Prey species preference and specialized feeding behavior in the

Prey species preference and specialized feeding behavior in the Mediterranean Octopus vulgaris. by Kaitlin Mae McConnell and Kathryn Scott, Fall 2010

Abstract Learned feeding behaviors or prey preference studies provide reason for further investigation of intelligence in animals. Octopus vulgaris in Corsica, France exhibits prey specialization. From daily collection and categorization of midden shells and also specific characterizations of individual lair sites it was found that the size of the octopus as well as spatial distribution (specifically north or south) does not play a crucial role in the composition of is diet. Depth of the lair, however, does relate to diet preference. O. vulgaris diets can be grouped by depth and by prey preference. Very shallow depths (10 ft. or less) have similar diets composed mostly of limpets while octopus in depths from 11-30 ft. seem to specialize more in abalone, clams and oysters. Some individuals who were located in close proximity to each other were also able to be grouped as having similar patterns of feeding preference. Introduction Many observational and experimental studies based on animal learning and feeding behavior are performed on marine animals, namely those considered 'intelligent'. Killer whales and sea otters are two particularly studied species for their specialized and intriguing feeding and foraging strategies. For example, a distinct difference was found in the feeding preferences between two sympatric populations of orcas that may be the cause of each group’s own foraging strategy (Ford et al 1998). Despite living within the same region, some orcas were found foraging individually and opportunistically fed on birds, while others hunted in pods and generally ate seals and sea lions. Similarly, sea otters in the Monterrey Peninsula, CA were found to have diets that varied widely between individuals, and individuals have even shown to become specialists in a single prey type (Estes et al 2003). Another marine organism that could be an exemplary model for feeding patterns and behavior is the common octopus, Octopus Vulgaris. Though Hamilton’s 1997 study investigating observational learning in octopus demonstrated the order’s intelligence, little work has been done specifically examining what factors contribute to O. Vulgaris’ diet and patterns of preference. Unlike the sea otters’ feeding pattern, which may be explained by the transfer of feeding behaviors from mother to young, the life history of O. vulgaris does not allow a similar period of learning from their parents because octopus larvae are released into the water column directly after hatching (Mather 1991). However, it is still possible that O. vulgaris may be capable of exhibiting the immense amount of learning and sensory/motor skills needed to specialize in any certain prey type. Thus, our study sought to advance the scope of understanding of variables that

McConnell and Scott 1

contribute to O. vulgaris’ patterns of preference for prey species and amount of prey, both as a species and amongst individuals, by asking these questions: 1. Does the amount and type of food in O. vulgaris diets differ as a function of:

a. size of octopus b. whether the lair is located in the north or south regions of our study c. depth of the lair We hypothesize that the size of the octopus will attribute to a more ample and specific diet. This is because we assume that a larger octopus is an older octopus, and therefore over the course of its lifetime has learned to efficiently specialize in a specific prey type. Similarly, we hypothesize that the depth of the lair is contributing variable because we believe that larger octopus thrive in deeper depths. However, we hypothesize that the location of the octopus in the north or south will not be a contributing factor to variance of diet because the habitat does not change between the two sites. 2. Can different O. vulgaris individuals be grouped based on their feeding

behaviors/preferences? We hypothesize that individuals of O. vulgaris will have specific prey preferences and that individuals can be grouped together based upon these preferences. We also believe that these groups’ patterns of preference may come about from overlapping of individuals’ specific location. We set out to explore the cause of variation by taking both observational and quantitative measurements of different qualities of the lairs of twenty-three O. vulgaris lair sites and their occupants, and also daily collected shells of eaten prey from each site. Materials and Methods Species Description Octopus vulgaris, the common octopus, has a worldwide distribution. They are most abundant in the coastal waters of the Atlantic and the Mediterranean Sea between 1-200 m (Wood and Day 1998). They reach between 30 and 91 cm in size and live a maximum of 1.5 years (Wood and Day 1998). In the Mediterranean, O. vulgaris is often found in fully or partially constructed lairs of rocks, man made items, and/or other natural formations along the shallow rocky shores. These refuges, also known as "dens" or "lairs," range from well developed (completely constructed by the octopus) to favored cracks in rocks, and are used as a protected place for O. vulgaris to sleep, eat, guard eggs, hide from predators, and rest throughout the day (Mather 2009). O. vulgaris hunt by locating small mollusks and crustaceans and boring a hole into the back of the shell, inserting their radula into the main body cavity and injecting the prey with a weakening venom before they remove and eat it in its entirety (Arnold and Arnold 1969). McConnell and Scott 2

