MAJOIDEA Samouelle, 1819

Wicksten, Mary K., 2012, Decapod Crustacea of the Californian and Oregonian Zoogeographic Provinces 3371, Zootaxa 3371, pp. 1-307 : 206-207

SUPERFAMILY MAJOIDEA Samouelle, 1819

Until recently, all of the spider crabs were included in one family, the Majidae . Garth (1958) placed the genera that occur in the entire eastern Pacific Ocean into seven subfamilies, three of which do not occur in the area of coverage. Drach & Guinot (1983) and Hendrickx (1995 c, 1999) elevated the existing eastern Pacific subfamilies (as presented by Garth) to family status, and added another family, the Inachoididae . I have followed this family arrangement in the text that follows. Martin & Davis (2001) listed seven families under the superfamily Majoidea , but did not elevate Garth's subfamily Oregoniinae to family status. Ng et al. (2008) and Ng et al. (2009) considered the family Pisidae to be poorly defined but included it as a subfamily, the Pisinae, of the Epialtidae . In a molecular analysis, Hultgren & Stachowitz (2008) found good support for the family Oregoniidae and Inachidae but a "close phylogenetic association" between the Epialtidae and Pisidae . However, their analysis was limited to only 36 species, mostly common and shallow water inhabitants of the north Pacific. The species of the epialtoids and pisids in the Californian-Oregonian region are distinct in morphology and habitat, and therefore are considered here to be separate families.

The spider crabs are common in California and Oregon, and range from the intertidal zone to the continental slopes. As the common name implies, the body is spider-like in that pereopods 2–5 usually are relatively long in comparison with the body. A rostrum usually is present. The second article of the antennae is well developed, but usually fused with the epistome and often with the front. The orbits usually are incomplete. The chelipeds are slender and agile at least in smaller individuals, and can be used in delicate picking and twisting maneuvers as well as in crushing.

Most species possess hooked setae on the dorsal surface of the body and walking legs at least during part of their life cycle, and can attach food or camouflaging materials to these setae. Well-camouflaged species commonly are called decorator crabs (regardless of their generic classification). The crabs usually attach materials that are flexible and common in their habitat. Attachment is mechanical. The crabs do not secrete bioadhesive materials to the material. Some species store uneaten food on their hooked setae; others usually camouflage themselves with inedible materials ( Wicksten 1993).

Spider crabs usually feed on algae, smaller invertebrates, detritus and dead animals. Species of Loxorhynchus prey on echinoderms as well. The crabs in turn fall prey to the sea otter Enhydra lutris (Linneaeus, 1758) ; large fishes including Anarrichthys ocellatus Ayres, 1855 ; and Scorpaenichthys marmoratus (Ayres, 1854) ; large cancroid crabs and octopuses.

Adult spider crabs reach sexual maturity after a terminal molt, and do not molt or regenerate lost or damaged appendages after reaching maturity. Adult males usually are larger than females. The males' chelipeds often are longer than those of females, and the chelae often are heavier, broader and brightly colored on the interior surface. Adult females have slender chelipeds and rounded bodies. The decorating habit often is lost in adult males.

The definitive work on spider crabs of the eastern Pacific is that of Garth (1958). Extensive synonymies and detailed information on anatomy and distribution can be found in this work.