This lack of cheliped mobility is one of the features distinguishing members of the Parthenopidae from the spider crabs of the Majidae, which have a broadly similar superficial appearance (Ng & McLay, 2003). Other distinct features of the family include the fusion of the third to fifth segments of the male abdomen* (Tan & Ng, 2007); also, while female majids have a high-domed abdomen that forms an entirely enclosed brood chamber for her eggs, the parthenopid female's abdomen does not entirely seal the eggs away from the outside world. The similar adult appearance of Parthenopidae and Majidae, with their triangular bodies and pointed snouts, lead most early authors to regard them as closely related, but the similarities are now thought to be convergent. The larvae of parthenopids are more similar to those of other families than majids (Yang, 1971), while phylogenetic studies do not support their association (Brösing, 2008).
The subfamilial classification of Parthenopidae was reviewed by Tan & Ng (2007) who recognised only two subfamilies of elbow crabs, the Parthenopinae and Daldorfiinae (earlier authors recognised more - some have been moved to other families, others have been synonymised). The two subfamilies are distinguished by only a single character, the relative length of the
antennal segments, and a more formal analysis is still required to test their distinction. A separate subfamily had previously been recognised for the very distinctive Indo-Pacific species
Lambrachaeus ramifer which has the front of the carapace extended forward into a long neck (Ng & McLay, 2003), but Tan & Ng (2007) placed this species in Parthenopinae, noting that it had been separated on the basis of its own peculiar autapomorphies rather than by lack of the features of other subfamilies.
*If you don't know where to find the abdomen of a crab, then look at the underside of one the next time you're able to. The much reduced abdomen is turned forwards and held on the underside of the cephalothorax. In males, it is a small, narrow segmented strip. In females, it is much larger and broader, and is used to hold her eggs.
REFERENCESBrösing, A. 2008. A reconstruction of an evolutionary scenario for the Brachyura (Crustacea) in the context of the Cretaceous-Tertiary boundary.
Crustaceana 81 (3): 271-287.
Ng, P. K. L., & C. L. McLay. 2003. On the systematic position of
Lambrachaeus Alcock, 1895 (Brachyura, Parthenopidae).
Crustaceana 76 (8): 897-915.
Tan, S. H., & P. K. L. Ng. 2007. Descriptions of new genera from the subfamily Parthenopinae (Crustacea: Decapoda: Brachyura: Parthenopidae).
Raffles Bulletin of Zoology Supplement 16: 95-119.
Yang, W. T. 1971. The larval and postlarval development of
Parthenope serrata reared in the laboratory and the systematic position of the Parthenopinae (Crustacea, Brachyura).
Biological Bulletin 140: 166-189.
Lift up one arm, and bend your elbow. Reach with your fingers to a point on your back, between your shoulder-blades. Scratch. Not only will that work wonders for any annoying tingle that you might have been feeling, but you have just demonstrated your superior flexibility to an elbow crab.
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