Comparison of prey utilization in the lab and field for Octopus vulgaris

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1 Comparison of prey utilization in the lab and field for Octopus vulgaris Caitlin Carrington, Khanhchi Dam, Nicole Magana, and Ian Roussopoulos, 2012 Abstract: The relationship between prey utilization in the lab and field was investigated for Octopus vulgaris individuals in Calvi, Corsica, France. Midden contents were collected from 21 dens surveyed near STARESO research station. Nine of the occupants of those surveyed dens were captured and subjected to preference trials in the lab. Prey selection in the field was found to be significant and could be grouped geographically. O. vulgaris individuals showed statistically significant preference behavior during lab trials. Utilization in the field and lab, however, did not mirror each other, supporting that O. vulgaris act as opportunistic generalists in the field, but specialists in the lab. Introduction: Optimal foraging theory states organisms minimize energy spent in prey capture (search time) and consumption (handling time) while also minimizing personal risk and maximizing caloric intake. Generalist predators achieve this by feeding opportunistically from multiple available prey items. Specialists achieve optimal foraging by reducing handling or search time through the particular specialization they have evolved to capture and consume their prey. Being a generalist predator has the advantage of allowing the organism to survive through environmental conditions that may reduce the diversity of prey available to it at a given time. Conversely, specialist predators have the advantage of more quickly and efficiently capturing prey than their generalist counterparts making them more effective predators when their particular prey type is available (McPeek 1996). Octopus vulgaris has been shown to be an extremely intelligent animal that is adept at problem solving (Hamilton 1997). This high level of intelligence makes them especially good hunters, and allows them to successfully capture a wide variety of prey (Ambrose and Nelson 1983). O. vulgaris is considered a generalist predator, but work has been done that shows individuals may specialize on particular prey items (Anderson et al 2008). This is interesting because generalist species tend not to include individuals who specialize in specific prey selection, and specialists tend not to be capable of utilizing a wide variety of prey. Other animals that exhibit this behavior, such as dolphins and primates, are highly intelligent and capable of capturing a wide variety of prey, but can also adapt intellectually to fill environmental niches (Meynier 2008, Chapman and Chapman 1999). To have an invertebrate animal showing similar behavioral patterns as these highly intelligent vertebrates is unique, and warrants investigation. To our knowledge, no study has been done that directly compares data on prey utilization by O. vulgaris in the field with prey preference lab experiments done on those same individuals. We investigated the foraging behavior of O. vulgaris through midden collections done in the field at the STARESO research station. We also used lab based experiments of prey choice to determine the level of diet preference that is statistically significant in O. vulgaris. By running prey preference experiments and comparing the results with midden compositions, we hope to more clearly show whether these animals are opportunistic predators, strictly generalists, 1

2 specialists, or show individual behavioral preference when it comes to selecting their prey. Specifically, we have three questions and predictions we are interested in addressing: 1) Does prey selection in the field vary geographically? We predict that individuals residing in close proximity with each other will exhibit more similar prey utilization than those from groups of individuals who are separated geographically. 2) Do Octopus vulgaris individuals exhibit prey preference? We predict that we will see a significant pattern among O.vulgaris individuals who are offered a choice in available prey, specifically that a particular prey item will be chosen more often than any other. 3) Will the prey preference shown in the lab be reflected in individuals from the field locations surveyed? We predict that one of the three prey items offered in the lab experiments will be consistently chosen regardless of the prey selection shown by that individual in the field. If our predictions are correct they will support the hypothesis that these animals may act as opportunistic generalists in the wild, but show preference in selecting prey when confounding factors such as prey availability and risk of predation are removed. Methods: Study Site This study was conducted at STARESO Research Institute of Oceanography (42 34 N, E) near Calvi, Corsica during October STARESO s harbor is bordered by granite cliffs on the South and a jetty on the North. The South side offers a shallow rocky-to-sandy bottom, while the North side offers a slightly deeper, rocky substrate. North of the harbor the substrate is composed largely of granite boulders and cobble, while the South is characterized by many coves containing smaller rocks. Species Description The common octopus, Octopus vulgaris, is frequently used in laboratory studies. It is distributed throughout the tropical and semitropical waters of the world (Wood & Day 1998). This includes the Mediterranean Sea and the waters off of Corsica where this study was conducted. O. vulgaris lives a benthic life from very shallow water to a depth of 200m, and lives for years (Wood & Day 1998). From arm to arm they can reach anywhere from 1 to 3 feet (Octopus vulgaris 1999). They commonly predate upon gastropods and bivalves at dusk, which has been confirmed both in the lab and by studying them in their natural habitat (Octopus vulgaris 1999). To kill and consume their prey, O. vulgaris can either pull the prey s shells apart, or drill into it with their radula (McQuade 1993). They are quite often difficult to spot because of their remarkable ability to camouflage. However, they have conspicuous dens underneath boulders or in cobble that are surrounded by middens, or the discarded shells or exoskeletons of their prey (Wood & Day 1998). These middens are commonly studied to examine not only what the octopus is consuming, but also to determine which prey species are present in that area (Wood & Day 1998). Field Methods Snorkeling and SCUBA surveys were done from 150m north of the harbor mouth to 70m south to find possible O. vulgaris dens. Candidate dens were given a random number from 1 through 30 and tagged with flagging tape (Figure 1). Once dens were located and tagged, surveys were done every 1-4 days over a two week period to collect midden piles and record den 2

