Sacculina carcini is a parasitic barnacle that infects the reproductive system of crabs, causing sterility. This type of infection is considered highly virulent by some definitions due to its complete reduction of host fitness.

Life CycleEdit

Female Sacculina larvae swim freely in the water until they encounter adult crabs, then attach and burrow themselves into the crab shell via a soft spot, such as a joint. The body of a larva will abandon its exoskeletion and liquefy, entering the crab's body through this opening. The larva will then migrate to the thorax of the crab, settling within the reproductive organs and sterilizing the crab. Because an infected crab no longer needs to exert energy for reproduction, it can often grow larger and appear healthier than uninfected crabs.

If a male larva also enters the same crab, the male and female Sacculina will undergo sexual reproduction and produce eggs. At the same time, the female Sacculina will manipulate the crab to produce female hormones, regardless of sex. This causes the crab to develop nurturing behavior and tend for the eggs as if they were its own. When the eggs hatch, clouds of young Sacculina larvae are released to find new hosts.

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