Crustaceans are one of the oldest arthropod groups, having been on the planet for at least 560 million years. As other groups went extinct, the crustaceans came to dominate the oceans, much as the insects (which probably arose from crustaceans around 400 million years ago) have come to dominate the land. They are by far the most morphologically diverse group of animals ever to have lived. They range in size from tantulocarids and others with body lengths less than 1/10 of a millimeter, to giant spider crabs with spindly legs nearly four meters (12 feet) across, and lobsters weighing over 19 kg (42 lbs.) (Wolff, 1978). Some parasitic forms are so modified as adults that they consist of nothing more than a network of cells growing within a hosts' circulatory system. Such species can only be recognized as crustaceans by their nauplius larvae.
Like other arthropods, crustaceans have an exoskeleton composed of chitin to which their muscles attach. In some groups such as the crabs and lobsters, calcium salts are incorporated into the chitin matrix to produce a hard, thickened shell. Because the exoskeleton is made up of non-living material that cannot grow with the animal, the animal must periodically molt out of the old exoskeleton and produce a new, larger one. At the time of the molt, the animal is in a great deal of danger. The process is very strenuous, and the new exoskeleton remains soft for several hours.
Crustaceans differ from other arthropod groups in that they have 5 head somites (body segments) and a nauplius larva. The head somites (in general) each carries a pair of appendages that are biramous (have two branches off of a single segment that is attached to the body somite). This can readily be seen in the antennae of pet shop hermit crabs. The first two head somites carry the first and second antennae (a.k.a. antennules and antennae respectively) which have a sensory function. The third head somite is where the mouth is located, and it is surrounded by a pair of mandibles that act as jaws. The last two head somites bear the 1st and 2nd maxillae (a.k.a. maxillae and maxillules) which help in tasting and manipulating food. The five head somites are fused together and are indistinguishable except for the placement of the appendages.
The nauplius larva hatches out of an egg, and initially consists of not much more than a single median eye, the first three head somites with their appendages, and a terminal somite (telson). Somites are added with growth, and at some point the larva metamorphoses into an adult (post larval) form. Nauplii are most easily studied by looking at newly hatched brine shrimp, Artemia sp. (a.k.a. sea monkeys) under a magnifying glass or microscope. The most primitive crustaceans have many somites with serially repeated paddle-like appendages. “Advanced” species have differentiated appendages adapted to specific functions (swimmerets, walking legs, pincers, etc.). These species often hatch out as larval or post larval forms more specialized than the nauplius.
This group is so named because the latter thoracic appendages are generally unmodified from the first three pairs (termed maxillipeds). Groups in this class generally have a body pattern of 5 head, 6 thoracic, and 4 abdominal somites (followed by a telson). Body segmentation is often reduced, and several larval characteristics are often retained into adulthood, such as the naupliar eye. This attainment of sexual maturity while still having larval characteristics is termed paedogenesis. In this group is where most of the truly bizarre parasitic forms are be found.
Commonly called seed shrimp, these animals are generally tiny (0.5-3 mm), and live completely inside a hinged, bivalved carapace. They are most commonly found on the bottom, but many planktonic forms also exist. This group has reduced segmentation so that only a maximum of 2 thoracic appendages can be found. Generally found in all marine and freshwater systems, some species in the study area inhabit the harsh environment of the Caribbean salt ponds, living in waters hot to the touch and having salinities 2-3 times that of seawater. Others found on the reefs come out at night and release species-specific bioluminescent signals, much like fireflies, in order to attract mates.
Living ostracodes have been classified into two major groups. Members of the superorder Myodocopida whose members are marine can be recognized by the notch at the front of the carapace valves through which the antennae pass while swimming. This group includes the deepwater, 3 cm long Gigantocypris, and the bioluminescent signaling species (found only in the Caribbean). Species in the superorder Podocopida can be found in both freshwater and marine environments. They do not have notched valves.
Copepods are perhaps the most important crustaceans in the world's oceans in that they are the predominant first-level consumers upon which the rest of the aquatic food chain is based. The subclass Copepoda is one of the largest crustacean groups, with at present 211 families, yet there are few copepod specialists in the world, and many species yet to be described. Most copepods are microscopic, or nearly so. The exceptions are some of the parasitic forms. Copepods are ubiquitous in both freshwater and marine systems, inhabiting both the water column and bottom habitats.
Barnacles are recognized by most shoregoers as the crust covering pilings and boat bottoms, yet it was not until the invention of the microscope that scientists could see the nauplius larva and confirm that barnacles were related to copepods and crabs, rather than to the mollusks they resemble. Free-swimming larval barnacles recognize chemical signals produced by adults and therfore preferentially settle on previously colonized substrates. Cement produced from glands at the base of the antennae attach the larva to the surface where it quickly metamorphoses into the adult form and begins to secrete calcareous plates around its body. Comb-like legs (cirri) are kicked through the water as each wave passes, tiny planktonic organisms that are strained by this basket of legs are pulled by the into the shell to be eaten (example). Because barnacles cannot move as adults, each individual is hermaphroditic (having both male and female reproductive structures). The animal's coiled penis, which is is much longer than the animal's body, is used to reach and mate with those barnacles close by.
When one thinks of a crustacean he or she most often thinks about one of the more than 30,000 species in this group. This class is is split into three subclasses, Phyllocarida, Hoplocarida (stomatopods), and Eumalacostraca (mainly peracarids and decapods). The eumalacostracans (Eu- meaning "true") and hoplocarids have a 5-8-6 (head-thorax-abdomen) segment body plan. The subclass Phyllocarida is thought to represent the primitive malacostracan condition by having 7 instead of 6 abdominal somites (plus telson), and serially repeated, paddle-like, biramous (two branched) appendages.
The fewer than 40 species in the only phyllocarid order, Leptostraca, are all benthic organisms, generally less than 1.5 cm in length. They are generally found in accumulations of decaying organic material although we have collected them from living algae in the BVI. They have a bivalved carapace covering the head and thorax, plus a distinctive hinged rostrum.