Hermit Crab Research Paper
Review of the Blueband Hermit Crab, Pagurus samuelis
Ashley Cameron, Portland State University
Invertebrate Zoology, Dr. Masta
Abstract Pagurus samuelis is a common intertidal species of hermit crab along the Pacific coast, occurring from Alaska to Mexico. It can be identified by the blue bands around the end of its walking legs. P. samuelis is in the family Paguridae, consisting of “right-handed” hermit crabs in which the right cheliped is larger and the abdomen coils clockwise. P. samuelis is a detritovore, but will opportunistically feed upon living plants or animal matter. Unlike other crustaceans, hermit crabs require the shell of another organism to protect their soft abdomen, which is not covered by the same chitinous exoskeleton …show more content…
on the rest of their body. These shells are often from gastropods; P. samuelis favors the shell of the black turban snail, Tegula funebralis. Shell seeking and shell care occupies a large portion of the crab 's life, as it is necessary for safety and survival.
Introduction
The blueband hermit crab, Pagurus samuelis, is named due to the blue coloration around the ends of its walking legs. It is classified in phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, and family Paguridae. Paguridae is one of two families of hermit crabs, with the other being Diogenidae. Paguridae is composed mostly of “right-handed” hermit crabs in which the right chela is larger than the left and the abdomen coils clockwise. Diogenidae are “left-handed” hermit crabs, though this is a loose classification and not all hermits in either family are left- or right-handed. P. samuelis does, however, exhibit right-handedness, and therefore can only occupy shells that coil clockwise like its abdomen. Hermit crabs are dependent on their shells for protection due to having a soft abdomen, unlike “true crabs” which have abdomens curled under their bodies and protected by a hard carapace. Many of the hermit crab’s behaviors are related to either finding or keeping a shell; since they do not create their own shells, it is a constant struggle to find a shell of the ideal size and shape for optimal protection and efficiency.
P. samuelis is an omnivorous detritivore but will occasionally prey upon small crustaceans and other invertebrates.(5) It is not especially aggressive and most competition is a result of shell-seeking. Like most crustaceans, P. samuelis has multiple larval stages before its fully-formed adult body. This species resides in upper intertidal zones along the Pacific coast from Alaska to Baja California.(10)
Anatomy and Physiology (See Fig. 1 on p. 5 for labeled anatomical image) Like other Malacostracans, Pagurus samuelis has a 5-segmented cephalon fused to its 8-segmented thorax forming a cephalo-thorax.
Family Paguridae is classified as an anomuran, which is in contrast to the brachyurans, or “true crabs”. Brachyurans have a reduced abdomen that is curled underneath the body and protected by the chitinous carapace. The hermit crab has a unique abdomen – it is unprotected by the carapace that covers the hermit 's cephalo-thorax. The abdomen of P. samuelis coils to the right, and can fit shells that coil clockwise, which is necessary for protecting the soft body. Like other crustaceans, P. samuelis has five pairs of appendages on its cephalon, including antennules (chemosensory function), antennae (for touch), mandibles and and two pairs of maxillae.(13) The compound eyes are on stalks. The thorax includes eight appendages, of which the first pair are modified into claws, or chelae/chelipeds. Like other anomurans, but unlike brachyurans, the hermit crab has its fifth pair of walking legs greatly reduced and uses them for keeping its gills clean, while the fourth pair of walking legs are used for holding on to the inside of its shell. In hermit crabs, only two pairs of walking legs are used for locomotion, along with the chelipeds.(13) There are also appendages on the abdomen called pleopods which are used by the female to attach and carry fertilized …show more content…
eggs. Internally, the digestive tract is formed by a foregut, a stomach, a midgut and a hindgut. The foregut is mostly used as a passageway to the stomach where food is broken down. Nutrients then pass to the midgut, filled with out-pockets called diverticula for digestion. The waste passes through the chitin-lined hindgut and is excreted out the anus.(13) Respiration occurs via gills, which are found on the bases of the walking legs on both sides of the body. The gills are thin and used for gas exchange, as the oxygen in water dissolves into the crab 's blood and attaches to hemocyanin, a copper-containing molecule.(13) The circulatory system is open, and is powered by a single-chambered heart located in the thorax. Hermit crabs have compound eyes composed of smaller ommatidia units, each focusing light individually. The number of ommatidia must be balanced – greater numbers of ommatidia leads to finer focus, but less light can enter each one. P. samuelis sees a mosaic of repeated images instead of the single one that humans see.(13) Hermit crabs do not have ears, but instead have microscopic hairs on their bodies. These hairs are used for detecting changes in water pressure, which are then transmitted to the central nervous system.