sorted and marked using fast drying fabric markers. Hermit crab shells were
30 marked using a blue mark er, hermit crab shells with pre ex isting holes were marked using a black marker, and gastropods with any abnormalities on their shells were marked with green marker. Gastropods with fully intact shells were left unmarked (Figure 2.3) Before each feeding all specimens, whether consumed or not, were removed. This made sure that no hermit crabs entered a shell from a previous trial. Each shell obtained was observed under a dissecting microscope and classified into four categories: no hole borings present, one hole boring present created by O. joubini hole(s) present not created by O. joubini and multiple holes present with at least one created by O. joubini Once classified, the location of the O. joubini hole boring (if present) was recorded as well as the shell type, size, and inhabitant of the she ll during consumption. Data were organized into the number of hermit crab shells with/without hole borings and the number of gastropod shells with/without hole borings. After initial data collection, prey preference was also recorded This information was based upon the prey items consumed first when present with varying choices, such as different prey types, similar prey species ranging in size, and a combination of the two. Aquaria were checked every few hours and the prey consume d versus the living prey was documented. Figure 2.3 Several shells marked. The two outer shells are unmarked because they were fully intact gastropod shells. The middle left shell was marked blue because it was a fully intact hermit crab shell. The mi ddle right shell was marked black because it was a hermit crab shell with existing holes.
31 Results 3.1 Basic behavior Based upon the behaviors obse rved, a table was created (T able 3.1 ) outlining them When foraging, both vision and tacti le sensation were used to find prey. Pouncing and gro ping were the modes of forag ing practiced when using tactile senses Sit and wait, or ambushing, was used in conjunction with vision. The octopuses were seen actively following their prey visually when the prey w ould approach their places of hiding. Defense behaviors were also noted. Octopus joubini had thick ink that stayed present for several minutes before dissolving. This ink was a dark purple to black color. When alarmed, one of two behaviors would occur : blanch or blending with surroundings or dark brown/red coloration with light brown and red spots. When threatened, several behaviors were exhibited besides inking. This included remaining still, flattening to the substrate or lower into a hole or crevi ce, jetting, positioning one or two arms straight, performing an All octopuses were seen consistently performing these behaviors in no particular order overall, though some preferred certain behaviors to others. Several types of locomotion were used. When moving along the bottom crawling and both bipedal and multi armed walking were performed. When swimming, forward and backwards swimming or jetting were performed.
32 Table 3.1: Shown below table outlining the basic lifestyle, activity levels, and overall foraging and defense behaviors of Octopus joubini. Functional group Behavioral unit Character states Habitat characteristics General habitat Reef areas, shoal grass, sandy bench Depth Intertidal zone (though can be found as low as 5m) Energy level Varying tidal energies Den microhabitat Within rock crevices and shells (bivalve/gastropod) Den occupancy Days months (depends upon competition and availability)(in lab, stayed in on den) Gather objects to den entrance Yes (rocks, shells, coarse substrate) Aggregation Occasionally (based on den and prey item density) Activity Circadian r h ythms Diurnal, nocturnal Peak hours of: Mating: throughout the night Foraging: throughout the night Den construction/excavation: After sunset into midnight Resting behavior Closed within shell, hidden with rock crevice Foraging behaviour Substrate foraged Sea grass sandy bench, reef flat, within rock crevices General means of detecting prey Tactile, Visual (either groping, pouncing, or sit and wait) Mode groping, pouncing, or sit and wait (mainly hides within den or rock crevice awaiting prey items) Means of opening prey pulling (crustaceans) and drilling (gastropods) Prey preference Crustaceans Defense Ink, when released from animal: Thick and mucusy; present for minutes Ink color purple/black
33 Functional group Behavioral unit Character states Other reaction to threat Remain still; flatten to substrate or lower into hole; jet; with one or two straight arms; arm autotomy two under substrate Burying behavior Occurs (mainly during predator defense) Locomotion Benthic Crawling, bipedal and multi armed walking Swim above substrate Forward swimming, backwards swimming/jetting 3.2 Object recognition Octopus joubini reacted to various objects presented to them in a way unlike most other octopuses recorded. A total of four trials were performed with each object. In every trial, the full 90 minutes passed without the octopus attempting to make any contact with the objects presented. After chec king on the aquaria several hours later, every octopus did one of two things depending upon the objec t. If presented with the rock, L ego piece, or glass marble, the octopuses would all eventually use them to block the openings of their dens. If presented with the plant, the octopus would use it for resting as well as ambushing prey. When interacting with any object, not just a new one, the front right tentacle was consistently used first in every object interaction observed. It was also found that famili arity affected the overall amount of time it took to move and interact with the object. The exact time is unknown, but in general, within several hours the rock would be moved and used. The plant was also used in the same amount of time. These objects a lso were already present in some form within these
