Habitat & community ecology
  The section on HABITAT & COMMUNITY ECOLOGY includes a selection of topics such as hermit-crab competition, considered here, and COMMUNITY INTERACTIONS, GENE FLOW, INTRASPECIFIC COMPETITION, and INTERSPECIFIC COMPETITION, considered in other sections.
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  Hermit-crab competition & shell selection
  Hermit crabs spend several weeks in the plankton as larvae before settling and metamorphosing.  At this time the juveniles seek out empty gastropod shells in which to live.  The shells protect their soft hind parts from predation and, for intertidal forms, from desiccation.  Hermit crabs base their selection both on size (an individual upgrades the size of shell periodically as it grows) and on species of shell.
Research study 1
  photograph of hermit crab Pagurus samuelis courtesy Peter Bryant, University of California, Irvinephotograph of hermit crab Pagurus hirsutiusculusThree hermit-crab species are found commonly on the shores around the Hopkins Marine Station, Pacific Grove, California.  Of 1873 individuals collected in one study, 50% consist of Pagurus samuelis in the high-intertidal area, and 40% of P. hirsutiusculus and 10% of P. granosimanus in the mid-intertidal region. photograph of hermit crab Pagurus granosimanusOf many gastropod shells available to be occupied in the area, those of Chlorostoma brunnea and C. (Tegula) funebralis are by far the most popular, especially with the large-sized P. samuelis and P. granosimanus (representing 76% and 89% of shells occupied, respectively).  The third species P. hirsutiusculus is smaller and tends to occupy smaller-sized shells, such as Littorina spp. and Calliostoma spp. (only 11% are found in C. funebralis shells).  Of course, the smaller and younger stages of all species tend to occupy smaller shells.  Bollay 1964 Veliger 6: 71. Photo of P. samuelis courtesy Peter Bryant, University of California, Irvine.
Research study 2

table showing % shell utilisation by several pagurid hermit crabs in Horseshoe Cove, CaliforniaAnother study published at the same time on the same species of hermit crabs in the Bodega Head region of California, gives more data on percentage shell utilisation. Note in the table that Pagurus hirsutiusculus appears to favour shells of Nucella ostrina, P. samuelis shells of Chlorostoma funebralis , and P. granosimanus shells of several snail species.  Since the same types of shells are used by all 3 hermit-crab species, a measure of overlap probably gives a fair estimate of degree of competition between the hermit crabs.  This is 30% between P. hirsutiusculus and P. samuelis, 46% between P. hirsutiusculus and P. granosimanus, and 40% between P. samuelis and P. granosimanus.

In tests at the Bodega Marine Laboratory, P. hirsutiusculus prefers N. ostrina shells over those of Chlorostoma and Calliostoma, and an individual will leave another type of shell to get into the preferred shell even if the latter is too small.  In contrast, the other 2 hermit-crab species show no particular preference for shell type, as long as the size is suitable.  In eviction-type experiments, P. hirsutiusculus is dominated by the other 2 species even when it has a size advantage.  This seems to suggest that for some reason the other 2 species favour other shells over those of N. ostrina.  Eviction comes after an inspection, and is followed by the evictor grasping the occupant’s antennae or walking legs with its chela, and smoothly sliding the occupant from its shell.  Since it is usually quite difficult to remove a crab from its shell (by hand), the author considers that some signal or other must be passing between the protagonists.  This makes sense evolutionarily because shell exchange is a common behaviour for hermit crabs and an easy eviction would have survival benefits for both participants.  Orions & King 1964 Pac Sci 18: 297.

NOTE  a total of nearly 2,000 crabs is represented in the data

Research study 3

A study in San Juan Islands, Washington on Pagurus hirsutiusculus shows that shells are indeed a limiting resource.  Addition of 1,000 empty snail shells to an isolated, rocky intertidal reef at monthly intervals over 1yr results in an approximate doubling of crab numbers while in a nearby control plot numbers of crabs remain about the same.  Vance 1972 Ecology 53: 1062.

NOTE  a total of 12,000 shell are added, made up of 90% Littorina sitkana (used by small individuals) and 10% Nucella lamellosa (used by large individuals).

Research study 4

photograph of hermit crab Pagurus beringanusTo assess the effects of shell limitation on populations of hermit crabs in San Juan Islands, Washington, 3 sites are sampled for numbers and sizes of 3 hermit-crab species, and for sizes and species of occupied and unoccupied shells.  Results show generally that empty shells are rare, and that all but small specimens of 3 species of hermit crabs Pagurus hirsutiusculus, P. granosimanus, and P. beringanus inhabit shells smaller than preferred.  The results support the conclusion that empty shells are a limiting resource for these hermit crabs.  Although some resource partitioning is demonstrated in the use of somewhat different arrays of shells by the 3 species, the author notes that the same shell species can serve any one of the 3 hermit-crab species in different but adjacent habitats.  The author suggests that the mechanism allowing coexistence apparently involves both shell-resource and habitat partitioning. Vance 1972 Ecology 53: 1062.