(13) Smell is also reliant on small sensory structures. Sensilla are found on the outside of the crab 's body with the highest concentration found on the antennules, the dactyls (last leg segment), and mouthparts. The sensilla on the dactyls can find traces of food in the substrate, and the antennules are extremely sensitive to chemicals in the water, especially food particles and pheromones. The movement of antennules forces water to pass over them and allows the receptors better access to the chemicals. Taste and smell are difficult to differentiate because they follow the same system. Therefore, the antennules are considered “smell” organs while the mouthparts are considered “taste” organs, even though they both use sensilla for chemoreception.(13) Crabs have mechanoreceptors within the flexible parts of their cuticle. Setae, modified sensory hairs, projecting above the carapace can yield information regarding water movement and vibration. The antennae are primarily used for touch. Hermit crabs reproduce via external fertilization. The female 's gonopores are found near the base of the third pair of walking legs while the male 's reproductive organs are found near the base of his fifth pair of legs. The female carries eggs attached to the outside of her body, but inside the protective shell, after fertilization.
Life History In the genus Pagurus, species are diverse amongst all of their life stages. In general, each species proceeds through multiple larval stages before becoming an adult. There are different larval groupings that each species is classified by depending on its shared developmental characteristics with other species. There is no female hormone in Pagurus samuelis, and presence of the androgenic hormone determines sex by stimulating development of testes.(13) All larvae begin as small, planktotrophic nauplius larvae, which is a shared characteristic of decapods.(8) In P. samuelis, this first stage is referred to as the zoeae I, and subsequent stages are zoeae II, III, and IV respectively. P. samuelis is separated from most other species in its larval group because of its setation of the second maxilliped in stage II-IV, and its lack of separation of the sixth abdominal segment and the telson in stage III-IV.(7) After the IV stage, the newly molted organism is known as a glaucothoe, which resembles a small adult. It is in this last stage that the young hermit crab begins to search for its first shell to occupy. The glaucothoe continues to grow and must keep replacing its outgrown shell into its adult life. Before hatching, the eggs are carried on the female’s abdomen, within the shell. The eggs are cemented to the female’s body and protected during development. This is different than “true crab” development; “true crabs” do not carry their fertilized eggs until they are ready for hatching.(13) P. samuelis larvae require water for survival. When the eggs are ready to hatch, the female must come partway out of her shell and move the eggs into the water, using her pleopods, for them to hatch into the zoeae I stage. It is unclear by what methods P. samuelis searches for a mate, but it is likely related to chemosense. Once a suitable mate is found, courtship may include the male lifting the female onto its back and carrying her around for a day or more, while knocking against her shell.(10) Fertilization is quick, as both male and female must come partway out of their respective shells, exposing them to danger. The male’s vas deferens is highly coiled to compact sperm into a spermatophore for protection during transfer to the female.(13) Egg-bearing females have been found from May to August along the Pacific shore, with higher temperature extending the season of reproduction.(13)(10)
Ecology and Evolution Along the Pacific Coast, in California, Pagurus samuelis is the most common hermit crab occupying upper tidal zones, out-numbering its relative hermit P. granosimanus by 10-to-1 in some areas.(10) They are able to share this space as different species of hermits prefer different species of shells for their homes. P. samuelis often inhabits the black turban snail, Tegula funebralis. The necessity of a shell for the life of the hermit crab creates a population bottleneck.(4) The adult crabs are limited by the availability of proper shells to use. In previous studies, adding more shells to the hermit crab habitat have increased the population numbers. (habitat) In addition to supplying more homes for crabs present in these environments, the addition of more shells recruits young crabs to the area.(4) The greatest increase in adult populations of hermit crabs occurrs in late summer, which suggests that the population may experience seasonal bottleneck.(4) In some coastal areas, new species of snails may be introduced and hermits can choose these new habitats as their preference. Overall, the population of P. samuelis depends on the populations of gastropods in its community, making conservation of both gastropods and hermit crabs tied together. Other species of hermit crabs have been known to choose shells with attached cnidarians on them for protection against predators. Finding hermits residing among anemones is not uncommon as it deters the predator octopi from the area.