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Parasitic Barnacle on a Crab

This crab has a parasitic barnacle rooted in its reproductive system.
(Hans Hillewaert)

This swimming crab (Liocarcinus holsatus) has a parasitic barnacle rooted in its reproductive system. This invasion cuts off all reproduction for an infected crab and can even cause a male crab to change behavior — males don't normally take care of crab eggs, but the infected male will take care of the barnacle as if it is a brood of eggs. Read more about other parasites that you can find in marine organisms.