NOTE  assessed in laboratory-choice experiments at Friday Harbor Laboratories.  The author does not list the shells available at the 3 sites, but the lab experiments show slightly different preferences by the 3 hermit-crab species:

P. hirsutiusculus: mainly Littorina sitkana with larger individuals preferring Nucella lamellosa
P. granosimanus
: mainly Littorina scutulata, Lirabuccinum dirum (Searlesia dira),
N. lamellosa and N. ostrina
P. beringanus
: mainly Amphissa columbiana, L. scutulata, L. dirum, N. lamellosa
and N. ostrina

Research study 5

Other studies at Friday Harbor Laboratories, Washington involving fights over shells by disparately sized hermit crabs Pagurus hersutiusculus, show that the probability of the larger crab getting the shell of the smaller crab through fighting increases with decreasing size of the larger crab’s shell.  Interestingly, the larger crab’s aggressiveness does not appear to change with shell adequacy, and the mechanism underlying the shell exchange seems to be for an individual to fight whenever an opportunity arises and then consider changing shells only after a subordinate crab relinquishes its shell.  This behaviour is different than shown by other hemit-crab species that fight harder with decreasing shell adequacy.  The author suggests that the different attitude of P. hirsutiusculus originates with the chronic short supply of shells in the San Juan Islands population.  Under these conditions an individual stands almost always to gain by fighting; hence, a high level of general aggressiveness is advantageous.  The author suggests that it is an evolved behaviour, but it seems here to be a learned one.  The idea is interesting, and it would be worth examining P. hirsutiusculus’ behaviour in other areas where shells are not in short supply. Vance 1972 Ecology 53: 1075.

NOTE  hermit crabs, including P. hirsutiusculus, apparently have a complex system of aggression by visual and tactile means.  Thus, through these forms of communication (along with use of claws), fighting tends most often to be ritualistic, and shell exchange is usually effected with a minimum of damage to the participants

Research study 5.1

Are isolated hermit crabs more or less inclined to aggressive behaviour than crabs in groups? This interesting question would be hard to test on animals in the field because of their natural propensity to aggregate, so researchers from the University of California, Berkeley hold individual hermit crabs Pagurus samuelis in isolation in the laboratory for periods of 3, 8, 12, and 30d and compare their levels of aggression1 with group-held crabs over 30min-observation periods. Results show, as expected2, that tendency to initiate and dominate a bout are both enhanced by degree of isolation (“isolates” initiate 76% of encounters and dominate in 64% of them). It is important to note here that “dominance” is not determined as one might think, by which crab wins the other’s shell, while this does happen, in the present study shell gain or loss does not in itself determine the outcome of a match. Rather, dominance is decided by subtracting the total duration of all “fear3” behaviours from the total duration of all “aggressive” behaviours, and a crab is considered to dominate only if the difference exceeds 60sec out of the 30min observation period. Courchesne & Barlow 1974 J Comp Phys A 75: 32. Photograph courtesy Douglas Mason, California.

photograph of hermit crabs Pagurus hirsutiusculus fighting over a shellNOTE1 aggression is measured as frequency and duration of bouts (fights), as well as latency of initiation of a bout, who initiated, who dominated, and final conclusion

NOTE2 previous studies on the same subject, but not on west-coast species, show that single-held crabs tend to initiate more fights than group-held crabs, but with no enhanced success

NOTE3 fear behaviours as defined by the authors include avoidance, retreat, and withdrawal into the shell (with several levels of each); aggressive behaviours include eviction, knocking/rocking/pulling/holding, and different degrees of cheliped extension and presentation. To a naive reader, choice of the descriptor “fear” to be the antonym of “aggression” seems inappropriate for reason of anthropomorphism (crabs are not sentient entities). Perhaps “non-aggression” would have been a more appropriate term. Another criticism of the authors’ terminology relates to the use of “eviction” as just one component of aggression. Surely, in a squabble between hermit crabs, eviction from the shell must represent an ultimate loss, and therefore should be the determinant of which crab wins and which crab loses. After all, in fights over shells, aren’t all the other “aggressive” behaviours as listed above simply requisite steps to ultimate victory measured by one crab replacing its shell? Finally, shell size in relation to crab size is not included here as a factor in determining dominance, yet in most other studies of hermit-crab aggression the need of an over-sized crab to replace a too-small shell tends to dominate

Example of aggression in pagurids:
two hermit crabs Pagurus hirsutiusculus
appear to be fighting over a third shell 1X

Research study 6

graph showing occupancy of shells of Nucella whelks by hermit crabs Pagurus granosimanus in San Juan Island, Washington
In another study at Friday Harbor Laboratories, Washington, 4554 snails (94% N. lamellosa and 6% N. ostrina) are collected over a 3-y period from a 200 m2 study area, fixed with small numbered tags, and returned to the area.  The research study actually is on the snails but, as it turns out, after their death the shells are used by hermit crabs Pagurus granosimanus and, because they are tagged, the fates of the shells and their involvement in the hermit-crab population ecology can be monitored.  The graph shows an initial surge in use of tagged shells as the study gets underway in spring/summer 1969, and then a gradual decline as the shells are degraded, or lost, or the tags fall off. The important thing with respect to hermit crabs is that all accessible shells are occupied in the study area; no empty shells are available for additional crabs.  The hermit-crab population size in this are therefore depends on shell availability.  Spight 1977 Biol Bull 152: 120.