(5) It is not known if P. Samuelis uses these defense mechanisms or not. Unlike shells, food does not seem to be a limiting factor, as many food choices are shared across species. Preferring detritus, P. samuelis will also occasionally consume pieces of larger organisms or tiny live organisms (such as krill).(5) Historically, crab groups diversified in the Cretaceous period (145 – 65 mya). Hermit crabs are among the oldest groups of crabs and early hermits found shelter in both gastropod and ammonite (now extinct) shells.(13) In the fossil record, it is likely that hermit crabs are under-represented. The calcified gastropod shell is left behind, but the hermit crab’s largely soft body does not lend itself as well to be fossilized. Most of the fossil record of hermit crabs includes pieces of chelae or gastropod shells.(11) However, fossilized shells are classified as gastropod fossils and not hermit crab fossils. One study examined the question as to why hermit crabs are so infrequently found in fossils with their shells. P. samuelis was used as the model of this study and it was found that when tidal pools flood and hermits are buried in sediment, the hermit has the option of leaving its shell behind and digging for the surface. The orientation of the aperture had an effect on the decision of the hermit to escape its shell or not, with 73.2% of hermits recorded to abandon their shells doing so when the aperture was upright.(11) Crabs who abandoned their shells exhibited a 95.1% survival rate as opposed to the 63.3% survival of those who remained in the shell after burial.(11) Heavier shells, which offer greater protection and growth potential, may be more difficult to maneuver back to the surface.
Shell Selection and Shell Behavior Shells are essential to the survival of the hermit crab and Pagurus samuelis is no exception. The shell provides protection from predators as the soft abdomen of the hermit crab makes an easy target. P. samuelis prefers Tegula funebralis shells as the gastropod is another common Pacific tidal organism. Finding a shell is largely accomplished via chemosense, sensing the calcium carbonate composition of the shell. P. samuelis is also attracted to shells that contrast with the background, such as a black shell on sand.(9) In a study with a control group versus blinded crabs, they both made initial contact with the shell in a similar amount of time, but the final selection took much longer in the blinded crabs, suggesting eyesight does play a significant role in shell selection.(9) Once contact with the shell is made, a routine behavior pattern is observed. The hermit orients itself to the shell by walking directly towards it and then grasps the shell with its first pair of walking legs. The cheliped is open and examining the shell and the second pair of walking legs grasp the shell for the crab to climb upon it. The crab examines the outside of the shell by crawling over it and running the cheliped across until it finds the aperture. The crab then uses the chelipeds and first walking legs to explore the inside of the shell, removing any debris and holding the shell with the second pair of walking legs. If satisfied, the crab then positions itself to be removed from its current shell and inserts its abdomen into the new shell.(9) Shell preference has been documented in multiple species of hermit crabs, though some have stronger preferences than others. In one study, Pagurus samuelis was found exclusively occupying Tegula funebralis shells, while the other hermit crab in the study occupied the shells of more than one gastropod species.(3) Size and weight are both important considerations for hermits seeking new shells. Lower weight means less energetic stress on the animal, while a larger size means there is more room to safely retreat into the the shell.(6) Small shells are correlated with reduced fecundity.(12) Acquiring a shell is done in different ways. The crab may find an uninhabited shell that it deems desirable. It is extremely unlikely that P. samuelis would find and kill a gastropod to take its shell. There is, however, competition within species for shells. If a crab finds a shell it wants that is inhabited by another crab, it will extend its cheliped in a threatening gesture.(2) The aggressor then lifts or turns the other crab so that the shell opening is facing upwards. Then, the aggressor begins knocking its own shell against the defender is a series of strong taps.(2) The attacker then may attempt to pull the defender out of its shell and will try to move into the new shell. Not all shell fights are detrimental as both crabs in ill-fitting shells may benefit from an exchange.(2) P. samuelis has little interspecific competition due to a difference in size and habitat between itself and other hermit crabs inhabiting the Pacific coast.(1) Another method of acquiring a new shell is to wait at a site with many discarded shells. Hermit crabs will form a line ranging from the largest crab to the smallest crab to trade up shells so each inherits the shell of the crab larger than itself.(13)
Conclusion As more is learned about the correlation between hermit crabs and their shells, more efforts can be focused in protecting their habitat. Pagurus samuelis depends on its preferred gastropod shell for survival, highlighting the importance of preserving all intertidal ecology.
