The biology of the fourteen species of spider crabs occurring in the Isle of Man area was investigated, particular attention being given to their distribution, food, moulting cycle and breeding. The distribution of a species is related to the type of bottom deposit, and members of a genus have complementary distributions. There is a variable degree of diet selection and food availability is an important factor in determining distribution. The moult of puberty, which in all cases is the terminal eedysis, is well defined in all species. There are two patterns of breeding, one continuous throughout the year, the other with a single annual hatching. With few exceptions the relative growth of the chelae is allometric, exhibiting marked sexual dimorphism.