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Calcinus Seurati
The Whitebanded Hermit
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Taylor Homan 2016
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Summary | |
Calcinus seurati, known as the whitebanded hermit crab, is a member of the family Diogenidae (Orrell, 2016), although it is not in fact a true crab. It is found on rocky shores and beaches in tropical waters, primarily in Hawaii, but some individuals have been found throughout Southeast Asia and the South Pacific (Chan, 1993). C. seurati’s distinguishing feature is the presence of white bands around the second and third pereiopods (Chan, 1993). While not classified as a threatened species, C. seurati faces threat of habitat loss due to climate change and ocean acidification.
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Physical Description | |
Hermit crabs, belonging to the class Malacostraca of the phylum Arthropoda, are a type of crustacean that have a segmented body which is broken down into twenty-one sections, also referred to as somites (Ruppert et al., 2004). These somites can be categorized in larger groupings by the head, thorax, abdomen, and tail (see Anatomy and Physiology, fig.4) (Ruppert et al., 2004). However, unlike most other crustaceans, what sets the hermit apart is the lack of a hard exoskeleton to cover the abdominal area. The exoskeleton of a hermit crab ends at the thoracic section of its body with the posterior carapace (Forest et al., 2000). The abdomen is protected only by a layer of cuticle, and the external shell which the hermit crab obtains and occupies (Tudge, 1995).
Calcinus seurati is a species of hermit crab belonging to the family Diogenidae (Orrell, 2016). This family is characterized by an enlarged left chela, earning the family the name “left-handed hermit crabs” (Tudge, 1995). C. seurati is a relatively small species, reaching a maximum shield length of approximately 6.5 mm at adulthood (Chan, 1993).
Also known as the Whitebanded Hermit, C. seurati is easily identified by the black and white bands present on the second and third pereiopods (Chan, 1993). Colour of living specimens can vary. Specimens frequently have a light grey or brown carapace, and darken in colour throughout the abdomen (fig. 2). However, individuals of a blue colour variation have been found and identified by the white bands present on the pereiopods (fig. 1) (Chan, 1993). Most whitebanded hermits (if not all) also have a notable colouration of the eyes, ocular penduncles, antennae, and antennular penduncles (Chan, 1993). Eyes are a vibrant blue, and the ocular penduncles are frequently bright orange distally, fading into a proximal light blue. This colour pattern is repeated in the antennular penduncles, which are bright blue proximally and bright orange at the distal ends. The antennae are coloured solidly orange.
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| Figure 1 |
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| Figure 2 |
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Ecology | |
Habitat
C. seurati, like other marine hermit crabs, lives primarily within the intertidal zone of tropical coasts (Chan, 1993). This can include rocky shores, beaches, and mangrove areas. Though classified as a marine species, C. seurati can spend limited time out of water, provided its gills are kept damp. This is an adaption that almost all marine hermit crabs have evolved, although less so than their terrestrial counterparts (Tudge, 1995).
Diet
The majority of marine hermit crabs, the whitebanded hermit included, have an omnivorous diet. This diet consists of algae, supplemented by scavenging on the carrion of dead or dying animals (Ingle & Christiansen, 2004). Opportunistic scavenging and predation such as this can extend to other invertebrates, and occasionally other hermit crabs, as well (Lancaster, 1988). Considering the relatively small size of C. seurati, it is unlikely that it predates upon larger invertebrates such as echinoderms, and instead focuses on smaller organisms such polychaetes or other small crustaceans.
Predators
Hermit crabs generally obtain empty shells previously belonging to gastropods that were preyed upon for protection. However, this leaves them vulnerable to the same predators that the gastropods faced (Elwood & Neil, 1992). These predators can generally be divided into two groups: predators that break the shell, and predators that extract the animal from it. Included in those that extract the crab are fish, octopuses, other crustaceans, and even some gastropods (Tirelli et al., 2000).
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Life History and Behaviour | |
Reproduction
Hermit crabs are gonochoric, and reproduce sexually. Males transfer sperm to the female not directly through the penis, but by use of the anterior pleopods (gonopods) (Ruppert et al., 2004). Sperm is transferred with the protection of a spermatophore, and it has no flagellum with which to move. Instead, the spermatophores attach to the surface of the female, and are fertilized externally when the eggs are released through the gonopods (Ruppert et al., 2004). In order to mate, the hermit crabs must partially leave their shells, as their ventral surfaces must be in contact when the eggs and spermatophores are released for reproduction to occur (Ruppert et al., 2004).