Figure 1:
Anatomy of a hermit crab.
References
(1) Abrams, P. A. 1987. Resource partitioning and competition for shells between intertidal hermit crabs on the outer coast of Washington. Oecologia 72(2): 248-258.
(2) Dowds, B. M., Elwood, R. W. 1983. Shell Wars: Assessment strategies and the timing of decisions in hermit crab shell fights. Behavior 85: 1-24.
(3) Hahn, D.
R. 1998. Hermit crab shell use patterns: response to previous shell experience and to water flow. Journal of Experimental Marine Biology and Ecology 228: 35–51.
(4) Halpern, B. S. 2004. Habitat bottlenecks in stage-structured species: hermit crabs as a model system. Marine Ecology Progress Series 276: 197-207.
(5) Hazlett, B. A. 1981. The behavioral ecology of hermit crabs. Annual Review of Ecology and Systematics 12: 1-22.
(6) Lively, C. M. 1988. A graphical model for shell-species selection by hermit crabs. Ecology 69(4): 1233-1238.
(7) McLaughlin, P. A., Gore, R. H., Crain, J. A. 1988. Studies on the provenzanoi and other Pagurid groups: II. A reexamination of the larval stages of Pagurus hirsutiusculus hirsutiusculus (Dana) (Decapoda: Anomura: Paguridae) reared in the laboratory. Journal of Crustacean Biology 8(3): 430-450
(8) Pechenik, J. A. 2005. Biology of the Invertebrates. 5th edition. McGraw Hill. 368-361.
(9) Reese, P. S. 1963. The behavioral mechanisms underlying shell selection by hermit crabs. Behavior 21: 78-126
(10) Ricketts, E. F., Calvin, J., Hedgpeth, J. W., Phillips, D. W. 1985. Between Pacific Tides. Stanford University
Press.
(11) Shives, J. S., Dunbar, S. G. 2010. Behavioral responses to burial in the hermit crab, Pagurus samuelis: Implications for the fossil record. Journal of Experimental Marine Biology and Ecology 388: 33-38.
(12) Vance, R. R. 1972. The role of shell adequacy in behavioral interactions involving hermit crabs. Ecology 53(6): 1075-1083.
(13) Weis, J. S. 2012. Walking Sideways. Cornell University Press.
Ashley Cameron, Portland State University
Invertebrate Zoology, Dr. Masta
Abstract Pagurus samuelis is a common intertidal species of hermit crab along the Pacific coast, occurring from Alaska to Mexico. It can be identified by the blue bands around the end of its walking legs. P. samuelis is in the family Paguridae, consisting of “right-handed” hermit crabs in which the right cheliped is larger and the abdomen coils clockwise. P. samuelis is a detritovore, but will opportunistically feed upon living plants or animal matter. Unlike other crustaceans, hermit crabs require the shell of another organism to protect their soft abdomen, which is not covered by the same chitinous exoskeleton …show more content…
on the rest of their body. These shells are often from gastropods; P. samuelis favors the shell of the black turban snail, Tegula funebralis. Shell seeking and shell care occupies a large portion of the crab 's life, as it is necessary for safety and survival.
Introduction
The blueband hermit crab, Pagurus samuelis, is named due to the blue coloration around the ends of its walking legs. It is classified in phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, and family Paguridae. Paguridae is one of two families of hermit crabs, with the other being Diogenidae. Paguridae is composed mostly of “right-handed” hermit crabs in which the right chela is larger than the left and the abdomen coils clockwise. Diogenidae are “left-handed” hermit crabs, though this is a loose classification and not all hermits in either family are left- or right-handed. P. samuelis does, however, exhibit right-handedness, and therefore can only occupy shells that coil clockwise like its abdomen. Hermit crabs are dependent on their shells for protection due to having a soft abdomen, unlike “true crabs” which have abdomens curled under their bodies and protected by a hard carapace. Many of the hermit crab’s behaviors are related to either finding or keeping a shell; since they do not create their own shells, it is a constant struggle to find a shell of the ideal size and shape for optimal protection and efficiency.
