The order Decapoda contains those animals most people recognize as crustaceans: shrimps, lobsters, and crabs. Decapods (meaning ten feet) have the last five pairs of thoracic appendages modified as walking legs while the first three pairs, the maxillipeds, function as mouthparts. If one pair of walking legs has enlarged pincers (chelae) they are referred to as chelipeds. Within this group we can easily see the evolution of the crablike form from the more primitive shrimp-like form via intermediates with progressively more reduced abdomens.
Decapods are subdivided into two suborders. The suborder Dendrobranchiata is characterized by shrimps with gills having two principle branches off of a main axis. These species can most easily be recognized by their having small chelae on the first three pair of walking legs. Massive chelipeds are never seen. Reproduction is primitive in that the eggs are released into the water and hatch as nauplii. Members of the second suborder, Pleocyemata, have either trichobranchiate (unbranched filaments radiating from a main axis) or phyllobranchiate (double leaf-like branches off the axis) gills. The walking legs can have chelae on various pairs, but never on each of the first three. Eggs are cemented to the pleopods (abdominal appendages) and brooded until they hatch. Larvae usually hatch at a stage more developed than the nauplius.
Shrimps
These are the most commercially important shrimp species worldwide. They are also the most often encountered members of the suborder Dendrobranchiata in shallow waters. The two families most likely to be seen in the BVI are Penaeidae and Sicyonidae. Penaeids are generally thinner shelled than sicyoniids, which have a rigid, stony looking integument (often marked with an eyespot in life). Species are commonly encountered on, but not restricted to, sandy bottoms.
(Suborder Pleocyemata)
This small group is characterized by having a slightly compressed lobster-like abdomen. Large chelipeds arise from the third pair of walking legs. The other pairs are achelate (lack pincers). Three shallow water species occur in the western Atlantic, and all can be found in the BVI. The most common is the ubiquitous red and white banded coral shrimp, Stenopus hispidus, generally seen in pairs on the reef.
The infraorder Caridea is the most speciose shrimp group in the world. Commercially important species include many larger temperate water species, and the large freshwater tropical species of Macrobrachium. In the tropical marine environment many small species have evolved specialized lifestyles, living commensally on various species of sponges, corals, and echinoderms. Carideans are easily distinguished from penaeids by the pleura (side plate) of the second abdominal segment, which overlaps the pleura of both the preceding and following segments in the carideans but only the following segment in the penaeids (and all other decapods). The second walking leg is chelate in the carideans, and may or may not occur as a massive cheliped.
This speciose caridean group is most easily recognized by the superficial lobsterlike appearance caused by a pair of massive chelipeds. These are specialized in the snapping or pistol shrimps (family Alpheidae) so that explosive pressure can be built up in the major cheliped, and when the pincer is snapped shut, an audible pressure wave is produced that can stun competitors, predators, or prey. The popping sounds filling the ears of snorkelers and scuba divers on the reefs of the BVI and other areas around the world are produced by these numerous shrimp. Most species live under rocks and rubble, although some live commensally within sponges. The carapace, which covers the eyes, acts as protective goggles in these habitats.
These shrimp are in some ways the most lobster-like of the animals we are calling shrimp here, having a flattened abdomen with a spreading tail fan, and large chelipeds as the first walking legs. They are rarely seen, living under rocks and rubble, or in burrows in soft bottoms, but some species, especially the ghost shrimps (family Callianassidae), may be very common in the BVI. Ghost shrimps form many of the conical pits and "volcanoes" covering subtidal sand bottoms. Their complex burrows may extend 2 meters deep into the sand.
The name "lobster" is reserved for members of two very different decapod infraorders, Astacidea and Palinura, both of which have bodies that are robust, dorsoventrally flattened, and covered by a hard integument or shell.
These are the so called clawed lobsters that have the first pair of legs modified as massive chelipeds. Representatives include the Maine lobster, Homarus americanus, and the freshwater crayfish. In the BVI there are a few species in the family Nephropidae.....
These are the spiny and slipper lobsters commonly fished commercially in the tropical waters of the world. Members of this infraorder lack chelipeds of any kind. The spiny lobsters (family Palinuridae) use their long antennae to ward off fish predators, while the flattened slipper lobsters (family Scyllaridae) use their paddle-like antennae to sense food hiding under shallow sediment. Since both spiny and slipper lobsters lack chelipeds they use their massive mandibles to crush mollusks and echinoderms (their main food items).