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Portunion conformis, an entoniscid isopod, infects the common shore crabs Hemigrapsus nudus and H. oregonensis along the west coast of North America (Kuris et al., 1980). Crabs are infected by a free-swimming larva that enters through the host’s branchial (gill) chamber. Female parasites mature and develop huge brood pouches that hold eggs and larvae. Male P. conformis are dwarf and live permanently on the surface of their mate (Veillet, 1945). Ovigerous (egg-producing) females release larvae back out of the crab’s branchial chamber, which then attach to a putative intermediate host (assumed to be a pelagic copepod) before returning to infect a new crab (Veillet, 1945; Kuris et al., 1980).
Infection by P. conformis causes female crabs to eventually become sterile, as their ovaries are destroyed. Male hosts are never castrated, but sometimes will start to look like female crabs with widened abdomens and more slender chelae (Kuris et al., 1980).