E-Fauna BC: Electronic Atlas of the Wildlife of British Columbia

Fabia subquadrata (Dana, 1851)
Grooved Mussel Crab
Family: Pinnotheridae

Species account author: Josephine Hart.
Extracted from Crabs and their relatives of British Columbia.
Photo of species

© Aaron Baldwin  Email the photographer   (Photo ID #2227)

E-Fauna BC Static Map
Distribution of Fabia subquadrata in British Columbia
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Species Information

Carapace smooth, soft or hard, depending on stage of development; immatures are soft, with membranous, unpigmented subquadrate carapace without longitudinal grooves posterior to eye orbits; ovigerous females are similar but do have the longitudinal grooves and a transverse groove between the eyes. Chelipeds and walking legs are subcylindrical and sparsely setose; dactyls have curved claws. Males and females metamorphose to a hard stage very unlike the soft stages. The females integument is calcified and the carapace smooth and subpentagonal with anterior margins densely pubescent, covering sulci and front projections. Chelipeds with patches of dense pubescence and tips of fingers crossed. Walking legs flattened, margins pubescent, with long rows of plumose swimming setae.


Carapace: male 7 x 7.3 mm, female 17 x 22 mm.


Soft stages: carapace translucent creamy white with frontial area slightly more opaque. In adult females the orange yolks of eggs in the ovaries may be seen through the integument. Cheliped opaque creamy white with yellow tinged fingers. Walking legs creamy white with yellow setae. Eyestalk translucent white; cornea scarlet with gold flecks. Hard stage: carapace opaque white with anterior areas mainly tan, and scarlet reticulations on cardiac and branchial areas. Pubescence a light tan. Chelipeds white with yellow and pale orange; carpus and fingers a bright orange with pale grey pubescence. Walking legs yellow with orange; claw brown and setae grey. Antennule, antennae and eyestalk orange; cornea black.


Commensal or parasitic in bivalve molluscs; only one crab per host. Hard stages also free swimming with plankton.



Akutan Pass, Alaska, to San Diego, California; intertidal to 220 m.
Distribution In British Columbia

Common in the mussel Modiolus modiolus, intertidally and dredged, but also found in Mytilus californianus, M. edulis, Tresus capax, Mya arenaria, Astarte compacta, Cardita ventricosa, Crenella columbia, and Kellia spp.


Pearce (1966) studied the biology of Fabia subquadrata from the waters of the San Juan Archipelago, Washington. The juvenile crabs settle to the bottom, after a free-swimming, plankton existence, and enter a molluscan host. There they develop a membranous integument and moult at least seven times. Then a dramatic change takes place when the next moult produces the hard stage described above. These crabs can swim well and apparently swarm and copulate in the early summer. The males are believed to die soon after but the females enter new hosts, moult and produce the soft stage again and lay eggs which are fertilized by the stored sperm. The hard stages were not recognized as the immature females and the males of Fabia subquadrata until 1928, more than 75 years after the mature female had been described. Because these crabs are given protection, take food and cause injury the gills of their host their sharp claws, they should be considered parasitic rather than harmless commensals, as some other pea crabs are.

Status Information

Origin StatusProvincial StatusBC List
(Red Blue List)
BC Ministry of Environment: BC Species and Ecosystems Explorer--the authoritative source for conservation information in British Columbia.

Additional Photo Sources

General References

Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC: Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. [Accessed: 2024-07-23 1:05:47 AM]
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