3 occupancy. Shells were collected within a 1m radius of each den. Surveys began with seven dens, and dens were added to the study for the first week until the final sample size of 21 dens was reached. The depth, GPS coordinates, and den composition was recorded for each site. Midden contents were counted, categorized into groups by prey type, and measured to the nearest millimeter along the longest axis using calipers. Where possible, bivalve shells were matched, and unmatched shells counted as single individuals, a method which is commonly used in midden studies (Mather 1991a). Shells were grouped by type as follows: abalone, Arca, clam, crab, limpet, Mytilus, other, oyster, scallop, and snail. Shells that could not be identified were grouped into other. Lab Methods Octopuses were captured throughout the study for lab trials. Individuals were caught at marked dens so midden data could be compared to lab results. A total of nine octopuses were caught from depths between 5 and 20 feet by SCUBA and skin divers who used a combination of aquarium nets and goody bags to coax each octopus from its den. Individuals were kept in one of three aquariums (tank 1: 100x50x50cm, tank 2: 60x70x127cm, and tank 3: 44x38x44cm) for a period of 3 to 5 days. No octopus was ever kept in the presence of another to reduce stress, aggression, and competition. Before release, five successful prey preference trials were conducted. A successful trial was defined as one in which the octopus consumed at least one of the prey options. Each octopus was presented with three prey options: mussel (Mytilus galloprovincialis), Arca noae, and abalone (Haliotis lammelosa). Abalone and Arca were chosen because they appeared in study middens at a high frequency, and because they were easily collected in the STARESO harbor by skin divers. Mytilus was found at a lower frequency and was chosen to act as the novel option. Using calipers, the length of each prey item was measured (mm) along the longest axis. The time of day and duration of the trials were recorded by stopwatch. Trials were run at dawn, dusk, and night due to the octopus inactivity during the majority of daylight hours. After the prey items were placed in random order equidistant from each other on one or the other side of the aquarium, the octopus choice was observed and behavior recorded for a maximum of fifteen minutes. The two prey items not chosen were removed either once the octopus began to consume its prey or once the trial was terminated depending on which would cause less disturbance to that individual. Prey items not chosen were placed in a holding tank with flow through seawater. If the octopus did not consume anything or made no movements within the allowed fifteen minutes, all three prey items were removed. The octopus was given a minimum of one hour in between trials to increase the motivation to eat. Before release, 6 octopuses were tagged by injecting a unique color of latex paint under the skin either on the mantle or between two of the arms. Subcutaneous latex paint tagging has been proven to have high visibility and retention rates, as well as cause minimal damage and mortality in fish (Wolfe and Marsden 1998). All octopuses were released within 2-10 meters of the den they were captured in when weather conditions allowed. Experimental tanks were emptied and cleaned between octopuses to prevent the next inhabitant from chemosensing the previous octopus or mucous trails from previous prey items. Octopuses which were recaptured less than 24 hours after release were released again, but 2 individuals were caught more than 24 hours after release and held for an additional 5 trials in the lab. Statistical Analysis In order to determine if dens could be grouped geographically by midden contents, the number of shells corresponding to each prey category was tallied for each den. The total number 3

