Brown Box Crab

Soft bottoms deeper than 18 m, or on rocky faces overlooking soft bottoms.

Geographical Range: Kodiak, Alaska to San Diego, CA

Depth Range: Low intertidal to 547 m. Usually deep. In the Oregon offshore fishery they are mostly caught at 128-165 m depth.

The most distinctive feature of large lithodid crab is the fact that when it pulls its legs under itself and folds its claws across its front, as it often does, a large notch on the carpus of its chelipeds combined with a smaller notch on the carpus of leg 2 lie beside each other, creating a prominent tubelike opening presumably used for breathing. Other features include the fact that its abdomen is completely calcified and its legs are not markedly longer than the carapace. The carapace has tubercles with sharp spines and has no pronounced depression on the posterodorsal surface. It is rounded posterolaterally and does not resemble an equilateral triangle, and it has no lateral extensions which cover its legs when viewed from above. The sides of the walking legs are smooth and fit together tightly when folded, while the dorsal surfaces have tubercles and spines. The merus of the chelipeds has lateral extensions on its inner dorsal margin, and the carpus and propodus curve upward and cover the mouth when they are folded against the body. The largest cheliped has large, blunt white teeth on the cutting surface, while the small chela has small sharp teeth. The rostrum is a sharp upturned spine with more spines near the base, somewhat like that of Rhinolithodes wosnessenskii. Color is red-brown or tan with purplish and white areas. The chelae are tan and mottled red, with white on the dorsal side and orange or red fingers and white tips. Carapace width to 18.5 cm or larger. Maximum size in males is larger than that for females.

How to Distinguish from Similar Species: The tubelike passage through the folded legs provides certain identification. Lopholithodes mandtii looks similar but it is more brightly colored and has blunt bumps rather than bumps with spines on its carapace. Rhinolithodes wosnessenskii and Phyllolithodes papillosus have a deep posterodorsal depression in their carapace. Oedignathus inermis has a soft abdomen.

Biology/Natural History: This species is said to feed by scooping up sediment with its claws, and also to feed on the clams it digs up. Will Duguid reports that in the laboratory adults of this species are strongly attracted to and feeds on brittle stars. They may also feed on urchins such as Strongylocentrotus droebachiensis. It is thought that octopus may be its main predator. It may bury itself in the sediment, except that the front with the foramen is exposed for breathing. The abdomen has knobby plates and Flora and Fairbanks say it is also partly soft and not held very tightly against the thorax. An intrmittent open coast fishery for this species exists off Oregon. This species is said to occur in large aggregations of mixed males and females on soft bottoms. Molting within an aggregation seems to be synchronous, but not synchronized with that of other aggregations. Eggs and larvae of snailfish, especially Careproctus melanurus, the blacktail snailfish, are often found among the gill filaments of these crabs. These may occur in large numbers and even may contribute to collapse of the gills, but usually they do not seem to cause any harm. Off British Columbia this species has a biennial cycle for brooding. Females molt and breed during mid-summer, then brooded their eggs and larvae for 18 months before releasing them as zoeae the second winter or early spring after breeding. Much of this long brooding period was due to the fact that the brooded eggs underwent a 12-month diapause in the gastrula stage. The females released the larvae gradually, averaging a period of 69 days. Brooding females have a mean carapace length of 8.9 cm and a minimum carapace length of 7.5 cm. Females which have been brooding for a number of months and post-breeding females often have extensive overgrowth of polychaete tubeworms, hydrozoans, and small bivalves. The distal legs and much of the underside has a black stain not seen on males or pre-incubation females.