NOTE  the author estimates that about 40% of the Nucella spp. populations die each year

NOTE  2 other species P. beringanus and P. hirsutiusculus also use the tagged shells, but to a much lesser extent than P. granosimanus

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photograph of hermit crab Pagurus hirsutiusculus in a Nucell lamellosa shell taken from a video

CLICK HERE to see a video of a hermit crab Pagurus hirsutiusculus exchanging shells. It transfers from a Nucella lamellosa shell to a Ceratostoma foliatum shell.

NOTE  the video replays automatically

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Research study 6.1

One would expect that hermit crabs would resist or tolerate temperature and desiccation stress in accordance with the intertidal height they naturally occupy. This idea is examined by a researcher at the University of California, Irvine for 4 co-occuring species in the San Luis Obispo area of mid-California, Pagurus samuelis (upper-mid intertidal), P. hirsutiusculus and P. granosimanus (mid-low intertidal), and P. hemphilli (shallow subtidal). The major experiment consists of exposing replicate groups of 20 individuals of each species in sand-filled plastic tubs in the open sun for periods of 20min. A shaded region (20% of tub area) is provided in each tub. Results show greatest aggregation and shade-attraction behaviour in P. samuelis, intermediate levels in P. hirsutiusculus and P. granosimanus, and lowest levels in P. hemphilli. Individuals that are over-sized with respect to size of their shells are less resistant to aerial exposure. Other similar experiments but of longer duration show that the higher-inhabiting P. samuelis survives significantly longer than the lower-inhabiting P. granosimosus. Smallest-sized individuals in these survival tests are more susceptible, as expected based upon less favourable surface area-to-volume relationships, than larger-sized ones, but for P. samuelis, at least, the superior ability of small individuals to seek out shade and protective crevices in the field allow them to live higher in the intertidal zone than expected. The overall pattern shown here suggests that the species most adept at aggregating and seeking shade, both temperature- and desiccation-resisting behaviours, and in otherwise being able to resist effects of aerial exposure, are the ones most able to exploit higher intertidal-level habitats. Taylor 1981 J Exp Mar Biol Ecol 52: 205.

NOTE photographs of all 4 species can be found in Research Studies 1 and 8 in this section

NOTE temperatures in the experimental tubs range from 34oC on the sand surface, 31oC 10cm above the sand, and 28oC in the shaded area

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Research study 6.2

If a hermit crab carries ectoparasites, it is quite likely that they compete with their hosts for food, and thus we can find a place for this next research study here.  The study is done by researchers at the Institute of Marine Science, University of Alaska on 9 species of pagurid hermit crabs.  Of almost 8600 hermit crabs examined, some 20% are found to carry parasites comprised of 10 species of polychaete worms and 3 species of amphipods.  As most of these are omnivorous, the authors consider their association with pagurids to provide benefit not only in protection from predators, but also access to their hosts’ foods.  The authors provide information on distributions that may be of interest to some readers.  Hoberg et al. 1982 Ophelia 21 (2): 167.

NOTE  the authors designate these symbionts as commensals, but this would be true only if the host crabs are not harmed or negatively affected in any way, which does not seem to be true

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Research study 7

map showing collecting sites in the San Juan Archipelago used in study of hermit crabsResearchers at Friday Harbor Laboratories, Washington undertake a large and ambitious investigation of shell-use patterns in 12 hermit-crab species in a subtidal assemblage, and quantify the extent of intra- and inter-specific competition for each.  There is so much data presented in the paper that only a brief summary can be presented here.  Results show that shells are in short supply in the field and that competition exists between several of the species.  Interestingly, in 5 species shell the supply is reduced significantly more by interspecific competition than by intraspecific competition, a finding apparently atypical of such hermit-crab assemblages. Fighting, which is a dominant feature of hermit-crab competitive interactions in other investigations, seems to play only a minor role in this species assemblage, even though the authors note that the competition is primarily exploitative. Not surprisingly, relative amounts of each type of competition differ for each species in the community.  Three species, P. beringanus, P. kennerlyi, and E. tenuimanus, characterised by their abundances and generalised preferences, appear to dominate the shell-usage.  Overall, however, the researchers note that the species in the assemblage are surprisingly not close to a limiting similarity, that is, exclusion, in resource use. If there is a shortfall in this ambitious enterprise it must stem from its sights being set too high, an attempt to be too all-inclusive. Even though almost every species is found to interact with every other species in laboratory confrontations, it is hard for the reader to appreciate the significance of, say, a shallow-water species interacting or not interacting with another species living at mid-depth or deep-depth – a species that it would never ordinarily encounter.  The authors would argue that you’ll never know until you try it, and they are correct.  The paper concludes with an engaging discussion of possible limitations of the study, with suggestions for future work.  Abrams et al. 1986 Oecologia 69: 429.