P. samuelis is an omnivorous detritivore but will occasionally prey upon small crustaceans and other invertebrates.(5) It is not especially aggressive and most competition is a result of shell-seeking. Like most crustaceans, P. samuelis has multiple larval stages before its fully-formed adult body. This species resides in upper intertidal zones along the Pacific coast from Alaska to Baja California.(10)
Anatomy and Physiology (See Fig. 1 on p. 5 for labeled anatomical image) Like other Malacostracans, Pagurus samuelis has a 5-segmented cephalon fused to its 8-segmented thorax forming a cephalo-thorax.
Family Paguridae is classified as an anomuran, which is in contrast to the brachyurans, or “true crabs”. Brachyurans have a reduced abdomen that is curled underneath the body and protected by the chitinous carapace. The hermit crab has a unique abdomen – it is unprotected by the carapace that covers the hermit 's cephalo-thorax. The abdomen of P. samuelis coils to the right, and can fit shells that coil clockwise, which is necessary for protecting the soft body. Like other crustaceans, P. samuelis has five pairs of appendages on its cephalon, including antennules (chemosensory function), antennae (for touch), mandibles and and two pairs of maxillae.(13) The compound eyes are on stalks. The thorax includes eight appendages, of which the first pair are modified into claws, or chelae/chelipeds. Like other anomurans, but unlike brachyurans, the hermit crab has its fifth pair of walking legs greatly reduced and uses them for keeping its gills clean, while the fourth pair of walking legs are used for holding on to the inside of its shell. In hermit crabs, only two pairs of walking legs are used for locomotion, along with the chelipeds.(13) There are also appendages on the abdomen called pleopods which are used by the female to attach and carry fertilized …show more content…
eggs. Internally, the digestive tract is formed by a foregut, a stomach, a midgut and a hindgut. The foregut is mostly used as a passageway to the stomach where food is broken down. Nutrients then pass to the midgut, filled with out-pockets called diverticula for digestion. The waste passes through the chitin-lined hindgut and is excreted out the anus.(13) Respiration occurs via gills, which are found on the bases of the walking legs on both sides of the body. The gills are thin and used for gas exchange, as the oxygen in water dissolves into the crab 's blood and attaches to hemocyanin, a copper-containing molecule.(13) The circulatory system is open, and is powered by a single-chambered heart located in the thorax. Hermit crabs have compound eyes composed of smaller ommatidia units, each focusing light individually. The number of ommatidia must be balanced – greater numbers of ommatidia leads to finer focus, but less light can enter each one. P. samuelis sees a mosaic of repeated images instead of the single one that humans see.(13) Hermit crabs do not have ears, but instead have microscopic hairs on their bodies. These hairs are used for detecting changes in water pressure, which are then transmitted to the central nervous system.(13) Smell is also reliant on small sensory structures. Sensilla are found on the outside of the crab 's body with the highest concentration found on the antennules, the dactyls (last leg segment), and mouthparts. The sensilla on the dactyls can find traces of food in the substrate, and the antennules are extremely sensitive to chemicals in the water, especially food particles and pheromones. The movement of antennules forces water to pass over them and allows the receptors better access to the chemicals. Taste and smell are difficult to differentiate because they follow the same system. Therefore, the antennules are considered “smell” organs while the mouthparts are considered “taste” organs, even though they both use sensilla for chemoreception.(13) Crabs have mechanoreceptors within the flexible parts of their cuticle. Setae, modified sensory hairs, projecting above the carapace can yield information regarding water movement and vibration. The antennae are primarily used for touch. Hermit crabs reproduce via external fertilization. The female 's gonopores are found near the base of the third pair of walking legs while the male 's reproductive organs are found near the base of his fifth pair of legs. The female carries eggs attached to the outside of her body, but inside the protective shell, after fertilization.
