Hermit Crab Species, Size, and Shell Type Distribution on Hurricane Island, Maine. By Rachel Hennessy

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1 Hermit Crab Species, Size, and Shell Type Distribution on Hurricane Island, Maine. By Rachel Hennessy Two species of hermit crab live in the intertidal zones surrounding Hurricane Island. Pagurus acadianus and Pagurus arcuatus both live in the low intertidal zone, make their homes in both periwinkle and dog whelk shells, and are roughly the same size. P. arcuatus is distinctly more common than P. acadianus, but their pattern of shell choice and location are essentially the same. Hurricane Island is a small island in the Gulf of Maine. A former granite quarry, Hurricane Island is surrounded by many of rocky intertidal environments, and a variety of intertidal organisms, including two species of hermit crabs. Hermit crabs are decapod crustaceans that use empty gastropod shells as armor. They insert their soft back ends into the empty shell, and carry it on their back as they crawl around. They can retreat further into the shell to protect themselves. As the crab grows, it will need to switch to a larger shell to accommodate its larger body. Hurricane Island is host to two species of hermit crabs; Pagurus acadianus and Pagurus arcuatus. Both species live in both vacated Nucella lapillus and Littorina littorea shells. My original hypothesis was that P. acadianus would in general be larger and more likely to settle in N. lapillus shells, whereas P. arcuatus would be smaller and more likely to settle in L. littorea shells. To test this I gathered hermit crabs from various intertidal zones around Hurricane Island, and recorded their species, shell type, and size. Part way into the experiment I also began recording notes about the environment where the crabs were collected. I discovered that there is no clear correlation between hermit crab species and shell type choice or size, the choice seeming to depend more on the actual size of the hermit crab rather than the species. It is suggested that the most important factor involved in hermit crab shell selection is not species or type of shell, but rather the quality and intactness of the shells available (Rotjan, Blum, Lewis, 2004). Alternatively, it is suggested that the determining factor in shell selection is the size of the potential host shell in relation to the hermit crab itself (Lively, 1998). During the second half of the experiment I turned my focus from sizing to an overall population and preferred habitat survey using a quadrat and transect line. In terms of population, P. arcuatus is the more common species of hermit crab. Both species of hermit crab inhabit similar environments, namely shallow intertidal pools surrounded by rocks with constant or near constant water flow.

2 Materials: -25x25 cm quadrat (made from duct tape) -Meter stick -Sample bag -Small ruler to measure shells -Waterproof notepad + pencil Methods: The original methodology of the study was catered to the goal of finding a correlation between hermit crab species and shell size, which turned out to be non-existent. I collected data by gathering every hermit crab I could find at a site, recording their species and shell type, and measuring the length of the shell in centimeters from the apex to the farthest point on the lip, and rounded to the nearest quarter centimeter. Species was identified using a field guide. P. arcuatus, common name hairy hermit crabs are characteristically hairy and gray-brown in color, often fading to a lighter color or pink at the tips of the claws. P. acadianus, common name acadian hermit crabs are not hairy beyond a few small tubules along the claws, and generally have bright red on the middle of their claws, fading to white at the outer edges. Because my method for catching crabs depended heavily on my ability to spot them, there is no real viable population data, at least in terms of shell type, because it was much easier to spot hermit crabs residing in N. Lapillus shells due to their size and white coloration. When the lack of correlation between shell type and species became clear, my focus shifted to surveying the overall population of hermit crabs. In order to minimize the variability inherent in catching and counting hermit crabs, I elected to use a small quadrat, so that I could scoop all organisms from a set location and count out hermit crabs from there, lowering the likelihood that I would pass over hermit crabs that were well hidden or mistake hermit crabs for snails. In order to count, I would set down my quadrat, record some basic information about the environment within the quadrat, move any rocks or foliage that may obscure my view of the quadrat inhabitants, and carefully gathered every shell and placed them in a safe space where I wouldn t lose them, such as a sample bag, a bucket, or in one poorly planned instance, my pocket. I then sorted through the shells, tossing back any snails or empty shells immediately, and measuring and recording size, species, and shell type of any hermit crabs. I returned hermit crabs to the quadrat as soon as I had recorded

