Reproductive biology

With the exception of one species of crayfish, decapods reproduce sexually; and in almost all cases the sexes are separate. A number of caridean shrimps are protandric hermaphrodites, which means that they mature as males first and later change into females; a few species retain male structures and become functional hermaphrodites. Some species of snapping shrimps live in colonies with only one reproducing "queen," much like social insects.

In species where there is intense competition for mates, the claws of males are often proportionately much larger than those of females. An extreme example of this disproportion is seen in the fiddler crabs (Uca spp.), in which one of the claws of the male is useless for feeding and functions only to attract females and duel with other males. Courtship and mating can take anywhere from seconds to weeks, depending on the species. Copulation in penaeid and caridean shrimps is often an instant affair, while mating in crabs is often a long process involving extended periods of guarding before and after mating. In these cases the female crab can only mate while in her soft-shell condition, so she must molt her exoskeleton immediately prior to mating. A female preparing to molt releases pheromones that attract males; the male will embrace and carry the female for days or even weeks preceding her molt. Male hermit crabs are often seen dragging a smaller female about by the shell in anticipation of mating.

Fertilization can be external or internal, depending on the taxon. Many female brachyuran crabs have internal receptacles for sperm storage where sperm can remain viable for years, and males have evolved a number of different strategies to help insure that it is their sperm that fertilizes the eggs. The male will often stand guard over the female while she is soft to prevent others from mating with her, or block her genital openings with a sort of "chastity belt" to prevent matings with other partners.

In all groups except the Dendrobranchiata the eggs are brooded on the female's pleopods (abdominal appendages) until they hatch. Females clean and aerate their egg masses, and a chemical cue from the eggs stimulates the female to violently shake the mass when the larvae are ready to be released. Parental care generally ends with hatching, although young crayfish often continue to associate with their mother for protection. Some tropical crabs that make use of the freshwater trapped in bromeliad plants practice true maternal care, bringing food to their developing young and protecting them from predators.

Decapods in cold and temperate regions usually release their larvae in the spring to coincide with plankton blooms, while those in the tropics often reproduce year round. In most cases development consists of several planktonic larval stages followed by a stage that makes the transition to a benthic ex istence; in crabs these are known as the zoea and megalops stages, respectively. The megalops has well-developed pleopods and can swim like a shrimp, but once it finds an appropriate place to settle it molts to the first juvenile stage and the pleopods are lost.

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