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Yellow-footed Hermit Crab

Rebecca Everett 2016

Summary

Clibanarius virescens, or the yellow-footed hermit crab, (Krauss, 1843) is an anomuran species of decapod in the family diogenidae. Like other species of hermit crab, they reside in gastropod shells to protect their soft abdomen. Hermit crabs have an exoskeleton covering their exposed legs, thorax, head and chelae (Ruppert et al., 2004). Given their asymmetrical bodies, hermit crabs are not considered to be true crabs (Jones & Morgan 2002). Crustacean body plans are highly organised and consist of specialised appendages and systems for respiration. Other systems included within their bauplan include nervous, circulatory, reproductive and digestive systems (Ruppert et al. 2004). Hermit crabs can live anywhere from the high tide level to depths of the ocean (Tudge 1995). Clibanarius virescens typically inhabit the soft mud of lower intertidal marine environments in the Indo-West Pacific and South Africa (Wait & Schoeman 2012). The species is under no imminent threat, and is not currently assessed by the IUCN (IUCN, 2016).

Physical Description

Overview

Hermit crab morphology, including that of Clibanarius virescens has been modified to allow for the superfamily of Paguroidea to become wholly reliant upon abandoned mollusc shells (Ruppert et al., 2004). Individuals’ favour inhabiting gastropod shells to shelter their large, soft, spirally twisted abdomen (Healy & Yaldwyn 1970). The shells serve as a defense mechanism protecting them from predation, competition and environmental stresses (Reddy & Biseswar 1993). When threatened, individuals retreat into their shell until a time where they sense they are no longer in danger (Healy & Yaldwyn 1970). In order for their paguroidean bodies to successfully adapt to the gastropod shell environment, hermit crabs became asymmetrical, and are thus not considered to be true crabs (Ruppert et al. 2004). Right-handed (dextral) shells are favoured due to the directional twist of the abdomen, however left-handed shells can also be inhabited (Ruppert et al. 2004; Imafuku & Ikeda 2014).

A chitin and protein composed exoskeleton covers their anterior, provides them with protection from both predators and desiccation (Warner 1977). The Clibanarius virescens exoskeleton is hardened by calcium carbonate and is periodically shed, allowing an individual to grow (Hickman et al. 2014). The olive coloured carapace is composed of the cephalothorax (the head and thorax), and the exoskeleton. The defining features of Clibanarius virescens within the carapace include two pairs of antennae: long blue biramous antennae attached under the eyestalks, and shorter antennules between the organisms eye stalks. Their compound eyes are on separate eyestalks, have terminal cornea are encircled by a white ring (Imafuku & Ikeda 2014; Tudge 1995). Located on the carapace are mandibles used in biting, as well as the maxillipeds, which are biramous mouthparts, used in transporting food to the mouth and in grooming (Warner 1977). The exoskeleton is not present around their abdomen (Warner 1977).

Clibanarius virescens can reach a maximum carapace length of 40 millimetres (Morgan 2002; Jones & Morgan 2002). As seen in other Decapoda, individuals have five sets of pereiopods attached to the abdomen: their chelipeds (claws), first pair of walking legs, second pair of walking legs, and two pleopods located on the left side of the body. The pleopods are the modified fourth and fifth pair of walking legs of other Decapoda, and are usually located on the potion of the body inside the shell (Tudge 1995). Located between the forth and fifth pereiopods are the gills (Hickman et al. 2014). Clibanarius virescens can be identified through its characteristic yellow bands on the ends of their pair of dark green chelipeds (which are also covered in white spines), second and third pereiopods, and on the distal end of their antennulae (Tudge 1995; Davie 2013). The genus Clibanarius is classified within the family Diogenidae given the sub equal chelipeds, separated eyestalks and terminal cornea, as seen in figure 1 (Tudge 1995). The appendages will be examined in greater detail in the following section.

Their uropod is responsible for securing the hermit crab to the inside of the gastropod shell, and is the feature that distinguishes hermit crabs from brachyurans (Healy & Yaldwyn 1970). The uropod is composed of two sides – the left and right. The left appendage is larger than that of the right in adaption to the curvature of the shells they inhabit, and both appendages are composed of grip surfaces adhering them to the inside of the shell (Healy & Yaldwyn 1970). The telson and anus is located at the anterior of the hermit crab, and is responsible for excretion (Jones & Morgan 2002).