Like other octopus species, O. vulgaris leave piles of discarded shells, the remainders of their meals, just outside their lairs (Ambrose and Nelson 2008). These piles, called middens, help to locate an octopuses’ home, and sometimes the octopus itself. Initial observations were taken via snorkeling and SCUBA regarding spacial positioning of several octopuses, noting that most often where there were middens, there was usually a semi-permanent to permanent octopus occupant. Site Description This study was conducted at STARESO Marine Research Station, located outside of Calvi on Corsica, France. The research was carried out in shallow subtidal (6 to 25 ft.) Mediterranean waters surrounding the station, generally within 100 ft. offshore. Substrate was generally granite bedrock with many cracks and crevices. There also were places of rocky rubble broken up by meadows of posidonia oceanica, a seagrass that acts as an important component of the local marine ecosystem. In order to test our questions, we identified a total of 23 dens in which to visit daily: ten lairs to the north of the station’s harbor and thirteen within and to the south. Sometimes, two lairs belonged to the same octopus (i.e. lairs 6 and 7 in the north), some lairs were shared amongst a few octopuses (i.e. lairs 3, 4 and 5 in the north), and some lairs were very obviously specific to a single octopus (i.e. 10 south). Of these twenty-three, an octopus was observed at twenty sites (marked in blue) while only at three sites was no octopus ever observed (marked in green). (Figures 1 and 2)

Figure 1: North Side McConnell and Scott 3

Octopus Lair Sites, STARESO, Calvi, Corsica, France Blue tags indicate observed octopus occupation, green indicates no octopus ever observed

Figure 2: South Side Octopus Lair Sites, STARESO, Calvi, Corsica, France Blue tags indicate consistent octopus occupation, green indicates no octopus ever observed

1. Is there a relationship between the mean amount of food the octopus consumes and: All shells were cleared from each den prior to the start of the study. On each subsequent dive all new shells were collected within a two meter radius of the den, considering these shells the remains of the octopuses' meals from the previous day. Shells were then separated and catalogued into several groups: limpets, oysters, clams, urchins, crabs, abalone, broken shells, and other. Shell fragments that were unidentifiable were added to the ‘broken’ category, while ‘other’ included any shell that did not fall into one of the other categories. We totalled each group and measured the length of the abalone shells using calipers. The mean abundance for each prey species at each den was calculated, but because new lairs were found throughout the course of the study and not all were surveyed daily, data was later standardized in PRIMER 6. a. size of the octopus When present, the size of the octopus was measured by measuring the distance between the horns of the octopus’ eyes. When possible, calipers were photographed with the octopus to use as a scale when analyzing the photos later. This size data, however, was relatively inconsistent and thus a qualitative size gradient was used to categorize the octopuses based on visual McConnell and Scott 4

observations - small, medium, and large. This was analysed with a PERMANOVA test against the shell collection data (described above) in PRIMER 6. b. whether the lair is located north or south A pairwise SIMPROF test run against the shell collection data evaluated whether there was any contribution of geographical location to diet variance. c. depth of the lair Depths of each den were recorded using SCUBA depth gauges. The depths were then separated into three categories: shallow (s=10 feet or less), medium (m=11-15 ft.), and deep (d=16 ft. and below). PERMANOVA (main and pairwise) and SIMPROF tests run against the shell collection data determined any significant relationship between depth and food preference. 2. Can different octopus individuals be grouped based on their feeding behaviors/preferences? A SIMPER cluster analysis (p=0.01) in PRIMER 6 was run with the shell collection data determined whether there were individual octopuses that shared similar diets. Results From the mean values of the number of each prey species consumed at all 23 sites it was found that limpets were the most abundant prey species, representing roughly 25% of the total diet. The dietary spread of other prey species was about 8-10%, with the exception of urchins being a very small contributor (Figure 3).

