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Under natural conditions hermit crabs Pagurus longicarpus, Pagurus pollicaris, Pagurus brevidactylus, and Pagurus brevidactylus exhibit shell preferences. This preference is important to shell utilization and survival of the hermit crabs. Shell availability is known to be a limiting factor for hermit crabs. Shell preferences of four hermit crab species, P. longicarpus, P. pollicaris, P. carolinensis and P. brevidactylus were determined for any shells inhabited. All crabs were collected by an epibenthic sled in Beaufort, North Carolina and transported back to the lab for identification. It was found that all four species actively choose shells based on architecture. Ilynassa was the most inhabited shell due to the greatest abundance, but it was not most preferred shell. P. longicarpus and P. carolinensis showed significant shell preference amongst shell types. P. longicarpus prefers shells with snail fur (fuzzy) more than their non-fuzzy counterparts. P. longicarpus prefers fuzzy Ilynassa more than any other shell, but inhabits mostly Terebra and Anachis. P. carolinensis prefers non-fuzzy shells. P. carolinensis inhabits Ilynassa more than any other shell, but prefers Anachis or Terebra. There were not enough P. pollicaris or P. brevidactylus to conduct a chi2 analysis but there were significant difference in shell preference.
Hermit crabs lack a hard carapace and must rely on empty gastropod shells to protect their uncalcified abdomens (arkive.org). The gastropod shells help protect against environmental stresses and predation (Jensen 1996, Bertness 1980). It is well known that many species of hermit crabs exhibit shell preference (Bertness 1980, Jensen 1996, Alcaraz 2012). However, there are many different factors that play into shell selection such as abundance of gastropod shells (Mantelatto et al. 2007), wave action (Arguellas 2009), shell weight (Contreras-Garduno 2009), or competition from other species of hermit crabs in the same area (Morrison 2006, Alcaraz 2012). Shell weight plays a key role in selection when wave action or predators are present. Shell hierarchies also influence shell selection between species (Bertness1980). With the absence of heavy waves or predators, hermit crabs will actively pick shells that are less than 50% of their body weight which will intern allow then to grow larger (Contreras-Garduno 2009).
We looked specifically at shell selection of four species of hermit crabs: Pagurus longicarpus, Pagurus pollicaris, Pagurus carolinensis and Pagurus brevidactylus. We began with a null hypothesis that hermit crabs will use different shell types in proportion to their availability in other words they do not exhibit shell type preference.
Materials and Methods
Hermit crabs were collected in Beaufort, North Carolina (34° 41.595’ N and 76° 39.441’ W) on 20-September, 2012. The area had low wave velocity and moderate predatory abundance. All crabs were collected during high tide by dredge. Dredging lasted for five minutes on each pull. Hermit crabs were then separated by hand and stored in empty fishing tackle boxes. Each crab was put into individual compartments to prevent shell exchange. Crabs were taken back to the Duke Marine Laboratory for shell and species identification. Shell type, crab species, presence of snail fur (fuzzy), and shell condition was recorded. After identification all hermit crabs were put back into a bucket filled with seawater and released. In total 391 hermit crabs were caught of which 94 hermit crabs occupied shells with snail fur.
P. longicarpus used fuzzy Ilynassa more than any other shell type (Figure 1). Over 35% of the crabs occupied this shell. The crabs did not use fuzzy Anachis or fuzzy Eupleura at all. The rest of the shell types were moderately occupied. P. longicarpus occupied fuzzy Ilynassa much more than predicted. They occupied this shell three and a half times more than predicted (Figure 1). The crabs used Ilynassa, Terebra, Anachis, and Nassarius less than predicted. There was a significant difference between actual and predicted use (X216 = 57.95; p<0.05).
P. pollicaris used Ilynassa, fuzzy Ilynassa, and fuzzy Eupleura the most (Figure 2). Over 69% occupied these shells. The crabs did not inhabit Littorina, fuzzy Terebra, Anachis, fuzzy Anachis, Epitonium, Urosalpix or Olivella at all. The rest of the shell types were moderately inhabited. P. pollicaris occupied Ilynassa and fuzzy Ilynassa much more than predicted (Figure 2). The crabs used Terebra, Anachis, Urosalpinx, and Olivella much less than predicted. There were not enough P. pollicaris collected to run a Chi2 analysis; however there seems to be a very significant difference between shell use and availability.
