Common Spider Crab
Libinia emarginata

Also known as the portly spider crab or the nine-spined spider crab, the common spider crab is a long-legged and slow-moving crustacean that covers itself in algae and small debris as a defense against predators.

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The shell of the common spider crab measures about four inches from front to back. When its legs are stretched out, the spider crab can reach a spread of one foot. (Photo courtesy of joo0ey/Flickr CC BY 2.0)

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Appearance

The common spider crab has a tan, triangle-shaped carapace (shell) that measure about four inches front to back and features a row of nine low spines (this differentiates the creature from the six-spined spider crab). Its shell is often covered in algae, debris and small invertebrates held in place by hook-like hairs. When its legs are outstretched, the common spider crab can reach a spread of up to one foot, while males grow larger than females. The crab has white, narrow claws that are slow and not very strong, and has a tapered snout and short eyestalks.

Feeding

These crabs are sluggish and unaggressive scavengers with poor eyesight. Sensitive tasting and sensing organs located on the tips of its walking legs allow the common spider crab to identify food in the water or mud as it walks. It will often feed on large starfish.

Predators

Spider crabs belong to a group known as "decorator crabs": the crab's main defense against predation is ability to camouflage itself with various spines and tubercles, algae, debris and small invertebrates. When startled, it will wave its pincers over its head as a warning sign.

Reproduction and Life Cycle

The common spider crab must molt to grow. When molting, this crab clings to top of eelgrass close to water's surface. They have been observed molting in large "pods" in the fall, hibernating in dense patches in the winter and mating in large groups in the spring. Females close to releasing eggs are held behind males and aggressively protected. Young hatch from eggs that are bright orange-red when laid but turn brown during development, which takes about 25 days.

Did You Know?

  • Spider crabs can tolerate and live in polluted waters and eutrophic, or low-oxygen, environments.
  • They often walk in a forward motion, although they are capable of side-stepping like other crabs.

Sources and Additional Information

Tags:
Invertebrates Arthropods Open & Shallow Waters