Figure 3: Percent of diet by prey species of O. vulgaris at 23 sites McConnell and Scott 5

1. Does the amount and type of food in O. vulgaris diets differ as a function of: a. size of the octopus There is no significant correlation between size and mean amount of or variance of prey species that it consumes (p=0.473) b. whether the lair is located north or south There is no significant correlation between geographical location (specifically, north or south STARESO) and mean amount of or variance of prey species that it consumes (p=0.165) c. depth of the lair Under the depth parameters shallow = 10 ft. or less, medium = 11-15 ft. and deep = 16 feet and below, 8 sites were shallow, 10 were medium, and 5 were deep. A PERMANOVA main test on these depth groupings showed that there is a significant difference (p=0.001) between the diets of O. vulgaris at different depths. A PERMANOVA pair wise test showed that the diets of shallow octopus were significantly different than both medium and deep diets (p= 0.017 and 0.008 respectively), but medium and deep diets did not differ from one another significantly (p=0.059), (table 2). Since medium and deep diets did not have statistically significant differences in their diets, we considered medium and deep together as one group and ran all further tests as shallow versus medium/deep (md). Table 2 PERMANOVA pairwise comparisons of shallow (s), medium (m) and deep (d). Groups

t

P(perm)

Unique perms

m, s

1.82

0.02

988

m, d

1.48

0.06

852

s, d

2.64

0.01

689

A PRIMER 6 SIMPROF test on this data showed that limpet was a very strongly significant prey species to differentiate between medium/deep and shallow depth diets with a contribution of 29.63%. The next most contributing species are not as significant, with oysters contributing 12.86% and crab contributing 11.6% to the dissimilarity between the two depth categories (table 3).


Table 3 SIMPROF results of percent contributions of prey species for medium/deep (md) and shallow (s) groups. Group md

Group s

Species

Av.Abund

Av.Abund

Av.Diss

Diss/SD

Contrib%

Cum.%

LIMPET

16

40.57

12.85

1.45

29.63

29.63

OYSTER

11.47

15.05

5.57

1.42

12.86

42.49

CRAB

12.92

8.37

5.03

1.27

11.6

54.09

ABALONE

8.3

5.07

3.97

0.88

9.16

63.25

OTHER

13.19

6.91

3.61

1.37

8.33

71.58

ARCHA

11.27

6.67

3.47

1.03

8.01

79.59

CLAMS

9.08

3.76

3

1.1

6.93

86.52

SNAIL

9.04

6.88

2.59

0.81

5.97

92.48

Figure 4 compares the contribution that each prey species makes to the total diet of O. vulgaris while figure 5 shows how much of the total number of each prey species eaten was consumed in the two depth categories. Limpets make up the largest portion of shallow O. vulgaris diet followed by oysters. Medium/deep diets are more balanced between prey species, but by a small margin limpets are still the highest contributor followed by other, oyster and crab (Figure 4). In terms of distinguishing prey species for shallow vs. medium/deep, though, limpets are much more prevalent in shallow diets. Clams, abalone and other are more distinguished in medium/deep waters than in shallow (figure 5).


Figure 4: Percent of total diet of O. vulgaris by prey species at medium/deep and shallow depths (red=md depth and blue=s depth)

Figure 5: Percentage of total prey species consumed by octopus at medium/deep and shallow depths (red=md and blue=s)


2. Can different octopus individuals be grouped based on their feeding behaviors/preferences? Again refer to Figure 3 on composition of O. vulgaris total diet. Though thus far we have been largely examining the collective diets of the 23 O. vulgaris sites within our study, a PERMANOVA analysis on total diet shows there is a significant difference between prey species that individuals of O. vulgaris eat, and possibly prefer. A SIMPER test in PRIMER 6 with a 0.01 significance value showed that the individual prey species preference could be divided up into six groups: a, b, c, d, e, f (figure 6) with similarities between individuals ranging from 47% to 80%. One group, b, contained only one member, and so was considered an outlier. Groups e and f are the largest groups and also have the highest similarities among members (80.73% and 76.63% respectively).

