Recent research has shed new light on the evolution of crabs.
Hermit crabs do not have crab-like bodies, but king crabs and (to a lesser extent) squat lobsters do. Yet, they are all members of the infraorder Anomura. Scientists have been debating the question of how a crustacean evolves into a crab-like form from a non-crab-like form (a process known to the scientists as carcinisation) for some time. Research published in a recent issue of the journal Systematic Biology by Ling Ming Tsang and coauthors has shed further light on the controversy, revealing that the crab-like body has evolved at least twice from separate symmetrical hermit crab ancestors.
The authors used 2664 base pairs of five nuclear protein-coding gene sequences (arginine kinase, enolase, glyceraldehyde 3-phosphate dehydrogenase, sodium potassium ATPase -subunit, and phosphoenolpyruvate carboxykinase) to construct a phylogenetic hypothesis of 46 species of anomurans (spanning 14 of the 17 known families).
They then mapped the four different body forms (symmetrical hermit crab, asymmetrical hermit crab, squat lobster and crab-like) onto the inferred phylogeny.
The authors found that although the anomurans were monophyletic (i.e. evolved from a common ancestor), the crab-like form and the squat lobster-like form have each evolved at least twice within the anomurans.
In each case, the transition is hypothesised to proceed from the long-tailed symmetrical hermit crab through the squat lobster form or the asymmetrical hermit crab form, and finally to the crab-like form. The hermit crabs were also shown to evolve adaptations to living in mollusc shells twice: once in an exclusively deep-water clade and once in the common ancestor of all other asymmetrical hermit crabs.
As the anomurans have been traditionally classified according to body form, the results of this study indicate that the current classification scheme requires significant revision if it is to reflect phylogenetic relationships.
For more information, see the paper: Tsang, LM, T-Y Chan, ST Ahyong and KH Chu (2011) Hermit to king, or hermit to all: multiple transitions to crab-like forms from hermit crab ancestors. Systematic Biology 60, pp. 616–629.
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