musselcrab

Musselcrabs are small, ovigerous crustaceans belonging to the family Pinnotheridae. These crabs are often found living commensally within the shells of bivalve mollusks, most notably mussels and oysters, though they can also inhabit other invertebrates such as worms and sea squirts. Their common name, musselcrab, directly reflects their frequent association with mussels.

The relationship between musselcrabs and their hosts is generally considered commensalism, where the crab benefits from shelter and access to food particles filtered by the host, while the host is typically not significantly harmed. In some instances, especially with dense infestations, the crab's feeding activity might slightly impede the host's ability to filter water, but severe negative impacts are uncommon.

Musselcrabs are characterized by their soft, often translucent bodies and reduced carapaces, adaptations that allow them to navigate and reside within the confined spaces of their host's shell. They possess small claws, suitable for manipulating food particles and for gripping the mantle of their host. Their diet consists mainly of plankton and organic detritus that the host filters from the water.

The life cycle of musselcrabs involves a free-swimming larval stage that must find and colonize a suitable host to complete its development. Once a host is found, the crab settles and grows, often remaining with that host for the duration of its life. Sexual dimorphism is common, with females typically being larger and more sedentary than the more mobile males, who will seek out females within the mollusk populations.