NOTE  12 of 14 subtidal species are used: Pagurus aleuticus, armatus, beringanus, capillatus, caurinus, dalli, kennerlyi, ochotensis; Elassochirus gilli, tenuimanus; Labidochirus splendescens; and Paguristes turgidus.  Sizes range from 5-20mm carapace length.  The 2 species not used are symbiotic with sponges or inhabit tubes of worms Serpula spp., respectively.  One of the 12 species used, L. splendescens, differs from the others in size and habit such that it has minimal need to find new shells as it grows

NOTE  the crabs actually live in quite diverse habitats: on hard and soft substrata, from the shallow subtidal region (collected using SCUBA) to deeper depths (depths not specified, but some to 100m, collected non-selectively by dredging).  Nevertheless, the authors find that each of the 12 hermit-crab species overlaps significantly in shell and habitat use with at least some other species

Research study 8

photograph of a hermit crab Pagurus hemphilliA continuation of Research Study 7 above at Friday Harbor Laboratories, Washington on shell-resource partitioning in 6 species of intertidal hermit crabs Pagurus collected at Mukkaw Bay, Washington reveals that for all species the magnitude of intraspecific competition for gastropod shells is much greater than interspecific competition.  Competition occurs when use of a shell by one individual causes other individuals to use shells of lesser quality.  Lesser quality shells are ones that are too small, have holes that expose parts of the body (especially the soft abdomen), or have excessive growths of algae or other organisms.  In an initial collection of 2100 crabs of all species, only 53 empty shells are collected, suggesting low shell availability and thus potential for competition.  The shells most competed for, especially by larger crabs, are those of Cholorostoma (Tegula) funebralis. If large crabs are offered a selection of empty C. funebralis shells in the laboratory, about 60% switch to new shells, but not always to larger shells.  Some switching is from low-quality shells to higher-quality shells that may be actually smaller in size.  Greatest similarity in shell-size use occurs among the 3 large, low-intertidal species P. granosimanus, P. hemphilli, and P. beringanus.  In a series of paired-species fighting experiments (using species with overlapping habitats and equal-sized individuals), where one of the species’ shells are broken off at the tips to expose the occupants’ abdomens, most crabs occupying “good-quality” shells retain those shells regardless of species of crab.  However, notable “evictions” are 20% of P. hirsutiusculus by P. samuelis, and 25% of P. hirsutiusculus by P. granosimanus.  Overall, even though the potential for interspecific shell fighting is great in typically crowded habitats at Mukkaw Bay, occupied shells can usually be retained.  Thus, shell fighting or interference competition seems much less important than exploitative competition.  Obviously, there is less potential for both intra- and interspecific competition in this population than in other populations with a greater variety of shells available to be used.  In pair-wise preference tests only P. samuelis and P. hemphilli prefer C. funebralis shells over other shell species, suggesting that interspecies competition for shells could be more severe were it not for the preponderance of Chlorostoma shells in the habitat.  Interestingly, effects of intraspecific competition are greater in mid- and upper- intertidal species than for the lower intertidal species.  Abrams 1987 Oecologia 72: 248.

NOTE species include Pagurus h. hirsutiusculus, P. granosimanus, P. beringanus, P. samuelis, P. hemphilli, and P. caurinus.  All species are also present on outer coasts of Oregon and British Columbia

NOTE the study also includes collections from Botanical Beach, British Columbia and from Pacific Grove, California, but these data are not included here

Research study 8.1

map showing collecting locations for study of carcinisation in hermit crabs Pagurus hirsutiusculusdrawing of hermit crab Pagurus hirsutiusculus showing measurements taken for study of its purported carcinisationAlong the lines of “I didn’t know that!”, a researcher at the Academy of Natural Sciences, Philadelphia writes about a phenomenon known as carcinisation, or the evolutionary tendency of certain crustaceans to become more crab-like, including in hermit crabs the loss of shell. This tendency is apparently shown by some shrimp- and lobster-like crustaceans, and possibly some shell-inhabiting anomurans1, including in this case Pagurus hirsutiusculus along the west coast of North America. The study involves collecting crabs at 3 locations in each of Alaska and southern California, representing the extremes of distribution, with intermediate collections being made in Puget Sound and northern California (see map). Each crab is measured as shown in the drawing. The main findings are that northern males are larger than southern (by about 70% based on length of anterior shield: means of 4.7 and 2.8mm, respectively), that shells available for northern populations are generally too small2, and that crabs in the northern populations often abandon3 their shells altogether. Overall, comparisons of anterior carapace dimensions indicate that carcinisation (specifically, broadening of the carapace, most notably the anterior part) is more well-developed in males from Alaskan populations than in males from southern-California ones, but why this should be is not explained by the author. In fact, the author never really offers any theories as to why carcinisation should be selected for in hermit crabs in the first place. Blackstone 1989 J Zool, Lond 217: 477. Drawing courtesy Josephine Hart Royal British Columbia Museum, Victoria, BC.

NOTE1 an example is the tropical coconut-robber crab Birgus latro, an anomuran that temporarily inhabits a shell during its transition from ocean to land and has, as an adult, a relatively broad carapace (see photograph below Right)

photograph of coconut-robber crab  Birgus latro crawling on a treeNOTE2 the author does not offer any possible reason(s) for this; for example, are northern crabs are simply too large for the shells available, or are different, larger species of snail shells used by members of the southern populations? Also, what species of shells are used in the different areas?

