The subjects of today's post, the Majidae, commonly go by the names of spider crabs or decorator crabs. The first of those names might sound like some people's ultimate nightmare, but I doubt that anyone could complain about the latter. Majids are characterised by having a carapace longer than wide, often with a covering of bristly hooked setae and relatively long legs (hence the name 'spider crab'). They get their alternate name of 'decorator crab' from the habit of many species of using the aforementioned hooked setae to attach algae and other bits of organic matter to themselves. The primary purpose of this adornment is to provide camouflage, and a decorated spider crab can be inordinately difficult to see when not moving. A secondary use of the crab's organic covering, however, is that they will also feed on material from it in times of need*.
*It is perhaps fortunate for Gaga that the question was never raised of her doing the same.
The circumscription of the Majidae is more than a little fluid: at times, it has been used to include all the spider crabs of the superfamily Majoidea, but the more common practice these days is to divide the majoids between a number of families. Unfortunately, authors have disagreed about what those families should be. Ng et al. (2008) united the subfamilies Majinae and Mithracinae within the Majidae on the basis of shared features such as a well-developed protective orbit around the eyestalk. However, a direct relationship between majines and mithracines is not currently supported by molecular (Hultgren & Stachowicz 2008) or larval (Marques & Pohle 1998) data, though both these latter data sources are themselves limited by the relatively small number of studied taxa. Two smaller subfamilies included by Ng et al. (2008) in the Majidae, the Planoterginae and the isolated species Eurynolambrus australis, have not yet been analysed molecularly. Eurynolambrus australis is a particularly unusual little majid, so much so that it looks more like a parthenopid than a majid. Eurynolambrus also lacks hooked setae and so does not decorate itself; instead, it relies for disguise on its resemblance in colour to the coralline algae amongst which it lives (and on which it primarily feeds, though it is omnivorous overall—Woods & McLay 1996). Ng et al. placed it in the Majidae nevertheless owing to the resemblance of its larval stages to those of Majinae.
The two main subfamilies, the Majinae and Mithracinae, can be distinguished by the development of the orbit around the eyestalk. In the Mithracinae, the orbit is broadly expanded both above and below (with the lower margin formed from an expansion of the basal antennal segment), almost entirely enclosing the eyestalk and giving the front of the carapace a distinctly broad appearance in dorsal view. In the Majinae, the basal antennal segment is not expanded to form an underside to the orbit, so the eyestalks are contained from above only (Davie 2002). The Majinae are most diverse in the Indo-West Pacific, with only a handful of genera found outside this region. Some majines are quite large: the Australian Leptomithrax gaimardii reaches a leg-span of about 70 cm. The Mithracinae are more pantropical inhabitants of shallow water reefs.
Davie, P. J. F. 2002. Zoological Catalogue of Australia vol. 19.3B. Crustacea: Malacostraca: Eucarida (part 2): Decapoda—Anomura, Brachyura. CSIRO Publishing: Collingwood (Australia).
Hultgren, K. M., & J. J. Stachowicz. 2008. Molecular phylogeny of the brachyuran crab superfamily Majoidea indicates close congruence with trees based on larval morphology. Molecular Phylogenetics and Evolution 48: 986-996.
Marques, F., & G. Pohle. 1998. The use of structural reduction in phylogenetic reconstruction of decapods and a phylogenetic hypothesis for 15 genera of Majidae: testing previous larval hypotheses and assumptions. Invertebrate Reproduction and Development 33 (2-3): 241-262.
Ng, P. K. L., D. Guinot & P. J. F. Davie. 2008. Systema brachyurorum: part I. An annotated checklist of extant brachyuran crabs of the world. Raffles Bulletin of Zoology 17: 1-286.
Woods, C. M. C., & C. L. McLay. 1996. Diet and cryptic colouration of the crab Eurynolambrus australis (Brachyura: Majidae) at Kaikoura, New Zealand. Crustacean Research 25: 34-43.