34 aquaria. Octopus took no less than six hours to interact with the marble, and a minimum of eight hours before the octopus would investigate the Lego piece 3.3 Drilling and pulling 3.3 .1 Gastropods Overall 140 gastropod shells were observed, representing eleven different species. The shells ranged in size of 1cm 4.5cm. Of these 140 shells, 130 were found to have drill holes. No shell over the length of 1.5cm was left intact. The abundance of each p rey represents the overall abundance of the prey items in Sarasota Bay. It was observed that 92.9% of the time Octopus joubini drilled gastropod prey items with which they were presented. These prey items are shown below in table 3.2. Table 3.2: Below describes each species of gastropod consumed, followed by ranges and number drilled. Common name Scient ific name Sizes (cm) Drilled Intact Florida crown conch Melongena corona 1.5 4.5 45 3 Carribean false cerith Batillaria (Lampania) minima 1 1.5 38 3 Common eastern dog whelk Nucella lapillus 1 1.5 23 4 Common Atlantic marginella Prunum apicinum 1 1.9 8 Ivory cerith/Dark cerith Cerithium sp. 3.25 4 Banded tulip Fasciolaria hunteria 3 3 Ebony turrid Crassispira sp. 1 3 Atlantic dogwinkle Nucella lapillus 2.5 2 Lightning whelk Busycon sinistrum 2.5 2 Pear whelk Busycotypus spiratus 4.25 1 Gem cyclostreme Marevalvata tricarinata 1 1
35 3.3 .2 Pagurus sp Overall 212 Pagurus sp. shells were observed, representing eight shell species. The shells ranged in size of 0.75cm 5cm. Of these 212 shells, 62 were found to have drill holes. It was observed that 29.2% of the time Octopus joubini drilled hermit crab prey items presented to th em. Table 3.3 show s the shell species inhabited by the hermit crabs, as well as size ranges and number drilled. A direct correlation between drill decisions and prey size was found. Once a prey item reached 2.25 cm, the prey item was drilled 100% of the time (Figure 3.1 a ) Table 3.3: Shell species inhabited by the hermit crabs, size ranges of t he shells, and number drilled. Common name Scientific name Sizes (cm) Drilled Intact Carribean false cerith Batillaria (Lampania) minima 0.75 1.5 40 98 Florida crown conch Melongena corona 2.5 5 10 Common eastern dog whelk Nucella lapillus 0.75 1.25 5 28 Ebony turrid 2 3.5 4 West Indian dove snail Columbella mercatoria 1 3 Fly speck cerith Cerithium muscarum 1 10 Ivory cerith/Dark cerith Cerithium sp. 0.75 1.2 8 Gem cyclostreme Marevalvata tricarinata 1 1.2 6
36 Figure 3.1: a) Percen t drilled based upon shell size inhabited by a hermit crab. n=212. This number represents all shell species of Pagurus sp. drilled. b) Observed shells laid out according to size, species, and drill hole condition. 0.00 20.00 40.00 60.00 80.00 100.00 0 1 2 3 4 5 6 Percent drilled Shell size (cm) Percent drilled based upon shell size ( Pagurus sp. )
37 3.3 .3 Drill Hole L ocations The location of the drill holes was only dependent upon the shell species and was not related to what type of prey item was inside the shell. Drill hole location was dependent upon shell shape, shell size and thickness, and shell species. The holes were made in the weakest area, mainly being the sutures approximately 2/3 ab ove the columella, depicted in F igure 3. 2. This was not the case in shells where the sutures are located close together near the apex, such as the Melongena shell shown in F igure 3.2. Figure 3.2: Top) Overall anatomy of a shell. Bottom) Melongena shell with sutures located higher up th e shell near the apex. Image from (in order): a) http://www.manandmoll usc.net/lesson_plan_anato my_files/gastropod_shell. html b) http://www.gastropods.c om/0/Shell_142 20.shtml
38 Figure 3.3: Family Melongenidae. Average drill hole locations on shells Drill holes indicated by arrows (note: not all drill holes represented, just average). Image from: http://www.sealifegifts.net/Florida Crown Conch 2in -287.html
39 Figure 3.4: Family Batillariidae Top left) Average drill hole locations on shells Drill holes indicated by arrows (note: not all drill holes represented, just average). Top right and bottom) Drill hole done by O. joubini on a false Caribbean cerrith. It can be seen that drilling along the sutures is preferred. Images from (in order): http://www.idscaro.net/sci/01_coll/index.htm ; Benjamin Merlino; Benjamin Merlino
40 Figure 3.5: Family Columbellidae. Average drill hole locations on shells Drill holes indicated by arrow s (note: not all drill holes represented, just average). Image from: http://shellmuseum.org/shells/shelldetails.cfm?id=90