3 their information in order to avoid double counting. After I finished a quadrat, I used my meterstick to select a new site around one meter horizontally down the intertidal, roughly the same distance from the actual tide mark. The distance from the tide mark ranged between 0 and 25 centimeters because in my earlier studies I had discovered that hermit crabs rarely lived outside of this range in the intertidal. The statistical test I used in analysis was a T Test. Results: Fig. 1 Caption: Shell type is the biggest factor in determining size, there is no correlation between species and shell type or species and size (P value for P. acadianus in L. littorea shell vs P. arcuatus in L. littorea shell 0.728, P value for P. acadianus in L. littorea shell vs P. arcuatus in N. lapillus shell 0.895)

4 Fig. 2 Caption: Hermit crabs only show up in certain environments, and overall P. arcuatus is more common than P. acadianus Site information: -Site 1: Still, Shallow pool, rocky bottom, coraline, periwinkles, thick reddish seaweed -Site 2: Shallow ledge, coraline, some leafy red seaweed, periwinkles, no crabs -Site 3: Deep crack, some mussels, coraline, white and red seaweed, shrimps, shell hash, no crabs -Site 4: Between two large rocks, some coraline, bare rock, some white seaweed, continuous water movement -Site 5: Fully submerged crack between several larger rocks, shell hash/sand bottom, some coralline and green seaweed, many periwinkles -Site 6: Series of rocky cracks between large stones, shell hash bottom, perris, rock whelks, some seaweed, no coraline, minimal water movement -Site 7: Same as site 6, deeper, more water movement -Site 8: Shallow crevice between rocks, some water flow, mostly bare rocks, barnacle spat, periwinkles and whelks -Site 9: Top of large rock, dry, barnacles, periwinkles, no crabs Fig 3 Caption: Hermit crabs living in the intertidal down by the galley are all, or at least primarily of the P. arcuatus variety, and most crabs are in periwinkle shells Site Information: -Site 1: Cracks, coraline, some red seaweed, constant water flow

5 -Site 2: Shallow crack, gentle water flow, mostly submerged, coraline bottom, red algae, periwinkles, no crabs -Site 3: Shallow pool, gentle occasional surge, coraline bottom, some red seaweed, periwinkles, no crabs -Site 4: Gentle water flow, shallow, coraline, green seaweed, ne hermit crabs but hermit c rabs are visible just a little closer to the water -Site 5: Deep crack, minimal coraline and seaweed, mostly bare rock, still water -Site 6: Totally dry, barnacle spat, some perris, red and green seaweed -Site 7: Same as site 6, more red seaweed -Site 8: Deep, sandy bottom, between large rocks, no snails or crabs -Site 9: Top of rock, dry, green seaweed, barnacles Discussion: If one looks at all my data gathered on hermit crab sizes, it would seem that the most common sort of hermit crab would be a P. arcuatus in a N. lapillus shell. However, this data is skewed because it is much easier to spot any hermit crab residing in a N. lapillus shell due to their size and color. Logically, it also does not make sense for the most common sort of hermit crab to be so large, as in general there are many more small and young individuals than older, larger ones. This is supported by my findings when I was using a quadrat, most obviously in the transect at the intertidal by the galley. The quadrat method eliminated, or at least significantly lowered the chance that I would overlook hermit crabs in periwinkle shells, and as a result a more accurate count was taken, showing that periwinkle shells were in fact at least as common, if not more common than whelk shells at that site. However, most common size of each shell type hermit crab depends less on a population count, so the data is at least somewhat significant. Both P. arcuatus and P. acadianus living in L. littorea shells had a size range of around 1 to 2.25 cm in length, but with most individuals being around 1.5 cm long. Alternatively, both P. arcuatus and P. acadianus living in N. lapillus shells ranged in size from 1.5 to 4 cm, but with most individuals being around 3 cm. There is not enough data to suggest that either species is larger or smaller, or prefers a certain shell type, just that in my study P. acadianus was less common. The study also suggests that the biggest factor in shell size is shell type, with N. lapillus being nearly always larger than L. littorea.

