Systematics

From a phylogenetic perspective the term ‘crab’ is misleading, describing a general body shape rather than a taxonomic group. Hermit crabs then, are not considered true crabs. Hermit crabs, Paguroidea, are a superfamily of the decapod infraorder Anomura. The true crabs, infraorder Brachyura, are considered to be the sister clade to the Anomura, with strong molecular and morphological support (Ahyong and O’Meally 2004). The differences separating the hermit crabs from the true crabs are obvious; hermit crabs have a large abdomen whereas brachyurans have a very reduced abdomen tucked beneath the cephalothorax and instead of the characteristic sidewise walk of crabs, hermit crabs walk forwards. Also, the fourth and fifth pereopods are reduced in anomurans (Saxena 2005).

Traditional groupings within the Anomura have been based largely on morphology. In addition to the Paguroidea (hermit crabs), extant anomurans are classified into six further superfamilies. The anomurans are considered the most morphologically diverse group within the decapods (McLaughlin 2010), but all can be described with one of the following body plans; crab-like, squat lobster, asymmetrical hermit crab and symmetrical hermit crab (Tsang et al. 2011). Which of these represents the ancestral Anomuran condition and how many times each generalised body plan arose has been the topic of considerable debate (Tsang et al. 2011).

The debate has largely focused around the king crabs, again not a true Brachyuran crab, currently superfamily Lithodoidea of Anomura (Tsang et al. 2011). Central to the debate is the long standing ‘hermit-king’ hypothesis, which argues that rather than being considered a sister group to the hermit crabs, king crabs evolved from within the Paguroidea, more specifically from an ancestor with an asymmetrical abdomen (Tsang et al. 2011). Recent genetic studies have strongly supported this claim (Tsang et al 2011) but have been opposed by some larval and morphological studies (McLaughlin et al. 2004). McLaughlin et al. (2004) argues for the reverse, that hermit crabs evolved from a king crab ancestor through loss of calcification.

The phylogeny amongst the hermit crab superfamily Paguroidea itself has also been disputed. Typically, seven families are considered, the largest of which is Paguridae, followed by Diogenidae, which is the best represented across the Great Barrier Reef (Tudge 1991) and contains the Dardanus genus. Diogenids are the left handed hermit crabs, having a larger left cheliped and are further characterised by a cervical groove in the carapace shield (Fraaije et al. 2012). The symmetrical hermit crabs, family Pylochelidae, are often considered the basal Paguroid, an intermediate between the ancestral anomuran and the highly modified abdomen of the other asymmetrical hermits (Tsang et al. 2011). Pylochelids, unlike other hermit crabs, shelter their straight, more calcified bodies in hollowed wood or polychaete (tube worm) tubes (Gherardi 1996). Molecular studies have increasingly provided support for the possibility that the asymmetrical body plan shared by the other six Paguroid families may have arose more than once (Ahyong and O’Meally 2004).

A recent molecular based study by Tsang et al. (2011) has, however, produced further interesting developments. This study concluded that hermit crabs, both symmetrical and asymmetrical, indeed arose only once and that the ancestral condition was symmetrical, as per previous hypotheses (Tsang et al. 2011). However, it was also concluded that almost all other anomurans, except for the mole crabs (superfamily Hippoidea), evolved from a symmetric hermit crab ancestor and furthermore, that within this clade, crab, squat lobster and asymmetrical hermit crab body plans have all arisen independently at least twice (Tsang et al. 2011). This includes the evolution of king crabs from Paguridea hermit crabs (Tsang et al. 2011), that is, support for the ‘hermit-king’ hypothesis, or perhaps more appropriately, the ‘hermit-everything else’ hypothesis.

The findings of Tsang et al. (2011) call for a major reshuffling of the anomuran phylogeny. In particular Paguroidea is now considered paraphyletic, with an assortment of what were traditionally the other anomuran superfamilies nestled within it. Similarly, Coenobitidae, the family of land hermits, have been suggested to have evolved from within Diogenidae, the family including Dardanus. Whether or not the findings of Tsang et al. (2011) are accepted by the scientific community, in whole or part, this recent research clearly demonstrates the uncertainty of the anomuran phylogeny. Dardanus megistos can accordingly be expected to change family and superfamily classification within the next decade.

Recent proposed phylogeny of Anomura based on DNA analysis by Tsang et al. (2011). This phylogeny suggests that aside from Hipppoidea, all other anomurans evolved from a symmetrical hermit crab body form. Further, the traditional grouping of all hermit crabs into superfamily Paguroidea no longer holds. Both Pylochelidae and Diogenidae were concluded to be paraphyletic by Tsang et al. (2011), though this diagram does not show sufficient detail to illustrate the latter. Storm Martin 2012, with reference to Tsang et al. 2011.