Development
The fertilized eggs are attached to the body of the female hermit crab, and kept within her shell for protection as they brood. Before the eggs hatch, the embryos develop into a protozoea stage and then a zoea stage, at which time they hatch into the water (Ruppert et al., 2004). The zoea stage is the planktonic larval form of the crabs, after which all of their appendages develop and they become megalopa (fig 3) (Ruppert et al., 2004). The megalopa are the final stage in the larval portion of their lifecycle. After this stage, the hermit crabs must find a shell to reside in, to protect their soft abdomen. As they continue to grow, they are forced to find larger and larger shells to accommodate their size (Ruppert et al., 2004).
Shell Selection and Competition
There are numerous factors involved when selecting a shell. Beyond just the size, hermit crabs have been known to examine the size and shape of the aperture with their chelae, the weight and colour of the shell, and the condition of the shell itself before accepting it (Elwood et al. 1992). If the shell is damaged or has holes from previous predators, some crabs will reject them, presumably because they indicate vulnerability to the same predators (Pechenik & Lewis, 2000).
Due to a lack of available or adequate shells, there is often competition between crabs for available resources (Elwood et al. 1992). This competition can escalate to the point where one individual will physically remove another from its shell, and then occupy the shell itself (Tirelli et al., 2000).
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| Figure 3 |
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Anatomy and Physiology | |
Respiration
The carapace of Calcinus seurati, like all marine hermit crabs, has an extension on either side containing the branchial chambers (Conlan n.d.). These extensions are called branchiostegites (Ruppert et al., 2004). Within the branchial chambers are as many as 24 pairs of gills (Conlan n.d.). The gills are aligned longitudinally through the branchial chamber, and serve as a division of the chamber into two sections (Ruppert et al., 2004). One section is referred to as the inhalant chamber, and the other as the exhalant chamber. Water enters in the inhalant chamber, and oxygen diffuses into the gills as the water moves further into the branchial chamber before turning directions and exiting through the exhalant chamber (Conlan n.d.). To restrict the amount of sediment that enters the chamber and covers the gills, a filter of setae is present at the entrance to the chamber (Ruppert et al., 2004).
Internal Transport
Once the oxygen has diffused through the gills, it is carried by hemocyanin throughout the hemal system to the rest of the body (Ruppert et al., 2004). Calcinus seurati has a circulatory system containing a heart, arterial system, capillaries, and venous sinuses (Ruppert et al., 2004). Hemocyanin is the respiratory pigment that carries oxygen in the blood. The heart is surrounded by the pericardial sinus, where oxygenated blood enters from the branchiopericardial vein, and is attached to seven major arteries that distribute that blood to the rest of the body via capillaries (Ruppert et al., 2004).
Digestion and Excretion
The decapod digestive system consists of a foregut, a midgut, and a hindgut. The foregut is made up of an oesophagus, where food enters initially, and a two-chambered stomach (the proventriculus), where it begins to be processed (Conlan n.d.). The first chamber the food reaches is the cardiac stomach, which contains three teeth that form the gastric mill (Ruppert et al., 2004). Decapods often eat quickly and with little consideration of the sediment ingested with their food. Muscles force food through the gastric mill into the second chamber, the pyloric stomach, which breaks down the large clumps into very small particles (Ruppert et al., 2004). The pyloric stomach filters the particles to again remove large pieces, after which the food moves through the intestines of the midgut, where chemical absorption occurs (Ruppert et al., 2004). The remaining waste moves to the hindgut, where it is expelled through the anus as faeces (Conlan n.d.). The abdomen of a hermit crab remains ensconced within the gastropod shell, however, and it must occasionally clean out the waste.
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| Figure 4 |
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Biogeographic Distribution | |
Calcinus seurati is found mainly around the Hawaiian Islands, in the Pacific Ocean. Other specimens, however, have been recorded throughout Southeast Asia, particularly Taiwan, as well as the Indo-Pacific region and the South Pacific Ocean (Chan, 1993).
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Evolution and Systematics | |
Despite its common name, the hermit crab is not actually a true crab at all, due to its exposed abdomen and tendency to walk forward, rather than sideways. Within the order Decapoda, the hermit crab belongs to the infraorder Anomura, the sister group to the true crabs, Brachyura (Conlan n.d.).