Pronounced sexual dimorphism is exhibited in Clibanarius virescens. Reddy & Biseswar (1993) found that males display larger carapace lengths, cheliped size and were larger in mass. The difference in size may be due to males having a faster growth rate than female individuals, and is particularly prevalent in environments with an abundance of large shells available (Wada 1999; Hazlett 1981; Contreras-Garduño & Córdoba-Aguilar 2006). Females may not be capable of growing as large as males due to females redirecting their energy towards egg production (Reddy & Biseswar 1993). Hazlett (1981) states that larger females are less successful at producing eggs compared to smaller females, due to the pressure of locating a shell large enough to store her eggs.

Figure 1

Detailed focus on appendages

Hermit crabs have five pairs of modified pereiopods. These appendages have been adapted to living within a shell, and are now exhibited as chelipeds (claws), two pairs of long walking legs and two pairs of reduced walking legs hidden within their shell. A thorough exploration was completed in an effort to explain Clibanarius virescens’ appendages including their chelipeds, pereiopods, pleopods, antennulae and antennae.

The first pereiopods are chelipeds, and are usually large in size, as illustrated in figure 2. Similarly sized pereiopods allow the claws to open horizontally in front of the individual (Jones & Morgan 2002). Chelipeds are appendages with multiple purposes. Feeding is a major function, through using their claw to clasp onto food (Warner 1977) Additionally, when an individual is inhabiting a new shell, they can use one or both of their chelipeds to alter the aperture of the shell by using the cheliped as an operculum (Ruppert et al. 2004). By altering the aperture, the individual will become better fixed to the shell. Spinous teeth and fine hairs, or setae cover the chelipeds, and assist the individual in sensing their surrounding environment, functioning as mechanosensors and chemosensors with particular sensitivity to calcium (Mesce 1993). Given hermit crabs inhabit calcium carbonate gastropod shells; it is thought that they use their setae to distinguish gastropod shells from objects such as rocks (Mesce 1993).

The second and third sets of olive to dark brown biramous pereiopods are much longer, strong, and are responsible for allowing movement along substrate (Healy & Yaldwyn 1970). Biramous appendages are the ancestral form, and have two branches (Hickman et al. 2014). The second and third pairs of pereiopods are also covered in setae that have been adapted for swimming (Schram & von Vaupel Klein 2012). These pereiopod pairs are composed of five segments: the ischium, the merus, the carpus, the propodus and the dactylus (figure 3) (Tudge 1995). The ischium is the innermost segment usually hidden within the shell, and is the joint connecting the appendage to the thorax (Chapple 2012). The propodus begins to demonstrate the characteristic yellowish white colour the species is named for (Chapple 2012). Dactylar are not present throughout the entirety of the Clibanarius genus, however are present on the distal ends of Clibanarius virescens’ second and third pairs of pereiopods, seen in figure 4 (Tudge 1995; Morgan 2002). Dactylar are yellowish-white in colour and are useful for gripping onto irregularly shaped surfaces. During energy events such as currents or waves, dactylar are an advantage (Warner 1977; Chapple 2002; Watling & Thiel 2013).

The pereiopods (forth and fifth set of walking legs) are severely reduced in comparison to other decapods (figure 5). They are located on the thorax inside the shell. The fourth pair of pereiopods is used to assist the organism with moving in and out of its shell (Chapple 2002). The fifth pair is essential to the maintenance of cleaning of the gills and removing excrement from the shell. The fifth pereiopods move anteriorly after grooming, where they are met by the maxillipeds. The maxillipeds remove dirt and faeces from the setae (Chapple 2002).

Females retain pleopods as they are used to carry their eggs, however they are lost in male individuals (Ruppert et al. 2004; Imafuku & Ikeda 2014). By inhabiting the dextral shells, the right side of the body of the hermit crab is compressed into the inner of the shell (Imafuku & Ikeda 2014). The compression of the right side of the body may explain why pleopods may only occur on the left side of the organism. As these pleopods and associated eggs are located on the left side of the body, the abdomen is pressed to the right side of the gastropod shell (Imafuku & Ikeda 2014). The right side of the body is compressed into the inner of the shell, and therefore there may not allow for the growth of appendages (Imafuku & Ikeda 2014). Given pleopods are only present in females, there is a subsequent skew of the uropod not seen in male individuals (Imafuku & Ikeda 2014).