41 Figure 3.6 : Family Buccinidae. Top left) Average drill hole locations on shells Drill holes indicated by arrows (note: not all drill holes represented, just average). Top right) Drill hole from O. joubini on a dog whelk. Bottom) drill hole locatio n on lightning whelk/pear whelk as indicated by an arrow. Ima ges from (in order): http://www.gastropods.com/5/Shell_3305.shtml ; Benjamin Merlino; http://en.wikipedia.org/wiki/Lightning_whelk
42 Figure 3. 7 : Family Fasciolariidae. Drill hole locations on shell Drill hole indicated by arrow. Image from: http://shellmuseum.org/shells/shelldetails.cfm?id=87
43 Figure 3.8 : Family Marginellidae. Average drill hole locations on shells Drill holes indicated by arrows (note: not all drill holes represented, just average). Image from: http://www.gastropods.com/7/Shell_1957.shtml
44 3.4 Prey preference When presented with similar species prey of varying sizes, blue crab size played no role in prey choice. Hermit crab size does play a factor, and the hermit crabs were consumed in order of largest to smallest each time. Gastropod prey items were also consumed in order of largest to smallest in each trial. When presented with varying prey of similar sizes, blue crabs were chosen first regardless of size. Hermit crabs were consumed next, followed by the gastropod. When present ed with varying prey of different sizes, blue crabs were again chosen first, rega rdless of if they were the smallest or largest prey. Hermit crabs were chosen second regardless of size, and gastropods were chosen last in each trial. These d ata are shown in figure 3.9 to 3.1 1 Figure 3.9 : Time it took for octopus to consume blue c rabs of varying length. No bar represents no consumption of prey item after 180 minutes. In trial one, one blue crab of 1.5cm, 3cm, and 3.5cm were used. In trial two, one blue crab of 2cm, 3.25cm, and 4cm were used. 0 20 40 60 80 100 120 140 1.5 3 3.5 2 3.25 4 Time (min) Length (cm) Blue crab varying sizes Trial 1 Trial 2
4 5 F igure 3.10 : Top) Time it took for an octopus to consume Hermit crabs of varying length. No bar represents no consumption of prey item after 180 minutes. In trial one, hermit crabs of 1.25cm, 1.5cm, and 2cm were used. In trial two, hermit crabs of 1.25cm, 1.5cm, and 3cm wer e used. Bottom) Time it took octopus to consume varying prey of similar sizes (1.5 2cm). The prey used were one blue crab, one gastropod, a nd one hermit crab. One of each was consumed. 0 20 40 60 80 100 120 140 160 180 200 1.25 1.5 2 1.25 1.5 3 Time (mins) Length (cm) Hermit crabs varying sizes Trial 1 Trial 2 Trial 1 Trial 2 0 20 40 60 80 100 120 140 160 180 Blue crab Gastropod Hermit crab Time (mins) Species Varying prey similar sizes (1.5 2cm)
46 Figure 3.11 : Top) Time it took octopus to consume varying prey (blu e crab, hermit crab, gastropod) of similar sizes (2.5 3cm). One of each was consumed. Middle) Time it took for octopus to consume varying prey of varying sizes. No bar represents no consumption of prey item after 180 minutes. In trial one a blue crab (3. 5cm), a hermit crab (1.5cm), and a gastropod (2.5cm) were used. In trial two, blue crab (2cm), a hermit crab (3cm), and a gastropod (1.5cm) were used Bottom) Time it took for octopus to consume gastropods of varying length. No bar represents no consumptio n of prey item after 180 minutes. 0 20 40 60 80 100 120 140 160 Blue crab Gastropod Hermit crab Time (mins) Species Varying prey similar sizes (2.5 3cm) 0 20 40 60 80 100 120 140 160 180 Hermit crab 1.5 Gastropod 2.5 Blue crab 3.5 Hermit crab 3 Gastropod1.5 Blue crab 2 Time (mins) Species and length (cm) Varying prey varying sizes Trial 1 Trial 2 0 20 40 60 80 100 120 140 160 1 1.25 2 1.25 1.5 2 Time (mins) Length (cm) Gastropods varying sizes Trial 1 Trial 2
47 Discussion 4.1Behavior Octopus joubini observed in this study exhibits many behaviors similar to most other octopuses. T hey do show some behaviors that few other octopuses normally do. When threatened, normally an octopus will raise its arms, flatten to the substrate, or even jet away leaving a trail of ink in its wake. When O. joubini is threatened, it may bur y under the substrate or block aperture of its home shell This has been seen in as well in juvenile O. vulgaris. When resting or presented with a threat, they will block their den openings (Katsanevakis and Verriopoulos 2004) This tends to not be seen as much on ce maturity is reached. In O. joubini, not only did this behavior occur consistently throughout maturity, it was found that it was used for foraging as well. Octopus joubini would hide beneath the substrate or within shells waiting for prey to come close Once in range, O. joubini would strike. Hanlon and Messenger (1998) described this behavior in only one other octopus, Octopus burryi When given a new object, O. joubini seemed wary This was not limited to one individual. Each of the three present ed with new items did not investigate them for some time after being introduced into the aquaria. These octopuses rearranged objects in their aquaria almost daily. When presented with new objects, they would move them and apparently use them in different ways use in the struct ure of a new den all together. This has been seen in many other octopus species and is a common practice in captivity. Mather and Anderson (2000) noted this behavior and in some o ccasions even equated some behaviors of investigating new This study could have been very much improved upon in terms of data collection.