Figure 6: Lairs grouped by similar diet preferences


Bar graphs of abundance (percentage) of each prey species in each of the groups’ diets, generated in excel, showed what factors differentiated these groups from one another (figure 7). The outlier, group (member) b, eats substantially more limpets than the other groups. Limpets and other make up the majority of group e’s diet and group f’s diet is relatively constant across all prey species, but with a low number of urchins.

Figure 7: Excel bar graphs of percent of diet by prey species of O. vulgaris groups The same color codes for group data from figure 6 were used to make new north and south maps, figure 8 and figure 9 respectively. These maps show that the different diet groups do, to some degree, geographically cluster themselves as well. Note specifically the large amount of members from group f (lime green) clustered in the harbor on the south side, the cluster of members of group e (light blue) on the north side, and the two members of group a (red) in the far north.


Figure 8: Groups represented geographically (north side)


Figure 9: Groups represented geographically (south side) Discussion Based on previous studies on ‘intelligent’ marine organisms’ feeding behaviors, like those performed on orcas and sea otters, our study set out to explore preference and variation between and amongst individual O. vulgaris. We believed that there would be a relationship between octopus size and feeding preference since a larger octopus is probably older and thus has had more time to perfect feeding habits. Furthermore, it initially seemed as if larger octopuses resided in deeper lairs, and thus we hypothesized that depth would influence feeding behaviors. Since habitat is consistent between north and south sites, we assumed that geographical location would have little to no affect on feeding preference. We were surprised to find that our data does not support our hypothesis about octopus size and food preference. Based on visual field observations, it seemed that larger octopus preferred abalones and smaller octopuses preferred oysters and limpets. But, significance tests in PRIMER 6 showed no relationship. This might imply that there is no change in feeding preference or any perfecting of feeding behavior throughout an octopus’s life. It could also imply that any preference for a specific prey item is probably developed at an early age. There was also no variance in diet between north/south locales, but this was less surprising given we did not make any in field observations that supported any relationship. This


might allow us to assume, though, that prey species have a generally consistent spatial distribution and range around STARESO. The SIMPER test allows us to assume that there is a noteworthy difference between the diet preferences of the groups shallow and medium/deep. In other words, we may assume that shallow O. vulgaris prefer to eat limpets while medium/deep O. vulgaris prefer clams, abalones, and other. The test does not provide any implications as to why this preference exists. It would have allowed for an interesting comparison if we had done simple distribution transects, or another such species diversity observation, to see what spacial ranges the different prey species occupy and in what relative abundance. This data could have been compared with the results regarding species preference at different depths. For example, Santina and Chelazzi (1994) found that limpets are generally shallow dwelling species and since, in our study, limpets are the most influential food item in terms of causing notable differences between shallow and medium/deep diets, we may assume that limpets are the most common prey species in shallow depth diets because prey and predator share a common habitat. No literature regarding abalone and clam (the most influential prey species in medium/deep depths in our study) distribution in the Mediterranean could be found. But, based on our own field observations, abalones seemed more common at depth. This could be the reason medium/deep dwelling octopuses ate more abalones. No field observations regarding clam distribution were made. We assume that prey species in higher abundance were those preferred by the members of that group. There are several variables that could account for this preference. It could be related to the spatial distribution of the prey species; i.e. O. vulgaris eat what is available. But, it could also be behavioral: considering octopuses as intelligent animals, it may be that learned feeding behaviors are the cause of the preference patterns we saw. Sea otters, for example, learn different feeding strategies from their parents (Estes et al 2003). We know that octopuses do not learn by paternal or maternal example, but it is possible that their feeding behaviors may be learned from different sources, such as opportunity. In other words, O. vulgaris might learn how to eat a specific prey because of an individual’s course of life; i.e it was one of the first species it learned how to eat, or it favored its taste, or it coincided with the octopus’ specific lifestyle and thus it stuck with that prey as a primary source of food for life. This is reflected in the data regarding orca foraging strategies described in the Introduction. Interestingly, based on figures 8 and 9, there seemed to be a correlation between groups and geographical location. This may be purely situational; lairs within the same general area have access to the same prey. But, more likely, the lairs that are grouped by prey preference and by geographical proximity belong to the same octopus or are shared by multiple locals. We attempted to do a lair fidelity study by tagging octopus, but were unsuccessful. Therefore, we can only make assumptions based on observations and photographs that the octopus occupying more than one nearby lair were either the same or different octopuses. If, however, it was found that the local distribution was one octopus to one specific lair, then we could more easily assume that O. vulgaris’ specialized feeding behaviors can be learned and shared. Despite our finding extremely significant data (with a p-value of less than 5%), it is impossible to ignore that there are several possible sources of error. Namely, the fact that we did McConnell and Scott 13