The antennulae (first antennae in figure 6) are used for smelling and tasting their environment (Waldrop et al. 2014). Individuals capture the olfactory cues in the air by ‘flicking’ their antennules through the water (Waldrop et al. 2014). By flicking their antennules, connective flows are generated around their antennules. As fluid moves across their antennules, fluid sticks to their aesthetasc hairs, were the olfactory cues could then be interpreted (figure 7) (Waldrop et al. 2014). The long blue antennae (figure 8) are used for touching the surrounding environment, and locating objects in the organism’s path (Healy & Yaldwyn 1970). Males often use their antennae when looking for a mate to detect pheromones released by female individuals (Ruppert et al. 2004). An additional purpose of antennae is using them to identify if a gastropod shell of interest is inhabited. This is achieved by touching the shell with their antennae to elicit a response from any individual inhabiting the shell. If there is no response, the individual knows that the shell is available to inhabit (Healy & Yaldwyn 1970).

Figure 2

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Figure 6

Figure 7

Figure 8

Ecology

The genus Clibanarius is primarily comprised of algal feeders, however they have also been observed to be opportunistic omnivorous predators and scavenger feeders (Benvenuto et al. 2003). They are often found in intertidal seagrass beds, mangroves coral reefs and other sandy substrates feasting on detritus, smaller invertebrates and animal tissues (Linnean Society of New South Wales 1906; Tudge 1995; Jones & Morgan 2002; Benvenuto et al. 2003; Sampaino Sant’ Anna et al. 2009; World Register of Marine Species 2016). Due to their small size, they can forage around the smallest areas of live rock in search for food and mates (Reddy & Biseswar 1993). They congregate in large numbers taking refuge under large rocks at low tide (Tudge 1995). Animals that have the power to crush the crab’s shell predate upon hermit crabs, with predators including fish, true crabs, octopus and birds (McGuire & Williams 2010). Clibanarius virescens play an important role within the ecosystem as, by being scavengers, they are recycling organic matter and energy back into the system (Jones & Morgan 2002).

Life History and Behaviour

Feeding

Development and Growth

Like other crustaceans, hermit crabs including Clibanarius virescens grow by moulting, a process controlled by the hormone ecdysone (Hickman et al. 2014). Once moulted, the crabs with their soft bodies are usually vulnerable to predators, however, this is not a problem faced by Paguroidea, as they can remain within there shell throughout the process (Healy & Yaldwyn 1970; Ruppert et al. 2004). Paguroidea do face a unique complication to the growth process due to the subsequent need to locate a larger shell to house them. Finding a new shell can be troublesome, as they rely upon shells that have already been discarded. In order to locate a new shell, they probe each shell they encounter with their antennae to test for any disturbance (Healy & Yaldwyn 1970). A shell will only be considered to be suitable once the individual deems its surface texture, weight, internal and external shape and aperture as appropriate (Healy & Yaldwyn 1970). When the individual is satisfied, it will align itself with the empty shell, quickly removes itself from the old shell, and inserts itself into the new shell (Healy & Yaldwyn 1970). The process of changing shells is extremely rapid, as the individuals are highly vulnerable to predators throughout the process (Ruppert et al. 2004). The shell selection behaviour is not a learned behaviour, and instead is ingrained into them from birth (Healy & Yaldwyn 1970).

Predominantly active at high tide, Clibanarius is known for travelling hundreds of metres per day, possibly providing individuals of this genus exposure to shells and other resources that are not exploited by other genus’ (Hazlett 1981). Shell size did not have an affect on the individual’s range of locomotion. The type of shell individuals inhabit did appear to have an affect on movement speed, possibly due to the degree of friction between the shell and sediment (Benvenuto et al. 2003). It is believed that the competition for preferred shells could be a limiting factor on Clibanarius virescens population numbers (Ruppert et al. 2004).