4 of shells found for each den on each sampling day was also tallied to determine good sampling days for each den. A good sampling day was considered a day where at least 5 shells were found at that den. Data from all other sampling days was dropped. Additionally, dens were omitted if the total number of shells found at that den throughout the entire study totaled less than 10 shells. A PERMANOVA test was run to determine the overall p-value for the relationships between midden contents at each den. The critical p-value for all tests was set at p=0.05. The PERMANOVA test included a pairwise comparison of each geographic group and gave a p- value for each pair. Dens were grouped by geographical location as residing north of the harbor mouth (N), south of the harbor mouth (S), the north half of the harbor (NH), or the south half of the harbor (SH). North and south harbor were delineated by a line which began at the ladder in the STARESO dock (42 34'48.85"N, 8 43'26.84"E) and ran at a heading of 150 to the harbor mouth. A transect line was put in place at the beginning of the study and left until the last sampling day. A SIMPER test was also run to evaluate prey type abundances and their contributions to the differences between each pair of den groups. A Bray-Curtis cluster analysis was done to determine the level of dissimilarity between each of the dens. Figure 1: Diagram of the den numbers used in statistical analysis and locations in and around the harbor at STARESO. The area north of the harbor mouth is labeled N, the area south is labeled S, and H denotes the harbor. To determine if preference was exhibited in the lab, data on selected prey, position of prey in tank, and size of all prey were recorded and analyzed using Analysis of Similarity (ANOSIM) which gave a pairwise comparison to detect differences between groups of prey preference. A Bray-Curtis cluster analysis was done to show the level of dissimilarity between groups. To compare the prey selection in the field to preference in the lab, we only included field data that was used in the statistical methods previously described. We also included only those 4

5 lab subjects that were captured from the analyzed dens. Both sets of data were formatted so they could be directly compared using a RELATE test. Results: A Bray-Curtis cluster analysis on our field midden data showed dens could be grouped geographically (Figure 2). North harbor (NH) and South (S) were the most similar, and north (N) was the least similar to the other groups. In light of this result, all other tests were run with respect to geographic group. The PERMANOVA test gave a p-value of for the overall test and significant p-values for three out of the six pairwise comparisons (Table 1). The SIMPER analysis showed Arca and abalone as the top two prey types contributing to the dissimilarity between groups of dens, by as much as 50% and 45% respectively, except for the comparison between the S and SH groups where crab and abalone were the top two contributors (Table 2). 50 Group average Transform: Square root Resemblance: S17 Bray Curtis similarity 40 Distance N SH Samples Figure 2: Bray-Curtis cluster analysis. The closer the horizontal lines are to the x-axis, the more similar the connecting groups are. NH S Table 1: The PERMANOVA analysis shows significant differences between the dens in each geographic region. P- values are given under the P(perm) headings. The lower portion shows p-values for each pair of regions. 5

6 Table 2: SIMPER analysis of four geographic pairings. Prey types in each group were compiled into average abundances and compared, and the percent contributions were determined. Contributors were ranked from highest to lowest for each pair. We ran an ANOSIM test to answer our second hypothesis about prey preference in the lab. It showed that captured octopuses significantly demonstrated a prey preference in our laboratory setting (p=0.001) (Table 3). All captured octopuses selected abalone during at least two out of five successful trials. The Bray-Curtis cluster analysis divides eleven sets of trial into three subgroups (a, b, and c) according to the similarity of their prey selections (Figure 3). Group a, consisting of three individuals, only ate abalone. Group b selected abalone four out of five trials. Group c chose only two or three abalone during their trials. Both groups a and b showed a strong preference for abalone compared to group c. In Figure 4, individuals from group a and b are scattered around the vector associated with abalone, while individuals from group c cluster around either Mytilus or Arca. Out of 55 trials, Arca was only chosen once. Table 3: An ANOSIM test shows a statistical comparison of prey selection for captured octopuses and yields a p- value

