Crabs

There are two kinds of crabs. The irregular-tailed crabs, or Anomurans, include the hermit, porcelain, and mole crabs. Some have a small, tapered abdomen, and others, especially the hermit crabs, have a somewhat coiled abdomen that is tucked into a discarded snail shell. The true crabs, or Brachyurans (short-tailed crabs), have a greatly reduced abdomen that is tucked forward beneath the carapace. Contrary to popular belief, crabs do not always scuttle sideways; they can walk forward or backward with equal facility. Afew of the fast moving crabs seem to use two pairs of legs when running. The blue crab (Callinectes sapidus) family, which also includes the familiar lady crab (Ovalipes ocellatus) have a paddlelike last pair of appendages and are fast and agile swimmers. Crabs vary greatly in size and shape. The smallest are pea crabs, which measure only a few millimeters across the carapace, while the legs of a Japanese spider crab can span up to four meters. The differences in size and shape have allowed crabs to colonize a wide range of habitats. Most of the 4,500 species of true crabs are marine, inhabiting the rocky intertidal zone or burrowing in sand and mud. Some species are benthic and others invade freshwater, especially in the tropics. Still others are terrestrial or semiterrestrial, often ranging far away from water, although they generally prefer moist places. A tree-dwelling tropical species feeds on coconuts. Ecologically, almost all crabs function as scavengers, feeding on dead or dying animals or organic debris of any kind. Many species are also active predators, capable of killing small fish and breaking open the shells of different mollusks. Porcelain crabs, for example, have one of their mouthparts equipped with long, feathery setae that are swept through the water. They catch small particles and transfer them to other parts of the mouth, where they are eaten. Organic matter is dismembered by the pincers and conveyed to the mouth by a series of appendages called maxillipeds and maxillae.Apair of comblike mandibles guard the mouth and chop the food into tiny pieces, which are then swallowed.Ashort esophagus leads to a gastric mill analogous to the gizzard of other animals, where food is ground up.

Crab Reproduction
Reproductive behaviors vary, but during courtship males may use various chemical, visual, or acoustic signals including pheromones, waving chelipeds back and forth, or drumming at the entrance to the female's burrow. Other males build pyramid-shaped mounds to attract females. Following courtship the male transfers sperm to her in a packet called a spermatophore. The eggs are attached to the female's abdominal appendages and form an egg mass between the abdomen and the thorax. The abdomen is therefore out of place and hangs downward when the female is carrying eggs. The eggs are usually brightly colored with carotenoid pigments, and can be red, yellow, blue, brown, or green. They range in size and number between species, but a large marine crab can lay up to a million eggs at a time. The tiny larvae are called zoea and are very common summer components of marine plankton. It takes one or two more molts before the crablike form is evident. Many of the semiterrestrial and freshwater species develop directly into miniature crabs.

Crabs as Food
Crabs are food for a variety of other animals, including fish and larger invertebrates. To protect themselves, some species carefully place sea anemones on their carapace, while others, such as the decorator crab, glue pieces of shell all over their carapace. Still other species, such as the pea crab, take refuge inside oysters and other shellfish, while boring crabs dig burrows into mud substrates or soft limestone rocks.