NOTE3 in this regard, one author implies that possession of small, light-weight shells by P. hirsutiusculus allows their abandonment during escape from predators, but does having one's soft, unprotected abdomen exposed to a pursuing predator make a lot of survival sense? Hart 1982 Crabs and their relatives of British Columbia BC Prov Mus Handbook No 40, 267pp.

An Indo-Pacific robber-crab Birgus latro crawls
on a tree. Its soft abdomen is tucked underneath.
Note the wide "carcinised" carapace 0.1X

Research study 9

Hermit crabs inhabit snail shells for protection against predators and other harm from the environment.  It is apparently the only occurrence in nature where a single structure acts without modification as armour for 2 different animals. As hermit crabs have no way to maintain or repair their acquired shells, do the shells give the same degree of protection against crushing predators as afforded their original owners?  A study on crushing resistance of shells of living Calliostoma ligatum and of conspecific shells inhabited by hermit crabs Pagurus spp. collected in the same locality of San Juan Island, Washington shows that they don’t.  In fact, hermit crab-inhabited shells are only about 60% as strong as the originals.  The authors suggest that the weaker state of dead shells likely owes to loss of mineral content of shells (up to 20% within a week) after the death of the snail, and to accumulated shell damage during use by the hermit crabs . The authors posit that hermit crabs may have been a significant factor in the evolutionary diversification of durophagous predators, supplying a prey identical in size and shape to gastropods but with significantly lower resistance to crushing.  LaBarbera & Merz 1992 Paleobiology 18: 367.

NOTE eating of hard-shelled prey; mostly used with reference to feeding habits of fishes

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Research study 10

photograph of hermit crab Pagurus hirsutiuscullus preferentially investigating a plaster replica of a Nucella shell over the real thingApart from size, what features of a shell might be perceived by a hermit crab searching for a home.  Is colour important? Or possibly scent?  This topic is investigated in Oregon for Pagurus samuelis and P. hirsutiusculus, species that prefer shells of Chlorostoma funebralis and Nucella canaliculata, respectively.  Tests on visual and chemical aspects of these shells indicate that P. samuelis relies mainly on visual cues for shell selection, and will actively track objects that resemble C. funebralis shells in colour, shape, and size.  In contrast, P. hirsutiusculus does not display tracking behaviour.  Both species respond to the chemical composition of shells, specifically their calcium content, and may use this ability to locate partially buried shells.  If shells are coated to mask this chemical cue, P. samuelis carries on its exploratory investigation, but P. hirsutiusculus does not initiate shell exploratory behaviour. Note in the photograph that P. hirsutiusculus preferentially invetigates a plaster shell over a nearby silicone-coated Nucella canaliculata shell. Each species appears to rely on shell cues that are distinguishing features of their preferred shells.  Mesce 1993 Anim Behav 45: 659.

NOTE during an earlier study at the University of Oregon the author determined that calcium-bearing objects such as shells elicit explorative and investigative behaviour in hermit crabs photo series showing a hermit crab Pagurus hirsutisusculus presented with a choice of real shell and plaster replicate, choosing the plaster replicatePagurus hirsutiusculus deprived of their own shells.  The author performs choice tests involving real shells, replica shells made of CaSO4 .  H2O (i.e., plaster), natural shells that are coated, and inanimate objects such as shell-sized pebbles.  In one experiment in which a crab is presented with a natural shell and a plaster replicate, it investigates both and ultimately chooses the replica shell, possibly because of the greater rate of dissolution of calcium from the plaster replica (see photographs on Right). To control for the faster dissolution of Ca from the plaster shells (vs. much slower dissolution of Ca from natural shells), the author cleverly saturates the seawater in the test aquarium with CaSO4 .  H2O and shows that the preference of the crabs for plaster shells is abolished.  Through a series of other small but ingenious experiments the author convincingly shows that the single inorganic substance calcium is the active factor at the surface of a shell that triggers shell exploration leading to selection of a shell by P. hirsutiusculusMesce 1982 Science 215: 93.

In this photo series, P. hirsutiusculus is presented
with a real shell and a plaster replica. It carefully
investigates both and ultimately selects the replica