7 50 60 Group average Resemblance: S17 Bray Curtis similarity Group b c a Similarity Chuck Norris Little Caitlin Little Caitlin Tiger Littlest Khanchi Samples Figure 3: Bray-Curtis Cluster analysis of prey selection for octopuses in the laboratory. The X-axis represents individual octopus IDs, and the Y-axis shows a scale of similarity. Cedric Little Jimmy Cedric Chubs Little Ian The Kraken The Kraken Arc Resemblance: S17 Bray Curtis similarity Chuck Norris 2D Stress: 0 Group b c a Cedric Littlest Khanchi Ab Little Caitlin Tiger Myt Chubs Cedric Little Ian Little Jimmy Figure 4: Bray-Curtis cluster analysis of prey selection for octopuses in the laboratory. The three vectors dividing the diagram signify the three types of prey. 7

8 The RELATE test was used to analyze our third hypothesis, and the Spearman Rank Correlation analysis yielded a significance level of 50.3%. Thus, the prey utilization in the field was significantly different from the prey preference exhibited in the lab. Discussion: The results from the field portion of our study made it clear that our subjects diets varied by region. The geographic groupings we chose during our sampling were both for convenience and because the habitat appeared to vary between regions. The south (S) and north harbor (NH) regions were characterized by small cobble and rocks (< 10cm) and dark-colored turf algae. The south harbor (SH) region was mostly composed of fist- to boulder-sized rocks (>10cm to 2m) and light-colored bushy algae. The north (N) region had highly variable substrate, and although the algae appeared similar to the SH region, it was patchy and less dense. It is reasonable to predict that such variable habitats might yield similarly variable densities and/or availability of prey types. We expect this to be a likely explanation for the differences we observed in midden composition, though our study did not include surveys or estimates of prey presence in these regions. Another likely cause could be individual preference. However, as we will discuss later, our lab results ruled out that possibility for this group of individuals. It is not surprising that our SIMPER test revealed abalone and Arca as the top two contributors in the differences in prey selection between most of our regions. As we collected middens, we noticed that most were primarily composed of either Arca or abalone, with few exceptions (one of the dens in the NH region heavily favored scallops, and another two in the same region always had crabs in their middens). It was rare that a midden pile would contain comparable quantities of both Arca and abalone, which provides more evidence that prey availability in each area might be variable. For the lab experiment, all captured octopuses showed a strong preference for abalone. Possible explanations might include the difference in caloric content and handling time among the three prey types offered. We noticed that the Arca used were much smaller in size compared to the other two prey items offered as well as what we found in the middens. Therefore, Arca might offer a lower nutritional value. Additionally, abalones seemed to be easier to handle because they are not enclosed in two shells like the bivalves. This might be another factor that explains abalone popularity among our lab subjects. Controlling for variables like size and caloric content of the prey could give a more definitive conclusion. Our study shows strong supporting evidence for the idea that Octopus vulgaris individuals act as opportunistic generalists in the field, but show preference in prey selection when confounding factors such as risk of predation and prey availability are removed, such as in the lab. Our results demonstrate that there was absolutely no relationship between what our study octopuses were choosing in the lab versus in the field. This is consistent with a key assumption of optimal foraging theory, which states that individuals will try to lower their risk of predation in lieu of pursuing preferred prey. Common predators of octopuses in the Mediterranean are moray eels and conger eels (Common Octopus 2010), which we observed to be prevalent in our study area. Thus, it is more beneficial for an individual to hunt within its home range, where it can easily navigate back to the protection of its den, regardless of whether its preferred prey is found within that range. This will lead to the octopuses not always consuming their preferred prey type, but instead selecting prey based on factors such as availability. An additional observation noted in the lab was a hoarding-like behavior displayed occasionally by some individuals. This would occur most frequently with recaptured individuals, or those who were nearing the end of their five trials. An individual would first select a prey item 8

