Although perhaps not much to look at, crabs are an interesting and diverse group of animals. This dungeness crab (Metacarcinus magister) recently captured in San Francisco Bay, for example, is the most economically important of the West Coast crab species. They are found all along the coast and have a specialized digestive tract organ called the gastric mill, which is a muscular tube with sharp spines that allows them to grind up clams and other hard-shelled crustaceans, their favorite food items. Dungeness crabs have an interesting mating ritual that includes several days of face-to-face cuddling. Once sufficiently warmed-up the female signals her readiness for action by urinating on the antennae of the male, then molting.
Crabs are found in all of the worlds oceans and live in saltwater, freshwater, and on land. The articulation of their five pairs of legs makes a sidelong gait more efficient, which is why most crabs walk sideways rather than forwards. Species range in size from a few millimeters to a leg span of up to 13 meters.
Photo source: FISHBIO