Libinia emarginata, the portly spider crab, common spider crab or nine-spined spider crab, is a species of stenohaline crab that lives on the Atlantic coast of North America.
Distribution
Libinia emarginata occurs from Nova Scotia to the Florida Keys and through the Gulf of Mexico. It lives at depths of up to , with exceptional records of up to . and, as with other decorator crabs, often clothes itself in debris and small invertebrates.
Similar species
Libinia emarginata is very similar to Libinia dubia with which it is largely sympatric. They can be told apart by examining the row of spines along the center of the carapace: in L. emarginata there are nine, while in L. dubia there are only six.
Ecology and behavior
Libinia emarginata lives on various substrates, at depths of up to . Adults are sluggish and not aggressive, and younger crabs are frequently covered with sponges and hydroids.
Unusually for crabs, L. emarginata preferentially walks forwards, rather than sideways, although they are also capable of sidelong movement. Its skeletal, muscular more closely resembles that of forward-walking species, rather than that of more closely related sideways-walking species.
L. emarginata will mate in large aggregations. These aggregations may function as a protective mechanism during reproduction.