48 Attempts to film these trials to eliminate researcher influence failed due to lack of equipment with proper night time capabilities. 4.2 Feeding When actively searching throughout their aquaria for food, th e octopuses both pounced and gro ped depending on the situation presented. In most cases, gro ping was used. The octopuses would investigate within shells to determine if there was prey present. In the wild, octopuses have been commonly known to exhibit similar foraging behaviors. Their decision to grope or pounce is mainly based upon the microhabitat being foraged (Hanlon and Forsythe 1997) Many larger rocks and corals will allow for excellent groping, while grass beds, sand flats, and unobstructed bottoms or smaller rocks can easily be pounced. This was then followed by both the collection and movement of the prey to the den, or the placement of the prey under the web near the mouth. When mostly smaller prey was present, several would be collected and brought to the den to be consumed. After ti me passed, this became the case less and less. The octopuses appeared to learn their tank limitations and constant feeding schedule, and became less concerned with collecting as much food as possible and bringing it back to their den. When handling prey, the amount of time spent handling the prey is directly related to the size of the prey item. As the size increases, so does the amount of time needed to consume the organism. Leite and colleagues (2009) noted that, in both the laboratory and the wild, h andling time increased proportionally to the prey size. When considering a time minimizing predator foraging, based upon this data larger prey may not always mean most beneficial in terms of energy consumption. As the prey item gets
49 larger, the shell thi ckness also increases. This means more time is spent drilling the shell and consuming the prey since there is more in general to consume. When drilling did not occur, prey size was still related to amount of handling time. Again, when the prey item was larger, it took more time to eat more. These factors must be taken into account in the wild. An octopus must choose whether to spend more time eating and vulnerable, but also gain more energy, or spend less time eating a smaller meal and obtain less ene rgy, but also have a lower risk factor from predation. Some shells of similar sizes but varying species took longer to drill. This can easily be explained by shell thickness and shape. For example, Nucella lapillus took somewhat longer to drill than c e r iths because they are thicker. Shell shape also played a factor. Although the crown conch was not as thick as some of the other shells, it took much more time because of the large crown shaped apex that t he octopus had to work around. This concept does not appea r present in the literature. It has been found that shell shape does have a large impact on prey preference. The method for consuming gastropods and Pagurus sp. did also vary slightly. Once drilled, the P agurus sp. was pulled from the shell th rough the aperture and then the This was similar to the performance of other octopus species consuming certain types of decapods (Cortez et al 1995; Hanlon et al 2008; Mather 1991) Once a gastropod s hell was drilled, it would be pulled then eaten. There was no need for a second step of removing from the carapace. When a Pagurus sp. shell was not drilled, the removal process was very simple. The octopus was able to pull the crab out of its shell wit hout t he use of drilling or envenomating In contrast, gastropods have such a strong muscular foot that it is nearly
50 impossible to pull these organisms from their shells without the use of toxins. In the few cases they were not drilled, several options w ere considered. The octopus could have directly injected its toxins into the siphon or body of a gastropod that had not closed itself within its shell quickly enough. They may have also been consumed after recently dying of other causes. There have been lab studies where only non living food was provided for octopuses. These organisms behaved and performed just as well a s octopuses fed a living diet (Fiorito 1999) 4.3 Drilling The percent overall drilled is represented by the ease of consumption of the prey item. The lower number of prey items drilled means it is easier to consume because it may not have to be drilled. In most of the drilled shells, a hole was located directly on or very near a suture in the body whorls. This is very well known to be the thinnest part of a shell. During initial tests, the drill holes were very sporadically located and seemed to have no r eal pattern to their placement. It has been found that many other octopus species drill in a similar manner (Nixon and MacConach ie 1988; Steer and Semmens 2003; Anderson et al 2007) It is also noted that the aperture of the shell is used as a reference point when drilling. Because of this, most drill holes are located on the sutures on the same face as the aperture. Octopus jou bini consistently drilled along these thin sutures near the aperture. The drill hole locations also suggest that this is used as a reference when drilling (Figure 4.1) These shells were commonly piled outside the octopus dens, just as other researchers have noted ( McQuaid 1993 Iribarne et al 1991)
