This is the rhizocephalan barnacle Peltogaster curvatus (arrow) on its host, the hermit crab Pagurus bernhardus. Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news ...
The parasite sacculina is also discussed. This member of the barnacle family attaches itself to crabs during moulting. It alters the crab’s behaviour so that it protects the sacculina and its eggs.