In addition to hermit crabs, Anomura also includes porcelain crabs, squat lobsters, king crabs, and others. Paguroidea (hermit crabs) is defined by the lack of hard exoskeleton covering the abdomen (Ingle & Christiansen, 2004). Diogenidae, the family Calcinus seurati belongs to, is further characterized by the enlarged left cheliped, earning the name left-handed hermit crab (Tudge, 1995). The species of the genus Calcinus are usually distinguished by their unique colour patterns (Chan, 1993). Unlike other marine taxa, the centre for Calcinus species diversification is not in the Indo-Malayan region, but instead near the Mariana and Tuamotu Islands (Malay & Paulay, 2009). C. seurati belongs to Clade VIII of Calcinus, with Calcinus laevimanus (Malay & Paulay, 2009).
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| Figure 5 |
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Conservation and Threats | |
The conservation status of C. seurati has not been evaluated in any official context. Though there is not enough data to determine the size of the populations living in the Indo-Pacific region, C. seurati is found very commonly throughout the Hawaiian Islands, and is not likely to be suffering threat of extinction.
Threats to the species are nonspecific threats currently faced by all tropical reef and intertidal ecosystems; habitat loss or destruction being the primary concern. Increase in sea temperature threatens to result in numerous habitats becoming uninhabitable by not only hermit crabs, but the entire intertidal ecosystem. In addition, pollution and land development are threats particularly to the mangroves (Zann, 1995), which tend to house large numbers of hermit crabs within the area.
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References | |
Chan, T. Y. 1993. Calcinus seurati Forest, 1951. BiotaTaiwanica. Online resource. Retrieved from http://crust.biota.biodiv.tw/pages/4973.
Conlan, K. E. Malacostracan crustacean. In Encyclopaedia Britannica Online. Retrieved from http://www.britannica.com/animal/malacostracan#toc33781.
Dixon, C. J., Ahyong, S. T., Schram, F. R. in press. A new hypothesis of Decapod phylogeny (Crustacea: Malacostraca). Crustaceana.
Elwood, R. W. & Neil, S. J. 1992. Assessments and decisions: a study of information gathering by hermit crabs. London: Chapman and Hall.
Elwood, R. W., Mclean, A., Webb, L. 1979. The development of shell preferences by the hermit crab Pagurus bernhardus. Animal Behavior 27, 940-946.
Forest, J., Laurent, M. de S., McLaughlin, P. A. and Lemaitre, R. 2000. The marine fauna of New Zealand: Paguridea (Decapoda: Anomura) exclusive of the Lithodidae. Wellington, New Zealand: National Institute of Water and Atmospheric Research.
Ingle, R. W. and Christiansen, M. E. 2004. Lobsters, mud shrimps and anomuran crabs. (Kermack, D. M., Barnes, R. S. K., Crothers, J. H., and Hayward, P. J. Eds.) Preston Montford, Shrewsbury, UK: The Field Studies Council.
Lancaster, I. 1988. Pagurus bernhardus (L.) – an introduction to the natural history of hermit crabs. Field Studies 7, 189-238.
Malay, M.C.(M).D. & Paulay, G., 2009. Peripatric speciation drives diversification and distributional pattern of reef hermit crabs (Decapoda: Diogenidae: Calcinus). Evolution 64(3), 634–62.
Orrell T. (custodian) (2016). ITIS Regional: The Integrated Taxonomic Information System (version Oct 2015). In: Species 2000 & ITIS Catalogue of Life, 26th May 2016 (Roskov Y., Abucay L., Orrell T., Nicolson D., Kunze T., Flann C., Bailly N., Kirk P., Bourgoin T., DeWalt R.E., Decock W., De Wever A., eds). Digital resource at www.catalogueoflife.org/col. Species 2000: Naturalis, Leiden, the Netherlands.
Pechenik, J. A. & Lewis, S. 2000. Avoidance of drilled gastropod shells by the hermit crab, Pagurus longicarpus at Nahant, Massachusetts. J. Exp. Mar. Biol. Ecol. 253, 17-32.
Ruppert, E. E., Fox, R. S. and Barnes, R. D. 2004. Invertebrate zoology: a functional evolutionary approach. (7th Ed). Belmont, CA, USA: Brooks/Cole.
Tirelli, T., Dappiano, M., Maiorana, G., Pessani, D. 2000. Intraspecific relationships of the hermit crab Diogenes pugilator: predation and competition. Hydrobiologia 439, 43-48.
Tudge, C. C. 1995. Hermit crabs of the Great Barrier Reef and coastal Queensland. Brisbane, Australia: School of Marine Science, University of Queensland.
Zann, Leon P. 1995. "Marine Pollution". Our Sea, Our Future – Major findings of the State of the Marine Environment Report for Australia. Australian Government, Dept of Environment, Water, Heritage, and the Arts.
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