Barnacles often attach themselves to Blue Crabs, especially in southern regions. The Striped Barnacle (Balanus amphitrite) and Turtle Barnacle (Chelonibia testudinaria) are found externally; a small goose barnacle, Octolasmus lowei, occurs in the Blue Crab's gill chamber, and the bean-shaped sacculinid barnacle Loxothylacus texanus attaches itself under the abdomen. A parasitic nemertean worm, Carcinonemertes carcinophila, is found on the gills of female crabs; on virgin crabs, it is pinkish, while on breeders it is red. (Gosner 1978)
Feeding experiments carried out by Harding (2003) suggest that the Blue Crab may be an effective predator of the Rapa Whelk (Rapana venosa), a large predatory gastropod from Asia that was discovered in Chesapeake Bay in 1998. Although mature Rapa Whelks are probably too large and well protected to be attacked by Blue Crabs, available data suggest that predation by Blue Crabs on juvenile Rapa Whelks might control whelk populations in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.
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