Morphology

Like all malacostracan crustaceans, the body of a hermit crab consists of twenty-one somites, or segments (Ruppert et al. 2004). This count includes the acron; the preoral, anterior-most segment, and the telson or tail segment, neither of which are considered true somites due to different embryological origins and the lack of carried appendages (Forest et al. 2000). This arrangement is comprised of five cephalic or head somites (in addition to the acron), eight thoracic somites and six abdominal somites before the telson (Ruppert et al. 2004). In malacostracans the five cephalic somites have fused with the three most anterior thoracic somites to form the cephalothorax (Forest et al. 2000). This fusion of segments is called tagmosis and the cephalothorax is then considered a tagma (Ruppert et al. 2004). The entire thorax of malacostracans is known as the pereon and the abdomen is referred to as the pleon (Ruppert et al. 2004).

Morphology of Dardanus megistos. Illustration from field observation, not intended to convey fine morphological detail. Illustration by Storm Martin 2012, with reference to Matthews 1953, Asakura and Hirayama 2002, Ruppert et al. 2004.

The 'keyboard crustacean' shows each body somite with its associated appendages. Concept adapted with modification from University of Bristol 2004

Some malacostracans have a carapace and this is particularly well developed in the decapods where it covers the entirety of both the cephalothorax and pereon (Ruppert et al. 2004). The carapace is formed by the fusion of the tergites, the dorsal exoskeletal plate of somites (Forest et al. 2000) (see exoskeleton) and provides additional protection, the most vulnerable points of an arthropod exoskeleton being the joints. In hermit crabs, which are adapted to shelter in gastropod shells or other protective coverings, hardening of the carapace is reduced (Forest et al. 2000), allowing the flexibility needed to withdraw into the shell easily. The anterior dorsal region of the carapace remains well calcified and is termed the shield (Forest et al. 2000). A diagnostic character of diogenid hermit crabs is a ‘v’-shaped cervical groove of the shield (Tudge 1995). Where the carapace of a malacostracan protrudes anteriorly it is termed the rostrum (Ruppert et al. 2004). This is most obvious in shrimps, prawns and lobsters but is very reduced in most hermit crabs, including Dardanus megistos. The hermits of Diogenidae, which includes Dardanus, have a short spine positioned between the eyestalks (Tudge 1995). This is part of the acron and so is not the rostrum, which is a protrusion of the carapace.

From left to right: antenna, antennule and eyestalk of Dardanus megistos. The eyestalks are of the acron and are not true appendages. The antennae are uniramous and the antennules are biramous, the exopod holding the aesthetascs, chemosensory setae. This illustration is not intended to convey fine morphological detail. Illustration by Storm Martin 2012 with reference to Forest et al. 2000

The reliance on a protective shelter is an important factor in the evolutionary history of hermit crabs, resulting in a distinct deviation from the typical decapod morphology. The abdomen is asymmetrical, soft and curled, adapted to coiling into the spiral of a gastropod shell. The telson is relatively small in hermit crabs and adapted to grasp onto the internal spiral of the host shell (Ingle and Christiansen 2004). Attempting to pull a hermit crab from its shell almost always results in the crab being torn in half (Ingle and Christiansen 2004), a testament to both its reliance upon its shelter and the strength to which its well adapted abdomen can hold on. The abdomen of all asymmetrical hermit crabs coils to the right and hence is better adapted to right-handed or dextril gastropod shells, though the less common left-handed shells are also utilised (Ruppert et al. 2004).

The ancestral crustacean is commonly hypothesised to have been composed of a linear series of repeating somites, each with a pair of jointed appendages (Ruppert et al. 2004). Some taxa have secondarily lost appendages from certain somites (Ruppert et al. 2004). Arthropod appendages are classed as uniramous or biramous, the former consisting of a single series of segments called a ramus and the latter branching into two such series or rami (Ruppert et al. 2004). Biramous appendages branch after the protopod, the two rami then considered as the exopod (external or lateral) and endopod (internal or medial). Additionally, epipods may attach to the protopod (Ruppert et al. 2004). The acron, the anterior most segment, is limbless but carries the eyes. In hermit crabs the cornea is supported by the ocular peduncle which is attached to the acron via the ocular acicle (Forest et al. 2000). The eyestalks are relatively soft and not highly mobile. The ocular peduncle of D. megistos is dark purplish red with a near black corneal ring and a thin white margin.

After the acron, the anterior most of the five cephalic somites holds the antennules, which are biramous and comprised of a three-segmented peduncle, the most distal of which holds the branched flagella (Forest et al. 2000). The antennules of D. megistos are of a similar length to the eyestalks but slightly longer. The peduncle is an orange-red, becoming more orange towards the flagella. The larger of the flagella, the exopod, is lined with aesthetascs, fine chemosensory setae (Forest et al. 2000).

Representation of second or third pereopod of Dardanus megistos. These are the primary appendages used for locomotion, ending with a strong point and so are not chelate. This illustration is not intended to convey fine morphological detail, but notice increasing density of setae distally and greater coverage of white spots dorsally. Illustration by Storm Martin 2012

Following the antennules the much longer antennae extend from the second cephalic somite. The antennae peduncle is similarly coloured to that of the antennules in D. megistos, with a light grey flagella. Unlike the antennules, the antennae are uniramous. The last three cephalic segments, together with the three most anterior thoracic segments (all part of the cephalothorax), house the external mouthparts. From anterior to posterior these are the mandibles, maxillules, maxillae and then the three pairs of thoracic maxillipeds. These are all biramous except for the mandibles and maxillules (Forest et al. 2004).

The remaining five somites of the pereon, each carry a pair of pereopods, or walking legs and hence the name decapods, meaning ten legs. The anterior most pereopods are referred to as the chelipeds or more commonly, the pincers. The Diogenidae are distinguishable from the other large hermit crab family, Paguridae, and most other hermit crabs, in that they are left handed. That is to say the left cheliped is larger than the right. This is true of D. megistos and is more pronounced in larger individuals, particularly in males. The chelipeds are the most powerful of the pereopods and are used in defence. Following the chelipeds, the second and third pereopods play the greatest locomotive role. These are not chelate, that is, do not end with a pincer, but instead tapered to a strong point. In hermit crabs, the fourth and particularly the fifth pereopods are reduced, usually remaining within the confines of the gastropod shell and hence are not used for walking. These appendages do however become important when the hermit crab attempts to right itself, providing anchorage within the shell. Further, the fifth pereopod has become specialised as a gill cleaning appendage, often resting within the gill chamber (Bauer 1981). For this purpose the fifth pereopod is chelate. The pereopods of D. megistos are red to a yellow-orange, covered with the characteristic black-lined white spots, especially along the dorsal portions of the limb. The first three pereopod pairs are covered with long setae, more densely so moving distally along the limb and giving a hairy appearance. The fourth and fifth pereopods are very sparsely covered with these long setae.

In most crustacean groups the appendages have been lost from the abdomen, but these are retained in malacostracans and being of the pleon, are termed pleopods. Pleopods are the ‘swimming legs’ of shrimps and in many malacostracans brood the eggs. Being housed in a shell, hermit crabs have less of a need for pleopods and consequently they are greatly reduced, particularly in males, retaining their egg brooding role in females. As the abdomen is coiled to the right only the left pleopods are retained (Poore 2004). Hermit crabs of Dardanus have four biramous pleopods, specifically pleopods 2-5 (Poore 2004). Following the pleopods are the uropods, biramous appendages of the last abdominal segment. In Dardanus these are reduced, the left being larger than the right (Poore 2004).