P. carolinensis used Ilynassa, Terebra, Anachis, Nassarius, and Urosalpinx more than any other shell (Figure 3). The crabs did not occupy Polinices or fuzzy Polinices at all. The remaining shell types were moderately inhabited. P. carolinensis occupied Terebra, Anachis, Nassarius, and Urosalpinx more than expected. They inhabited fuzzy Ilynassa less than expected, and the remaining shells were similar to predicted values (Figure 3). There was significant difference between actual and predicted use (X216 = 18.86; p<0.05).
P. brevidactylus used Terebra, Anachis, and Ilynassa shells the most (Figure 4). About 70% of the crabs occupied these shells. The crabs also inhabited Nassarius, Littorina, Urosalpinx, and Olivella. They did not inhabit the rest of the available shells, including shells with snail fur. Crabs occupied Littorina, Terebra, Anachis, and Nassarius more than predicted availability (Figure 4). Anachis was used twice as much as predicted. Crabs used Urosalpinx, fuzzy Ilynassa, and Eupleura less than predicted. Fuzzy Ilynassa was not used at all. There were not enough P. brevidactylus to conduct a Chi2 analysis; however there seems to be little significant difference of shell use to availability.
We rejected our null hypothesis. All four crabs seem to actively choose shell type unequal to availability. Overall, the main shell occupied was Ilynassa. This is probably due to the large abundance of this shell, compared to other shell types, in the sample area. The biology of the crabs plays a part in their shell type selection. While some species overlap in shell selection, others need specific lengths, or opening size to house their body and claws.
P. longicarpus prefers fuzzy Ilynassa, to other available shells. They actively choose any form of Ilynassa over other shells. P. longicarpus, also known as the long-wristed hermit crab, needs a shell long enough to hold it but also wide enough to fit their claws. In general, they choose long shells with wide openings such as, Terebra or Ilynassa, over all others. These crabs also seem to prefer shells with snail fur.
P. pollicaris do not select shell type in proportion to availability. These crabs choose fuzzy shells over their non-fuzzy counterparts when available. Furthermore P. pollicaris chooses large shells with very wide openings over all other types. The flat-clawed hermit crab choose fuzzy Ilynassa, fuzzy Eupleura, or Polinices over other shells because the opening allows the crab to pull its claws over the opening, acting as a pseudo-operculum.
P. carolinensis was by far the most abundant crab caught, and they do not select shell type in proportion to availability. They seem to avoid shells with snail fur. These crabs, unlike P. pollicaris, choose the non-fuzzy counterpart of Ilynassa, Anachis, and Terebra. Due to the abundance of P. carolinensis, they inhabited almost every shell type. The only shells they did not inhabit were Polinices and fuzzy Polinices, due to the shells large opening.
P. brevidactylus did not select shell type in proportion to availability, much like P. carolinensis, these crabs avoid fuzzy shells. They exhibit a clear preference to Ilynassa, Anachis, and Terebra. They avoid shells with large, wide openings such as Polinices, and choose shells with narrow openings and spiral tops such as Urosalpinx or littorina.
Overall wave action and shell architecture are the most significant factors to hermit crab shell selection in our sample area. According to Alcaraz and Arce, wave action plays a significant role in the distribution of shells and hermit crab shell selection. Areas with low wave velocity elicit lighter and more conical shells while areas of higher wave velocity elicit heavy, fat shells. Our data supports this hypothesis. Our study area is a subtidal zone with very little wave action. Therefore, light conical shells such as Terebra or Ilynassa dominated the shell abundance in our habitat and were the most inhabited shells. Hermit crabs inhabit theses light shells because it allows for increased growth and clutch sizes (Contreras-Garduno 2009). The durability of a heavy shell is not needed because the waves do not erode or break down the shells very fast (Arguelles, Araceli et al. 2009)
Snail fur is a colonial hydroid, Hydractinia echinata, commonly found on gastropod shells (Mercando 1980). Mercando and Lytle found that snail fur was a common occurrence on shells inhabited by P. longicarpus and P. pollicaris, but it did not colonize on shells inhabited by P. brevidactylus or P. annipules. However, it was not found to colonize on the same type of shells inhabited by other hermit crab species such as P. brevidactylus.
It is hypothesized that the living hydroid could be a means of shell partitioning within a hermit crab community, thus reducing competition for shells (Mercando 1980). A living Hydractinia colony seems to be a stimuli for differential shell selection, and not the rough textured surface of a dead Hydractinia colony (Jensen 1970, Mercando 1980).
It can be concluded that many environmental pressures contribute to the clear shell preference of sympatric hermit crab species in Beaufort, N.C. The main factors of shell selection include shell architecture and presence of snail fur. Further research should be conducted on the selection mechanism behind the preference of shells with snail fur over shells without.
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