Sacculina carcini - Parasitic Barnacles

Life Cycle

Direct, single-host parasite & parasitic castrator of crustaceans, specifically brachyuran crabs and green crabs (Carcinas maenas)

Free-swimming female larvae of this parasitic barnacle species will seek out and penetrate the outer protective layer of a host species, injecting a vermigon into the hemolymph (the fluid plasma in invertebrates that functions like blood). The parasite develops internal "root" systems throughout host hemolymph. When it matures, the reproductive apparatus breaks through the integument, the protective layer of the host, and becomes external at the crab's abdomen to enable sexual reproduction with free-swimming adult male S. carcini.

Journal of Crustacean Biology, Volume 38, Issue 4, July 2018, Pages 413–419, https://doi.org/10.1093/jcbiol/ruy044

Host Information

Since S. carcini has a direct life cycle infecting Carcinas maenas, its only predators are species that prey on the crabs as well. As has been observed in numerous other species, predation may, over time, lead to upward incorporation of a host for S. carcini if many favorable adaptations allow its survival in that predator species. However, for now, predation is a detriment to both S. carcini and its hosts.

The green crab is invasive to the pacific northwest, and this, combined with parasites' high reproductive output, means that S. carcini does not face much difficulty in transmitting between hosts--the parasite is even a welcome addition in some ecosystems for helping control green crab populations.

Distribution

S. carcini follows its host distribution. Green crabs are invasive to most major coasts.

https://homes.msi.ucsb.edu/~lafferty/PDFs/Invasives/Goddardetal05.pdf