6 The population and habitat study provides slightly more interesting data. The hermit crabs of Hurricane Island, regardless of species, tend to reside in certain types of habitats over others. In both of my transect lines, hermit crabs were noticeably absent in any environment that was ever totally unsubmerged (Fig 2 site 9 and Fig 3 sites 6 and 9). Hermit crabs were far more abundant in crevices and pools of water surrounded by rocks, with at least some water flow in and out, as is the case in Fig 2 sites 4, 5, 6, 7, and 8, and Fig 3 sites 1 and 5. Hermit crabs have also shown up in standing tide pools that have abundant green seaweed, as is the case in Fig 2 site 1 and in a previously recorded site at the intertidal south of Gibbon s Point. I believe that the reason for hermit crabs congregating at these particular habitats is for the protection they provide, and the satisfaction of the need for constant water coverage. Rocky cracks are filled with little nooks and crannies for the crabs to hide in, while thick seaweed and algae provides cover and camouflage. It is evident in all of my collected data that P. arcuatus is more common than P. acadianus, but it is perhaps most significantly illustrated in Fig 2. The data from Fig 2 comes from an approximately 9 meter transect line along the intertidal of Two Bush Island. In this line I passed through several areas with high hermit crab populations of a variety of sizes, including individuals of both species. Out of the total 66 hermit crabs I recorded, only four were P. acadianus, the rest being P. arcuatus. It is possible that the reason for such a distinct average and mode size of shell may depend only partially on actual crab size. Another important factor is the size of shells available. Most live L. littorea are around 1.6 cm, and most live N. lapillus are around 3 cm. Hermit crab shell choice is obviously based at least partially on size but shell availability is also likely a factor. When there is only a limited range of shell sizes, hermit crabs will have to take whatever shell fits best. Competition between crabs for the best shells is also a recorded phenomenon that likely factors into shell choice (Rotjan, Blum, Lewis 2004). An interesting further study would to be to compare hermit crab body size and health to shell type and size. Another possible study would to be to compare local snail populations and sizes to hermit crab populations and sizes, as snails are the source of the shells that hermit crabs wear. As for actual information gathered in my study, it is clear that hermit crabs have a preferred habitat within the rocky intertidal. They need to be constantly at least partially

7 submerged and some sort of protection, either by way of rocky crevices between stones or underneath a cover of vegetation. The significance of a hermit crab population study is simply a mode of observing environmental health. If any one species fluctuates significantly, it is an indicator that something is amiss. By taking a census of hermit crab populations on Hurricane Island, we can get a better idea of the island s ecosystems as a whole.

8 Bibliography Rotjan, Randi D., Julia Blum, and Sara M. Lewis. "Shell Choice in Pagurus longicarpus Hermit Crabs: Does Predation Threat Influence Shell Selection Behavior?" Behavioral Ecology and Sociobiology 56.2 (2004): JSTOR. Web. 3 June < Walker, Sally E. "Hermit Crabs as Taphonomic Agents." PALAIOS 4.5 (1989): JSTOR. Web. 3 June < Brown, Jennifer L., Brian A. Hazlett, and Carl H. Kaster. "Factors Affecting the Shell Assessment Behaviour of the Hermit Crab, Calcinus tibicen (Herbst, 1791) (Decapoda, Paguridea)." Crustaceana 64.1 (1993): JSTOR. Web. 3 June < >. Folino, Nadine C., and Philip O. Yund. "The Distribution of Hydroid Sibling Species on Hermit Crabs in Estuaries in the Gulf of Maine." Estuaries 21.4: JSTOR. Web. 3 June < Lively, Curtis M. "A Graphical Model for Shell-Species Selection By Hermit Crabs." Ecology 69.4 (1988): JSTOR. Web. 3 June < Trott, Thomas J. "Cobscook Bay Inventory: A Historical Checklist of Marine Invertebrates Spanning 162 Years." Northeastern Naturalist 11.2 (2004): JSTOR. Web. 3 June < Grant, William C., Jr. "Notes on the Ecology and Behavior of the Hermit Crab, Pagurus acadianus." Ecology 44.4 (1963): JSTOR. Web. 3 June < >.