Common Spider Crab
The Common Spider Crab is also known as the Portly Spider Crab or Atlantic Spider Crab. The shell or carapace of this crab is round and spiny, with nine small spines running down the center. This long, legged; slow moving crustacean is able to walk forward and side step (most crabs can only walk by side stepping.) The legs and pincers of the male spider crabs can be nearly twice as long as those of the females. The spider crab’s claws are different from those of other crab claws. They are narrow, long pincers that are slow and not as strong as other crabs; however, the larger males have big claws that can deliver quite a pinch. Spider crabs attach bits of algae, shell, and seaweed to the many fine, sticky hairs covering their bodies for camouflage. This is why they belong to a group known as decorator crabs.
This spider crab can be found living in all types of substrates, but most commonly in bays and estuaries on mud and sand in the shallow water. They range from Nova Scotia, to south Florida, the Florida Keys and Gulf of Mexico. These crabs are very tolerable to polluted or eutrophic environments, meaning environments where there is little oxygen.
Spider crabs are omnivorous and use the ends of their claws to scoop up bits of detritus and algae. Spider crabs are non-threatening and somewhat lethargic scavengers. They have poor eyesight; however, they do have sensitive tasting and sensing organs on the end of each walking leg. This allows them to identify food in the water or in the mud as they walk over it.
Like all other crabs, spider crabs molt in order to grow. The females stop molting after they become sexually mature and remain the same size for the rest of their lives. The males typically grow larger than the females and can be up to 9 inches from claw to claw. They have been observed molting in large “pods” during the fall, hibernating in dense patches in the winter and mating in large groups during the spring. When the female is ready to release her eggs she is heavily protected by the male by being held behind him. The young hatch from eggs that are a bright orange-red when laid but turn brown during development, which takes approximately 25 days.
When startled, spider crabs will waive their pincers over their heads in a beckoning gesture to warn off potential predators. The main defense against predators is the use of camouflage by covering themselves with various spines and tubercles, algae, debris and small invertebrates. Gulls and other shore birds are the main predators as well as fish and sharks.