51 Figure 4.1: These five pictures show drill holes located in the same area along or very near the sutures of the body whorls of a false Caribbean cerith. Holes indicated by arrows. Image from: Ben Merlino
52 As Pagurus sp. prey items became larger, the need to drill increased. After a certain length, it became too difficult to pull the hermit crabs from their shell and drilling had to occur. This may be for several reasons. The hermit crabs may become too dangerous to pull out after a certain length. They may also be too strong to pull from their shells. Steer and Semmens (2003) found similar data with Octopus dierythraeus. As both the biv alve and gastropod prey increased in size, so did handling time and necessity to drill. It was also found that, again, smaller prey were more beneficial in terms of energy consumption. A larger size range of shells would have allowed for more accurate data. Similar numbers of shells of varying sizes would have also shown a more accurate view of these octopuses and their drilling behavior. The size and species obtain ed were all based on the abundance in Sarasota Bay and finding more diversity may have required more time than there was available. Althou gh this is the case, the data are consistent with that of each individual octopus as well as the literature. In general, the octopuses learned the weaknesses of each shell species and effect ively drilled in the weakest point. In fact, the octopuses that were recently hatched had little to no trouble finding the weakest points of the prey presented and there was substantial evidence of this (Figure 4.2) n the early stages of feeding and shells generally had several on them. These holes were known to be trial holes because they rarely ever actually pierced the shell fully. After a short period of time, all octopuses effectively drilled near the weakest p oints regardless of octopus size, age, or prey size. This type of learning that these octopuses are capable of
53 Figure 4.2 These shells show examples Multiple attempts are visible and it can be seen that most of these holes never fully penetrate the shells. Holes indicated by arrows.
54 allows for much more effective foraging and overall quicker consumption of prey items. This shorter handling time increases the overall chance of survival of the octopus because it is spending less time being vulnerable while feeding. After a certain point, the octopuses did not attempt to pu ll open certain prey, mainly the gastropods. 4.4 Prey preference When presented with varying prey and varying prey sizes, the choices made were based upon one major idea: the easier it is to consume a prey item, the more valuable it is. Blue crabs, r egardless of size, were the first most desired prey. An octopus does not have to use up an extended period of time attempting to consume them. Because they have no incredibly thick shell, it is much e asier to quickly consume them. After a certain size ( 3.5 4cm), the octopuses would no longer consume the blue crabs. The reason why may become apparent when looking at time minimizing predators. It would take far too long for a small octopus to consume a prey item roughly its size. It would be much more b eneficial to spend less time eating smaller prey. The next desired prey item was the hermit crab. This may not be as easy as the blue crab to eat, and may even require drilling, but there is a chance that they will not. As the data have shown, only 29.2% of hermit crabs are drilled, though the larger ones must be drilled. The least desired prey was the gastropods. Though these are very easy to obtain, they require a large amount of effort and energy to consume. Furthermore, gastropods have been known to use chemoreception to detect predators ( Marko and Palmer 1991; Alexander and Covich 1991; Covich 1994 ). The gastropods may be
55 consumed the least because they actively avoided O. joubini while it was using ambush foraging. The gastropo ds would be able to sense the octopus and avoid it. This need to expend a large amount of energy to feed makes these not alwa ys worth feeding on in the lab. In the wild however, data show a slightly different view of prey preference. Most middens hav e bivalve shells, gastropod shells, and crustacean carapaces (Mather 1991; Hanlon and Forsythe 1997) Bivalves generally appear to be more desired than crustaceans (Hanlon and Messenger 1998) And the number of shells inhabited by hermit crabs is unknown One explanation can be seen in how octopuses forage. When presented with a high value prey such as a crab, they may take the risk and consume that prey item right on the spot where it was found. Other organisms such as bivalves and gastropods can be e asily carried back to the dens and consumed in safety there. Bivalves are also potentially easier to eat than gastropods because these may be pulled open in some cases.
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