Life History In the genus Pagurus, species are diverse amongst all of their life stages. In general, each species proceeds through multiple larval stages before becoming an adult. There are different larval groupings that each species is classified by depending on its shared developmental characteristics with other species. There is no female hormone in Pagurus samuelis, and presence of the androgenic hormone determines sex by stimulating development of testes.(13) All larvae begin as small, planktotrophic nauplius larvae, which is a shared characteristic of decapods.(8) In P. samuelis, this first stage is referred to as the zoeae I, and subsequent stages are zoeae II, III, and IV respectively. P. samuelis is separated from most other species in its larval group because of its setation of the second maxilliped in stage II-IV, and its lack of separation of the sixth abdominal segment and the telson in stage III-IV.(7) After the IV stage, the newly molted organism is known as a glaucothoe, which resembles a small adult. It is in this last stage that the young hermit crab begins to search for its first shell to occupy. The glaucothoe continues to grow and must keep replacing its outgrown shell into its adult life. Before hatching, the eggs are carried on the female’s abdomen, within the shell. The eggs are cemented to the female’s body and protected during development. This is different than “true crab” development; “true crabs” do not carry their fertilized eggs until they are ready for hatching.(13) P. samuelis larvae require water for survival. When the eggs are ready to hatch, the female must come partway out of her shell and move the eggs into the water, using her pleopods, for them to hatch into the zoeae I stage. It is unclear by what methods P. samuelis searches for a mate, but it is likely related to chemosense. Once a suitable mate is found, courtship may include the male lifting the female onto its back and carrying her around for a day or more, while knocking against her shell.(10) Fertilization is quick, as both male and female must come partway out of their respective shells, exposing them to danger. The male’s vas deferens is highly coiled to compact sperm into a spermatophore for protection during transfer to the female.(13) Egg-bearing females have been found from May to August along the Pacific shore, with higher temperature extending the season of reproduction.(13)(10)
Ecology and Evolution Along the Pacific Coast, in California, Pagurus samuelis is the most common hermit crab occupying upper tidal zones, out-numbering its relative hermit P. granosimanus by 10-to-1 in some areas.(10) They are able to share this space as different species of hermits prefer different species of shells for their homes. P. samuelis often inhabits the black turban snail, Tegula funebralis. The necessity of a shell for the life of the hermit crab creates a population bottleneck.(4) The adult crabs are limited by the availability of proper shells to use. In previous studies, adding more shells to the hermit crab habitat have increased the population numbers. (habitat) In addition to supplying more homes for crabs present in these environments, the addition of more shells recruits young crabs to the area.(4) The greatest increase in adult populations of hermit crabs occurrs in late summer, which suggests that the population may experience seasonal bottleneck.(4) In some coastal areas, new species of snails may be introduced and hermits can choose these new habitats as their preference. Overall, the population of P. samuelis depends on the populations of gastropods in its community, making conservation of both gastropods and hermit crabs tied together. Other species of hermit crabs have been known to choose shells with attached cnidarians on them for protection against predators. Finding hermits residing among anemones is not uncommon as it deters the predator octopi from the area.(5) It is not known if P. Samuelis uses these defense mechanisms or not. Unlike shells, food does not seem to be a limiting factor, as many food choices are shared across species. Preferring detritus, P. samuelis will also occasionally consume pieces of larger organisms or tiny live organisms (such as krill).(5) Historically, crab groups diversified in the Cretaceous period (145 – 65 mya). Hermit crabs are among the oldest groups of crabs and early hermits found shelter in both gastropod and ammonite (now extinct) shells.(13) In the fossil record, it is likely that hermit crabs are under-represented. The calcified gastropod shell is left behind, but the hermit crab’s largely soft body does not lend itself as well to be fossilized. Most of the fossil record of hermit crabs includes pieces of chelae or gastropod shells.(11) However, fossilized shells are classified as gastropod fossils and not hermit crab fossils. One study examined the question as to why hermit crabs are so infrequently found in fossils with their shells. P. samuelis was used as the model of this study and it was found that when tidal pools flood and hermits are buried in sediment, the hermit has the option of leaving its shell behind and digging for the surface. The orientation of the aperture had an effect on the decision of the hermit to escape its shell or not, with 73.2% of hermits recorded to abandon their shells doing so when the aperture was upright.(11) Crabs who abandoned their shells exhibited a 95.1% survival rate as opposed to the 63.3% survival of those who remained in the shell after burial.(11) Heavier shells, which offer greater protection and growth potential, may be more difficult to maneuver back to the surface.