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Research study 11

map showing study sites used in investigation shell use by hermit crabs Pagurus samuelis in Californiaphotograph of a fossilised black turban shell ChlorostomaSnail shells have a poor fossil record and this may be partly explained by their use by hermit crabs.  A taphonomical study at Bodega Marine Laboratory, California shows that hermit crabs Pagurus samuelis favour shells of Chlorostoma funebralis and, moreover, the use of these shells by the crabs may negatively affect the shell’s “fossil longevity”.  First, Chlorostoma shells occupied by hermit crabs have an array of bionts, some unique to crab-occupied shells.  While living snails mostly have clean, biont-less apertural areas (84% of shells), those inhabited by P. samuelis have much more fouling (61% of shells). Interestingly, the authors can determine whether the bionts settle during the life of the hermit crab or after by the pattern of wear caused by the body of the hermit crab.   Note in the photograph of a fossilised Chlorostoma shell that movement by the crab has kept a portion of the apertural area free of fouling (see arrow).  The fossilised bionts thus provide a record of the last inhabitant of the shell, whether snail or hermit crab.  By tethering empty shells in different habitats for several months, the authors show that longevity at rocky-shore sites is affected mainly by physical processes, whereas longevity on mudflats is influenced mainly by mistaken predation by shell-crushing crabs such as Cancer productus.  Surprisingly, much of the potential taphonomic “signature”of the bionts is retained on the shells even after tumbling on a high wave-energy shore because of protection conferred by the barnacles and worms settling in concavities of the shell and within the aperture itself.  Walker & Carlton 1995 Palaeo 114: 197.

NOTE  taphonomy is the study of the fate of remains of organisms after death

NOTE  in this areas the bionts are mainly barnacles and spirorbid worms

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Research study 12

Confirmation that shell selection by hermit crabs is dependent on chemical and not visual cues is made using hermit crabs  Pagurus samuelis at the Bodega Marine Laboratory, California.  The investigators first note that when presented with a choice between its original shell and a matched Chlorostoma shell not previously inhabited by a hermit crab, P. samuelis will choose its original shell.  The same crab is able to differentiate its original shell from a similar shell previously occupied by another P. samuelis.  However, if the original shell is boiled, the preference is lost.  Is it the scent of itself the crab is attracted to, or a combination of its own scent and something to do with the shell?  Results of preference tests to find this out indicate that it is the latter – that the cue is in some way associated with the shell.  Benoit et al. 1997 Mar Freshw Behav Physiol 30: 45.

NOTE  the favourite shells of P. samuelis in the Bodega Bay area are those of black turban-snails Chlorostoma funebralis

NOTE  the crabs are removed from their shells in a novel way: by placing them on ice for 30min, then gently extracting them.  After 24h in seawater the shell-less crabs are good as new

Research study 13

Hermit crabs appear to remember the shell-types they formerly occupied.  Studies in Bodega Bay, California show that the lower-shore-inhabiting Pagurus granosimanus prefers to occupy shells of Olivella biplicata or Chlorostoma funebralis (or C. brunnea depending upon the site), while the sympatric and somewhat higher-dwelling P. samuelis prefers shells of C. funebralis.  When P. granosimanus is removed from its shell and offered a choice of equal-sized Olivella and Tegula funebralis shells, it invariably selects the shell-type it was in when collected from the field.  Thus, the prior shell occupied appears to play a role in future shell selection.  Hahn 1998 J Exper Mar Biol Ecol 228: 35.
NOTE  if you wonder how this is done without too much damage to the occupant, there are several methods in use: one is to drill a small hole at the apex part of the shell and poke the hermit crab’s soft posterior with a pin; another, more certain method, and the one used in the study cited here, is to heat the apex of the shell with a small flame.  A third method, likely even more stressful to the crab than the other two, involves brief exposure of the crab to freshwater which “relaxes” it enough to permit its extraction from the shell with forceps. A fourth method, used in Research Study 12 above, uses cold treatment - which may be least stressful of all. A fifth method is to crush the shell in a vise. A last method, as used in Research Study 15 below, is to anaesthetise the crab before pulling it out

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Research study 13.1

A publication by several California researchers on shell competition in the west-coast species Pagurus samuelis begins with the premise that little is known on intraspecific aggression in hermit crabs, which actually fails to credit numerous previous studies (see Research Studies 7 & 8 above, just as examples). Their finding, for example, that shell-less crabs are more aggressive than shelled ones is not new, but what does seem to be new is that males and females do not differ significantly in level of aggression, and this adds at least something to the study’s interest. Absher et al. 2001 Mar Fresh Behav Physiol 34: 117.

NOTE neither paper is cited by the present authors

NOTE measured by number of bouts in a 10min period, bout duration, and latency to respond

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Research study 14

map showing study sites used in investigation of shell use in hermit crabs Pagurus spp. in CaliforniaAvailability of habitat is an important determinant of adult population size for all animal species, and in most marine invertebrates whatever constitutes good adult habitat should also be good for the juveniles.  However, if the juveniles require a habitat different from the adults, then there may exist an early bottleneck with important consequences for later population size.  This idea of habitat bottlenecks at juvenile/adult size is tested with hermit crabs Pagurus samuelis and P. hirsutiusculus at 9 sites in California by increasing the abundance of juvenile "habitat" (Littorina shells) and/or adult "habitat" (Chlorostoma shells) and measuring the effect of each on adult population size. With seasonal settlement in March-April, recruits grow large enough to occupy juvenile-sized shells in 1-2mo, and large enough to occupy adult-sized shells by June-July. At each site all added shells are tethered with monofilament line to eyebolts set within tidepools.  At weekly intervals the extent of occupation of tethered shells is recorded and the tethers cut to free the hermit crabs.  New empty shells are added at this time. Results show that while juvenile population size is definitely limited by lack of juvenile habitat, there is no bottom-up effect on adult population size.  So, addition of empty juvenile-sized shells has no significant effect on adult population size, nor does addition of empty adult-sized shells have a significant effect on adult population size, although a positive trend is evident in the data.  The author suggests that adult population size in these hermit-crab species is most likely limited by recruitment. Halpern 2004 Mar Ecol Progr Ser 276: 197.