Reproduction and Larvae

Being larger than females, male paguroidea demonstrate “aggressive intrasexual behaviour” when female individuals are present (Hazlett 1981). Males rock, stroke and tap on female’s shells before copulation (Hazlett 1981). Larger males could be selected for through an increased mating success or success between male-to-male competition (Hazlett 1981; Wada 1999). Another possibility is female preference for larger males, however research on this theory suggests it is less likely (Contreras-Garduño & Córdoba-Aguilar 2006). Paguroidea are lecithotrophic and undergo metamorphosis in their direct and complete zoea larval development cycle (Anger et al. 2004). When the developing zoea larvae are released, the mother supplies them with a large amount of yolk allowing the larvae to have a large dispersal (Warner 1997). The larvae swim through the beating of their maxillipeds. Zoeal larvae develop spines orientated antero-posteriorly, and thus resemble shrimp (Warner 1977). Paguroidean larvae are commonly distinguished by the spade-shaped telson, which forms the uropod. Much of the larval life is planktonic, and can last for several months while the larvae moult between two and six times before settling (Warner 1977).

Anatomy and Physiology

Overview

Arthropoda are a coelomate phylum, meaning that they are triploblastic – they have an endoderm, mesoderm and ectoderm (Ruppert et al. 2004). The paguroidean body’s anterior is covered by a hard exoskeleton, created by a carapace. The posterior of the anomuran body, the abdomen, is encased in a thin, non-segmented cuticle that provides support to the body, but offers little protection (Healy & Yaldwyn 1970; Warner 1977; Ruppert et al. 2004;). The large, soft abdomen houses the gonads, liver and renal organs. The placement of these organs differs to that of other decapods, where these organs are usually located in the thorax (Healy & Yaldwyn 1970).

Digestive System

Paguroidea have adapted their mandible, maxilliped and maxillae appendages to be suitable for herbivory, crawling and burrowing behaviours (Ruppert et al. 2004). The digestive process begins when the maxillipeds and maxillae have manipulated the food, and has been deposited into the ventrally located mouth. The food travels down the predominantly straight gut by moving into the foregut, including the oesophagus and abdominally located stomach, where it is ground up (Ruppert et al. 2004; Poore 2007). The stomach grinds the food using the chitinous ridges and teeth lining the walls of the stomach (Ruppert et al. 2004; Poore 2007). The food then begins to pass through the endodermal mid-gut, where digestion enzymes are secreted and hydrolysis and hydrolysis product absorption occurs (Ruppert et al. 2004). The hindgut then reclaims water, forms and stores faeces and then excretes waste through the anus. The anus is located at the base of the telson. Waste is removed from the shell through transporting it to the pereiopods, which are cleaned by maxillipeds (Chapple 2002; Ruppert et al. 2004).

Circulatory System

Crustaceans have an open circulatory system; meaning that there are no veins, and blood is not separated from the interstitial fluid (Hickman et al. 2014). Their primary single chambered heart made of striated muscle, which is located in the dorsal region of the abdomen, near the gills (Ruppert et al. 2004; Hickman et al. 2014). Crustaceans’ heart pumps blood throughout the body to expose tissues to the essential gasses. The blood travels from the heart, throughout the body by arteries, circulates through the haemocoel before returning to venous spaces and re-entering the heart (Ruppert et al. 2004). The heart contains at least one opening called an osculum, which allows the haemolymph to re-enter the heart after travelling throughout the body (Ruppert et al. 2004; Hickman et al. 2014).

Respiratory System

Gills located between the forth and fifth pereiopods initiate the Clibanarius virescens’ respiration cycle as water enters the respiratory chamber at the base of the cheliped (Poore 2007). The gills, protected by the soft carapace edge, resemble featherlike projections and are covered by a very thin cuticle (Hickman et al. 2014). While paguroideans can respire for a short period of time in air, it is crucial that they keep their gills damp while out of the water (Poore 2007). Oxygen and carbon dioxide come into contact with the surface of the gills, and diffuse between the organism’s blood and seawater, and are transported around the body by the blood in veins and sinuses (Ruppert et al. 2004; Poore 2007).