9 with one arm, then grab the remaining prey items and tuck them under its mantle. This behavior prevented us from removing the remaining prey items as per our experimental procedure. This is compelling as it appears the animals were learning the pattern of the experiment and remembering that items were being taken away at the end of each trial. This suggests that they made an association between selecting a prey item and the subsequent removal of the remaining prey items. This may reflect behavior in the field when an octopus individual is attempting to hunt in the presence of other predators or competitors. Though O. vulgaris is a well-studied species, there is still much to learn. One possible direction for future research would be to examine the strength of the correlation between midden composition and prey availability. This would be accomplished through invertebrate transects carried out in areas that encompassed the home ranges of octopuses and comparing that data to the midden compositions of those individuals. This sort of study would be useful for investigating the distance octopuses travel to hunt in this area of the Mediterranean, as well as how the surrounding area s invertebrate composition influences prey selection. One study claimed the hunting grounds of O. vulgaris to extend as much as 100m from the den (Mather 1991b). None of the dens in our study were more than 80m away from its nearest neighbor, and many were less than 10m apart. If the hunting ranges of the octopuses in our study were overlapping, we would expect their midden compositions to be much more similar. However, prey availability cannot be verified without further study. By knowing exactly how much of each prey type is available we would be able to more confidently estimate the distances these octopuses travelled to retrieve prey and the degree of prey specialization present between octopus individuals. Our study was unable to include all possible prey types found in the middens at STARESO and the surrounding areas. To address this, surveys of midden composition could be done again, and trials of similar experiments could be run in the lab but using all the prey types found during the midden surveys, as well as one or two completely novel options. It would be informative to create a hierarchy of preferred food, and compare it among individuals to see what, if any, intraspecies diet similarities exist. For this, researchers should do a bomb calorimetry test on all prey types to see how the calorie content of the prey varies between size and species. Through this, we can see if the octopuses prefer prey based on calorie content, or if there are other factors controlling prey preference in the lab such as handling time or previous experience. Lastly, we propose a further study to document changes in midden composition over time. Over an individual s lifetime, this may show the learning process in prey selection, or seasonal differences in prey usage. Learning the patterns associated with octopus prey selection would allow us to more effectively administer a conservation effort on their behalf should it become necessary. Paring midden compositions with data on changing environmental factors, such as water temperature, over several octopus generations could shed light on the versatility of this species diet and predatory behavior under stress. As a model species of specializing generalist predators, this may also set standards for other organisms similarly affected by environmental change. 9

10 Acknowledgements: We would like to sincerely thank Pete Raimondi, Giacomo Bernardi, Jimmy O Donnell, Gary Longo, and Kristin de Nesnera for their help and guidance throughout our research. We would also like to thank the Marine Ecology Field Quarter class of 2012 for their help in locating and capturing octopuses and collecting middens. Finally, merci beaucoup to the staff at STARESO Research Institute of Oceanography for housing our research effort. Literature Cited: Ambrose R.F. and B.V. Nelson Predation by Octopus vulgaris in the Mediterranean. Marine Ecology 4: Anderson, R.C., Wood, J.B., and J.A. Mather Octopus Vulgaris in the Caribbean is a Specializing Generalist. Marine Ecology Progress Series 371: Chapman, C.A. and L.J. Chapman Implications of small scale variation in ecological conditions for the diet and density of red colobus monkeys. Primates 40(1): "Common Octopus - Octopus vulgaris." The Robinson Library. N.p., Web. 6 Dec Hamilton, G What is this octopus thinking? New Scientist 154(2085):30-5. Mather, J.A. 1991a. Foraging, feeding and prey remains in middens of juvenile Octopus vulgaris (Mollusca: Cephalopoda). Journal of Zoology 224: Mather, J.A. 1991b. Navigation by spatial memory and use of visual landmarks in octopuses. Journal of Comparative Physiology 168: Mather, J.A., Leite, T.S. and A.T. Batista Individual prey choices of octopuses: are they generalist or specialist? Current Zoology 58(4): McPeek, M.A Trade-offs, food web structure, and the coexistence of habitat specialists and generalists. The American Naturalist 148(1):S124. Academic Search Complete. Web. 6 Dec McQuaid, C.D Feeding behaviour and selection of bivalve prey by Octopus vulgaris Cuvier. Journal of Experimental Marine Biology and Ecology 177: Meynier, L., Pusineri, C., Spitz, J., Santos, M.B., Pierce, G.J. and V. Ridoux Intraspecific dietary variation in the short-beaked common dolphin Delphinus delphis in the Bay of Biscay: importance of fat fish. Marine Ecology Progress Series 354: Octopus vulgaris, Encyclopedia of Life. Web Oct

11 Wolfe, R.K. and J.E. Marsden Tagging methods for the round goby (Neogobius melanostomus). Journal of Great Lakes Research 24(3): Wood, J. and C. Day Octopus Vulgaris, the Common octopus. The Cephalopod Page. Web October