NOTE  the author notes that both hermit-crab species use Littorina and Chlorostoma shells almost exclusively in the study locations.  Because the crabs when small could not be distinguished to species, the author eventually combines data for both species for statistical analyses

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Research study 15

histogram showing types of snail shells used by hermit crabs Pagurus hartaehistogram showing shell preferences of the hermit crab Pagurus hartae in Barkley Sound, British ColumbiaResearchers at the Bamfield Marine Sciences Centre, British Columbia investigate shell preferences in an unusually tiny species Pagurus hartae (carapace length: 6mm) occupying subtidal under-rock habitats. Pagurus hartae inhabits 9 types of shells in the field, with Calliostoma ligatum (42%), Amphissa columbiana (27%), Alia carinata (10%), Nassarius mendicus (8%), and Homalopoma luridum (5%) being the most preferred (see histogram on Left). 

Crabs to be used in experiments are first anaesthetised, then drawn with care from their shells.  To test their shell preferences, the shell-less crabs are presented cafeteria-style with 4 suitably-sized shells of the 5 species just listed and left for 12h to make a selection. The results, lumped into 3 shell-shape categories for convenience, indicate that Homalopoma is the shell of choice, even though it is one of lowest-ranked shells of field crabs (see histogram on the Right). The data presentation is a bit tricky to follow. Shell selection, as just noted, is given by the height of the 3 bars. Note, though, that the histogram bars also show what shells were being occupied by the crabs prior to their use in the experiments. Thus, Calliostoma shells are only selected by crabs previously in Calliostoma shells. However, other crabs also previously in Calliostoma shells display no fidelity to this snail species and select other shells offered. Overall and unlike in other hermit crabs, there appears to be no significant fidelity for a certain shell-species that was occupied in the field prior to testing.  Low-spired shells are known from studies on other hermit-crab species to confer better protection from predators, and this may explain P. hartae's preference for the thick, globose shell of HomalopomaSato & Jensen 2005 Crustaceana 78: 755.

NOTE  anaesthetic is one drop of oil of cloves diluted in 25ml seawater

NOTE  before being used shells are cleaned and boiled in freshwater for 20min to kill any associated organisms and to remove any residual chemical residues

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Research study 16

schematic showing test arena for determining shell selection in hermit crabs Pagurus granosimanushistogram showing effects of shell condition and perceived competition and predation on shell-selection by shell-less hermit crabs Pagurus granosimanusAn animal surrounded by food generally tends to become more picky about what it selects to eat.  Similarly, hermit crabs become more selective when empty shells are abundant, but does this change with the perception of presence of more conspecific competitors or predators?  This is tested at Friday Harbor Laboratories, Washington using Pagurus granosimanus, shells of whelks Nucella lamellosa, a preferred host in the area of collection, and predatory crabs Cancer productus.  Each test involves a selection by shell-less hermit crabs of undamaged, slightly damaged, and more damaged, empty whelk shells.  Test conditions are 1) plain seawater Histogram showing frequency of shell-switching in hermit crabs being tested for shell preference(control treatment), 2) seawater infused with the scent of conspecifics), and 3) seawater infused with the scent of crabs and a few pieces of dead P. granosimanus (predation). The schematic on the Left shows the design of the competition treatment. Here, a test Pagurus is being presented with varyingly damaged shells in an arena screened off from numerous potential competitors. Test crabs are allowed 20min to make their selection.

Results show that hermit crabs regardless of treatment preferentially select undamaged shells (see histogram upper Right). However, frequency of switching varies significantly between the different treatments, with most switching occurring in the control treatment and least in the competition treatment. The data show that hermit crabs will to some extent adjust their behaviour in response to perceived environmental threats. Bulinski 2007 J Shellf Res 26: 233.

NOTE  the shells are removed after brief exposure of the crabs to 35oC seawater

NOTE  the shells are collected from living hermit crabs, then some are damaged by drilling either a 2-mm (“slight” damage) or a 5-mm (“more” damage) hole into the whorl near the aperture 

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Research study 16.1

Which is more important for a hermit crab: its shell for protection, or food to assuage its hunger? This question is addressed for starved (8-15d) and shell-removed Pagurus samuelis by scientists at Loma Linda University, California in 15min cafeteria-style choice tests. Results show that histogram showing choices made by hermit crabs Pagurus samuelis under different treatment conditions of being shell-less and/or starvedshell-removed1 and shell-removed+starved crabs significantly prefer empty shells2 (Chlorostoma funebralis) over food (100 and 78%, respectively; see histogram). In comparison, control crabs+ shells and starved crabs+shells preferentially select3 food over empty shells when given a choice. Protection of a shell is a stronger motivation than finding food. Billock & Dunbar 2009 J Mar Biol Assoc UK 89 (4): 775.