Nervous System

Decapodan central nervous systems are very compact compared to those of other arthopods, with a high degree of ganglion fusion (Warner 1977; Hickman et al. 2014). They have a brain located between their eyestalks composed of two supraesophageal ganglia, which are responsible for nerves within the eyes and antennae (Hickman et al. 2014). The supraesophageal ganglia are therefore responsible for registering the setae on the chelae and pereiopods and therefore must interpret smell and taste (Hickman et al. 2014; Waldrop et al. 2014). Additionally, there is the subesophageal ganglion, which is composed of five ganglia responsible for the supply of nerves to the mouth, the appendages and oesophagus (Hickman et al. 2014). A thoracic ganglion is located between the muscles at the bases of pereiopods with nerves in the appendages and associated muscles (Warner 1977).

Reproductive System

Clibanarius virescens’ reproductive system is located within its shell, in the abdomen (Healy & Yaldwyn 1970). Gonads are paired and are located just under the carapace (Warner 1977). Male gonads are larger than female gonads (Ruppert et al. 2004). Paguroideans are gonochoristic, with female individuals having their reproductive gonopores at the base of their third pair of pereiopods (Schram & von Vaupel Klein 2012). When the female’s ovary and male’s androgenic glands release their respective hormones, paguroideans fertilise externally (Warner 1977; Ruppert et al. 2004). Females retain their eggs on her pleopods to protect them from desiccation and predation (Ruppert et al. 2004).

Biogeographic Distribution

Clibanarius virescens live in the coastal intertidal to the shallow sub tidal regions in Australia. Individuals have also been identified in Asia, the east coast of Africa, and Fiji, seen in figure 9 (Poore 2004).

Figure 9

Evolution and Systematics

As seen in figure 10 Clibanarius virescens is a part of the phylum Arthropoda, a phylum defined by the moulting of a segmented chitinous exoskeleton. The phylum is the largest of all phyla, holding 80% of extant species. They are bilaterally symmetrical and triploblastic (Ruppert et al. 2004).

The subphylum Crustacea is distinguished from other Arthropoda subphylum by possessing two pairs of antennae (Ruppert et al. 2004). Many species have a well-defined head and trunk. Species are cephalised, with an exoskeleton and paired, jointed appendages. A head is composed of five segments, and each holds a pair of biramous appendages (Ruppert et al. 2004).

Class Malacostraca is the most successful of the four Crustacea classes. The class consists of orders of animals that have functional body regions, ambulatory legs and free-living larvae (Ruppert et al. 2004). The thorax is compiled of eight segments, while the abdomen is composed of seven segments. Appendages are attached to the thorax and abdomen (Ruppert et al. 2004).

Decapoda species comprise of approximately 25% of all crustacea species. Species are defined as having eight pairs of thoracic appendages. The first three segments of the thorax are fused, forming a cephalothorax, with the associated appendages forming the maxillipeds (Ruppert et al. 2004). There are five pairs of thoracic appendages, with the enlarged first pair of pereiopods forming chelipeds (Ruppert et al. 2004).

The taxa Anomura is comprised of non-true crabs such as hermit crabs, mole crabs and porcelain crabs. The last pair of walking legs are much smaller than the other pereiopods (Jones & Morgan 2002). Anomura are theorised to be the ancestral hermit crabs, and are thought to be the link between shrimp and true crabs (Ruppert et al. 2004). They originally resided within crevices and holes for protection against predators, and eventually, with the modification of the abdomen to fit gastropod shells, became reliant upon the molluscan shells for protection (Ruppert et al. 2004).

Paguroidea are hermit crabs with a coiled, soft abdomen protected within a gastropod shell, dating back to the lower Jurassic (Warner 1977). They are abundant on mudflats, mangrove forests and coral reefs (Jones & Morgan 2002).

The family Diogenidae is paraphyletic, with individuals in both the Coenobitidae and Paguridae evolutionary lines. Diogenidae are the most common hermit crabs within Australia (Jones & Morgan 2002). They are usually bright in colour (McLaughlin 2003).

The defining characteristic of the genus Clibanarius is the similar size of their clawed legs, their green or brown carapace and longitudinal coloured stripes on their legs (Jones 2002).

Figure 10

Conservation and Threats

IUCN Red List of Threatened Species has not yet assessed Clibanarius virescens (IUCN, 2016). Due to the species large worldwide distribution, the species thought to not be threatened.

References

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