not visit each site every day alters the validity of the repetition of our data, even though we did standardize all data. Often, the cause of our not visiting each site was weather related. When there was a storm and the water was too turbulent for us to dive, there was also most likely some degree of mixing/adding/removing of lairs’ shells which would have altered our final shell collection data. Furthermore, it is also possible that we did not properly or adequately clear all shells prior to beginning our study, or even on subsequent days, also lowering the strength our the shell collection data. On some occasions, it was apparent we had erroneously collected shells that had not been prey, i.e. snail shells that still had a hermit crab in them. This further hinted at the discrepancies in the methods. One such reason could be that moray eels put shells near the lairs. Morays have similar food preferences to octopuses, and on at least two occasions we saw a moray occupying an octopus’ lair. Lastly, as mentioned before, there were three sites at which we never saw an octopus, but from which we still collected shells. We still considered these sites in our analysis but this may not have been an accurate assumption. As previously discussed, morays also occupy octopus dens and so these octopus deficient lairs may strictly belong to an eel. It is also possible to assume, but rather unlikely, that the octopus was simply missed each time we visited. It is still important to further examine the causes of Octopus vulgaris feeding behavior. As an intelligent and dynamic organism, they can serve as a model for other species and provide insights into the mechanics of lesser-known ecosystems. SOURCES Ambrose, Richard F. and Bobette V. Nelson (2008) Predation by Octopus vulgaris in the Mediterranean. Marine Ecology 4 (3): 251-261. Arnold, John M. and Kristin Okerlund Arnold (1969) Some aspects of hole-boring predation by Octopus vulgaris. American Zoology 9 (3): 991-996. Estes, J.A., M.L. Riedman, M.M. Staedler, M.T. Tinker and B.E. Lyon (2003) Individual variation in prey selection by sea otters: patterns, causes and implications. Journal of Animal Ecology, 72: 144-155. Ford, John KB, Graeme M. Ellis, Lance G. Barrett-Lennard, Alexandra B. Morton, Rod S. Palm, and Kenneth C. Balcomb III (1998) Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters. Canadian Journal of Zoology, 76 (8): 1456-1471. Hamilton, G. (1997) What is this octopus thinking? New Scientist 154 (2085): 30-5.


Mather, Jennifer A. and Ron K. O’Dor (1991) Foraging strategies and predation risk shape the natural history of juvenile Octopus vulgaris. Bulletin of Marine Sciences 49 (1-2): 256269. Mather, Jennifer A. (2009) ‘Home’ choice and modification by juvenile O. vulgaris (Mollusca: Cephalopoda): specialized intelligence and tool use? Journal of Zoology, 3 (1): 359-368. Santini, Giacomo and Guido Chelazzi (1994) Glycogen content and rates of depletion in two limpets with different foraging regimes. Comparative Biochemistry and Physiology Part A: Physiology 111 (2): 271-277. Wood, James and Catriona Day (1998) Octopus Vulgaris, the Common octopus. The Cephalopod Page http://www.thecephalopodpage.org/Octopusvulgaris.php 9 December 2010.


Prey species preference and specialized feeding behavior in the

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