NOTE1 done using a table-mounted vise (the spelling “vice” used by the authors is probably not a Freudian slip; rather, it is likely that one of the authors or perhaps the editor of the British journal JMBA simply substituted a spelling that is commonly used outside of North America for the same vise device)

NOTE2 these empty “choice shells” are matched as closely as possible in size to the shell currently inhabited by the test subject

NOTE3 in a later publication the same authors show that shell acquisition by P. samuelis is mainly by tactile means (not visual, as might be expected), whereas food-finding is mainly by chemoreception of water-borne cues (not visual or tactile, as might be expected, at least for the latter). Billock & Dunbar 2011 J Exp Mar Biol Ecol 398 (1-2): 26.

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Research study 17

photograph and drawing of a fossilised hermit crab Palaeopagurus vandenegeliWest coast hermit crabs commonly live in sedimenty areas where they are at risk of burial through action of storms, river deposition, bioturbation, and other influences. Behaviour and survival during burial are investigated by researchers from Loma Linda University, California for the intertidal species Pagurus samuelis, with special interest in explaining why the fossil record is generally so sparse in hermit crabs. Experiments involved burying hermit crabs of known sizes at depths of 2, 4, and 6cm for 24h periods in either aperture-up or aperture-down orientations. Oxygen concentrations within the sediments during the treatments decrease by 75% within 15min and by 98% over 24h. In all treatment depths 77% of the hermit crabs are able to escape from burial; with the exception of a single crab, those that don’t, die. Common behaviour of the crabs is to abandon their shells within 10-30min of burial with most (67%) doing this from the aperture-up orientation (compared with 24% from the aperture-down orientation). As shell abandonment in the field would incur increased risk of predation, the authors speculate that it may, in large part, explain the dearth of fossilised crabs within their own domiciles. Shives & Dunbar 2010 J Exp Mar Biol Ecol 388: 33. Photograph and drawing courtesy Palaeontology 2003 46 (1): xx.

NOTE a photograph of this species can be found in Research Study 1 above

If you are unfamiliar with fossilised hermit crabs, here is
Palaeopagurus vandenegeli
in an ammonite shell from the Cretaceous,
about 100my in age, along with an artist's impression 1X

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Research study 18

histograms comparing shell-species preferences for hermit crabs Pagurus hirsutiusculus and P. granosimanusScientists have been curious about the dynamics of shell selection by hermit crabs for decades.  The latest entry of scientific papers comes from researchers at Bamfield Marine Sciences Centre, British Columbia who are interested not just in shell preferences in laboratory and field in the sympatric species Pagurus hirsutiusculus and P. granosimanus, and competition between them, all well-studied subjects, but also in the ontogeny of shell usage, a less well-studied subject.  The authors first determine in laboratory cafeteria-style tests that both species have well-defined preferences (see histograms, horizontal dashed lines indicate percentage occupancy expected if the shell types are randomly selected), and later confirm that these laboratory preferences are reasonably close to field preferences.  Since hermit crabs are usually so aggressively competitive in acquiring one-another’s shells, it is interesting that similar-sized individuals of the 2 species largely ignore the shell types used by the other species (at least during intermediate growth stages), a  behaviour noted by the authors to favour coexistence.  Note also in the histograms that shell preferences of both species change ontogenetically, from broader usage when individuals are younger and smaller, to narrower usage when older and larger.  Not surprisingly, the largest individuals of both species are mostly inhabiting shells of the largest available snail species in the habitat, namely, Nucella ostrina, N. lamellosa, Lirabuccinum dirum, and Chlorostoma (Tegula) funebralis.  As a consequence of this ontological change, both pagurid species may go through temporal bottlenecks in shell availability during their lifetimes that could differentially limit their abundances.  The researchers note that abundances of the 2 pagurids in 7 Barkley-Sound study-sites are poorly correlated and conclude that that they are therefore likely controlled by other independent factors.  The researchers provide an abundance of data relating to shell selection of the 2 pagurids, not considered here.  Straughan & Gosselin 2014 J Exp Mar Biol 451: 1. 

NOTE  this may seem a logical strategy, based on the presumably greater protection provided by a larger, thicker domicile; however, there must be cost tradeoffs involved between protection conferred by a larger shell and the costs involved in obtaining it and carrying it around.  Has this been examined for pagurids? 

NOTE  the authors choose to use the older name Tegula funebralis for this species, rather than Chlorostoma funebralis, but until the proper name is sorted out the latter will continue to be used in the ODYSSEY

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Research study 19

New shell or old, comfortable, worn-in shell? Choice tests by researchers at the University of California, Berkeley suggest that male west-coast hermit crabs Pagurus samuelis, at least, have no significant preference for either used or brand-new shells (results are roughly 50 vs. 50%). As these results differ from those of previous studies on marine and terrestrial hermit-crab species, much of the authors’ discussion relates to possible reasons why. Laidre & Trinh 2014 Crustaceana 87 (7): 856.

NOTE either their original shell or the same species of shell from a conspecific crab; new shells are ones never occupied by a hermit crab before; new and used shells are matched in size as closely as possible

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