Sand Crab & Parasitic Barnacle

Portunus pelagicus (sand crab) & Sacculina (parasitic barnacle)

This Sand Crab is infected with a parasitic barnacle. The leathery sac protruding from beneath the crab’s abdomen is the egg-producing body of the barnacle living inside the crab. Hormones secreted by the barnacle cause male crabs to become females, and once the infection takes hold the crabs are unable to reproduce. ‘Good parasites’ are those which have evolved over millions of years to have minimal impact on the survival of the host – it is in the parasite’s best interest to keep its host alive. ‘Bad parasites’, where the host is severely harmed or killed quickly, are probably new partnerships that have not had time to stabilise and evolve.

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