While working in Pleasant Bay in Chatham last week, I saw numerous pairs of common spider crabs (Libinia emarginata) in the shallow water. Spider crabs have quite a different look from other local crabs. Their carapace is rounder with a distinctive beaklike protrusion. Their eight walking legs and two claws are long and narrow giving them a very spider-like appearance. This species is also called “nine-spined spider crab” due to the nine spines, or bumps, running down the center of its carapace, which differentiates it from another local spider crab species, Libinia dubia, which only has 6 spines. 

The entire dorsal surface of a common spider crab’s carapace is covered with hairs, to which algae, detritus and other encrusting organisms often cling. In fact, spider crabs will actually add items to their back intentionally to add to the overall look; fittingly spider crabs belong to a larger group of crabs called decorator crabs. Although this gives spider crabs a rather messy or dirty appearance, there are advantages to looking like a pile of mud or seaweed. Since they are slow, weak-clawed and unable to dig a burrow for safety, common spider crabs rely on their camouflage to provide some degree of protection from predators.

Like all crabs, common spider crabs must molt in order to grow. However, the females stop molting after becoming sexually mature and will stay the same size for the rest of their lives. Because of this, males typically grow larger than females. The largest males can grow to have carapaces approximately 4 inches wide and leg spans up to almost a foot. During the summer when common spider crabs mate, the male will hold the female behind himself. This hold is achieved through an interweaving of their hind legs. In this position, the males will aggressively defend their female from other males. After copulation, the female can produce up to four consecutive broods from the sperm stored from a single mating.