These fascinating crustaceans are from the order Decapoda. Decapods come from a variety of different environments. A decapods body is composed of a series of hard segments called somites. These somites are either fused together, or linked by a flexible join forming the limbs. They have two sets of jaws in front of the mouth, and have several appendages near the mouth that act as feeding aids. The limbs usually start with a pair of developed arms with claws. Behind these are 4 pairs of walking legs, and as the claws are classed as legs they total 10, hence Decapoda. Some shrimps use the legs under the abdomen for swimming. However the true 'long tailed decapods', like all crayfish and some shrimp, can only swim backwards using the tail, not the pleopods. They are also used to fan freshwater to the gills (and may even bear gills themselves), food to the mouth, and females use these pleopods to carry their eggs.
The body is covered with a solid shell primarily made up of chitin, but also contains some calcium carbonate for strength (especially in crabs). This shell is held together by an underlying layer of skin (the endoskeleton), and together forms the crustaceans exoskeleton. Its muscles are also attached to this outer shell.
Decapods have a fast growth rate and need to moult their exoskeleton to grow. A soft new shell is continually grown under their existing exoskeleton and as the moult draws near a hormone is released from sacks in the eye stalks. This both encourages the moulting process, and suppresses their appetite. Then they pull themselves out of the back of their old shell, unfortunately sometimes some limbs can get stuck in the old shell. Shrimp and crayfish emerge by a split in their neck, and crabs via the back of the carapace. After they have moulted the new exoskeleton is soft and wrinkled, and they become very lethargic (due to the amount of energy used). During this time they grow and replace lost limbs (usually smaller, but sometimes completely). and the new shell is quickly 'pumped up' to allow for more growing space. They are vulnerable to attacks from tank mates when soft, and the shell can take between a few hours and a few weeks to fully harden, depending on species. They do this by absorbing calcium carbonate whilst the chitin hardens. This whole process can be very stressful for the animal.
Many shrimp are protandric hermaphrodites which means they begin as males then change into females with age. Lifespan varies from 2 to 4 years. Some individuals change to female before becoming a sexually mature male. It is not known what influences the 'decision' to skip the functional male portion of their life cycle. Some shrimps are active predators, feeding on smaller crustaceans and other invertebrates.