Shell Selection and Shell Behavior Shells are essential to the survival of the hermit crab and Pagurus samuelis is no exception. The shell provides protection from predators as the soft abdomen of the hermit crab makes an easy target. P. samuelis prefers Tegula funebralis shells as the gastropod is another common Pacific tidal organism. Finding a shell is largely accomplished via chemosense, sensing the calcium carbonate composition of the shell. P. samuelis is also attracted to shells that contrast with the background, such as a black shell on sand.(9) In a study with a control group versus blinded crabs, they both made initial contact with the shell in a similar amount of time, but the final selection took much longer in the blinded crabs, suggesting eyesight does play a significant role in shell selection.(9) Once contact with the shell is made, a routine behavior pattern is observed. The hermit orients itself to the shell by walking directly towards it and then grasps the shell with its first pair of walking legs. The cheliped is open and examining the shell and the second pair of walking legs grasp the shell for the crab to climb upon it. The crab examines the outside of the shell by crawling over it and running the cheliped across until it finds the aperture. The crab then uses the chelipeds and first walking legs to explore the inside of the shell, removing any debris and holding the shell with the second pair of walking legs. If satisfied, the crab then positions itself to be removed from its current shell and inserts its abdomen into the new shell.(9) Shell preference has been documented in multiple species of hermit crabs, though some have stronger preferences than others. In one study, Pagurus samuelis was found exclusively occupying Tegula funebralis shells, while the other hermit crab in the study occupied the shells of more than one gastropod species.(3) Size and weight are both important considerations for hermits seeking new shells. Lower weight means less energetic stress on the animal, while a larger size means there is more room to safely retreat into the the shell.(6) Small shells are correlated with reduced fecundity.(12) Acquiring a shell is done in different ways. The crab may find an uninhabited shell that it deems desirable. It is extremely unlikely that P. samuelis would find and kill a gastropod to take its shell. There is, however, competition within species for shells. If a crab finds a shell it wants that is inhabited by another crab, it will extend its cheliped in a threatening gesture.(2) The aggressor then lifts or turns the other crab so that the shell opening is facing upwards. Then, the aggressor begins knocking its own shell against the defender is a series of strong taps.(2) The attacker then may attempt to pull the defender out of its shell and will try to move into the new shell. Not all shell fights are detrimental as both crabs in ill-fitting shells may benefit from an exchange.(2) P. samuelis has little interspecific competition due to a difference in size and habitat between itself and other hermit crabs inhabiting the Pacific coast.(1) Another method of acquiring a new shell is to wait at a site with many discarded shells. Hermit crabs will form a line ranging from the largest crab to the smallest crab to trade up shells so each inherits the shell of the crab larger than itself.(13)
Conclusion As more is learned about the correlation between hermit crabs and their shells, more efforts can be focused in protecting their habitat. Pagurus samuelis depends on its preferred gastropod shell for survival, highlighting the importance of preserving all intertidal ecology.
Figure 1:
Anatomy of a hermit crab.
References
(1) Abrams, P. A. 1987. Resource partitioning and competition for shells between intertidal hermit crabs on the outer coast of Washington. Oecologia 72(2): 248-258.
(2) Dowds, B. M., Elwood, R. W. 1983. Shell Wars: Assessment strategies and the timing of decisions in hermit crab shell fights. Behavior 85: 1-24.
(3) Hahn, D.
R. 1998. Hermit crab shell use patterns: response to previous shell experience and to water flow. Journal of Experimental Marine Biology and Ecology 228: 35–51.
(4) Halpern, B. S. 2004. Habitat bottlenecks in stage-structured species: hermit crabs as a model system. Marine Ecology Progress Series 276: 197-207.
(5) Hazlett, B. A. 1981. The behavioral ecology of hermit crabs. Annual Review of Ecology and Systematics 12: 1-22.
(6) Lively, C. M. 1988. A graphical model for shell-species selection by hermit crabs. Ecology 69(4): 1233-1238.
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