Shrimps Crabs and Lobsters: Decapoda

Decapods show many complicated and even amazing behaviors. For example, Caribbean spiny lobsters form a long row of up to 65 individuals that march single-file toward deeper water. It is not well understood why they do this, although it may have something to do with avoiding storms during the winter. Juvenile red king crabs often gather together into mounds that may contain thousands of individuals. This behavior is believed to prevent them from being eaten by predators.

Some species form special relationships with other organisms. Certain shrimps establish cleaning stations where fishes line up to have their parasites removed. Fish parasites are worms, crustaceans, and other animals that live on their bodies and eat their blood and other body fluids. Other shrimps share a burrow with a fish. The fish watches for danger as the shrimp builds and maintains the burrow.

Some species make signals that are seen or heard by other members of the same species. Decapods that spend some or most of their lives on land produce sounds by rubbing one part of their body against another. Marine and freshwater species release special chemicals, or pheromones (FEH-re-moans), into the water along with their waste through special glands on the antennae. The pheromones are used to attract mates.

Except for one kind of crayfish, all decapods require males and females to reproduce. In some shrimps, the adults mature first as males and later develop into females. A few species keep both male and female reproductive structures and are called hermaphrodites (her-MAE-fro-daits).

Decapods have many kinds of courtship. In fiddler crabs the male has claws that are much larger than those of the females. Useless for feeding, the oversized male claws are used only for fighting with other males and attracting females. Depending on the species, mating is very brief or may occur only after males and females have spent long periods of time together. In these cases the female can only mate just after she molts, or sheds her external skeleton (exoskeleton). As they prepare to molt, adult females release pheromones to attract males. A male will sometimes grasp the female with its legs for several days or weeks until she finally molts. Sperm is transferred to the female as a fluid or inside packets. The sperm is deposited directly into the reproductive organs of the female or into a special storage sac in her body. In some species the male stands guard over the female to prevent other males from mating with her.

Most female decapods hold their egg masses with their pleopods. They keep the eggs clean and make sure that plenty of oxygen-carrying water is circulated around them. Just before hatching, the eggs release a chemical that tells the female to shake the mass to help release the larvae (LAR-vee), or young animals, into the water as they hatch. After hatching, parental care is rare. Young crayfishes will stay with their female parents for protection. In some tropical crabs that breed in freshwater trapped at the bases of plants growing on tree limbs, the females provide food for their larvae and protect them from predators.