The crustaceans group of marine arthropods contains about 35,000 species and include such animals as water fleas, barnacles, crabs, lobsters, shrimps and wood lice (pill bugs). It also includes tiny zooplankton, which live near the surface of the sea and occupy an important position i aquatic food chain.
The exoskeleton, which is composed of chitin and in some species (such as crabs) is hardened with calcium salts, is molted periodically to enable the animal to grow. This is an important process in the life of crustaceans (and other arthropods) and occurs in a strict sequence of events. First, useful materials, such as calcium salts, are reabsorbed into the body; the new cuticle is then laid down underneath the old one; the animal swells up causing the old cuticle to split, and finally the new cuticle is hardened. Before the new cuticle hardens the animal is easy prey for predators and usually seeks shelter during this period.
In many crustaceans an outer shell, or carapace, covers the exoskeleton of the thorax or anterior trunk segments. Typically, the body consists of a head, thorax and an abdomen, but often the head and a number of thoracic segments are fused to form a cephalothorax. The head region has 2 pairs of antennae, which is characteristic of crustaceans. It also has one pair of mandibles, which in most species are heavy and have grinding and biting surfaces, and two pairs of maxillae. The nervous system of these animals is well developed, and the sensory organs include eyes and various tactile receptors. Crustaceans usually have separate sexes, but some have both male and female. The young hatch from eggs which are usually brooded and take the form of free-swimming planktonic larvae with fewer appendages than the adult.
This subclass of mostly freshwater animals have earned their name (meaning "gill feet" ) fro their thoracic appendages which are modified for respiration as well as filter-feeding and locomotion. The group consists of four orders:
Brachiopods are filter-feeders which collect food particles on the bristles on the trunk appendages. The sexes are usually separate.
These two subclasses of tiny crustaceans contain freshwater and marine species, and planktonic (shallow-water) as well as benthic (deepwater) types. The ostracods are similar to the conchostracan branchiopods in that they have a bivalve hinged carapace. The head is more developed than the remainder of the body, and its appendages are modified for crawling, swimming, and feeding. Most ostracods are filter-feeders, but some species are scavengers, predators or parasites.
Most copepods are marine and occur in huge numbers, making them of great economic and ecological importance, because they form a large part of the diet of many fish. The body is short and tubular, with a trunk that is usually composed of 6 segments. The planktonic copepods usually live in the upper 650 to 975 feet of the ocean.
Barnacles belong to the only group of sessile crustaceans, the Cirripedia. The subclass derives its name form 6 pairs of feather-like trunk appendages (cirri) that protrude from the shell, which are sued for filter-feeding. All barnacles are marine, and most are found attached on rocks, shells or driftwood. Most barnacle species are hermaphrodite, but cross-fertilization is common.
The order Decapods contains most of the large crustaceans in the subclass Malacostraca; they include lobsters, crabs, crayfish and shrimp. More than 8,500 species have been described and most are marine. All decapods have 8 pairs of thoracic appendages; the rear 5 are modified for walking and are the origin of the name decapod; the front 3 pairs serve as feeding appendages (maxillipeds). In some decapods the first pair of walking legs is heavier and stronger than the others and has chelae (pincers).
The abdomen is composed of 6 segments and a tail (telson). Decapods have compound eyes on jointed movable stalks, and the central nervous system is well developed. Their wide variation in color usually depends on the habitat and the pigment-producing chromatophores in the exoskeleton. Aquatic decapods breathe with gills. An elaborate courtship ritual is typical of many decapods prior to copulation and odorous chemicals called pheromones play an important role in their sexual behavior. In many species the eggs are laid soon after copulation and are cemented onto the swimmerets (abdominal appendages) of the female.
Lobsters, like some crabs, have a single huge claw that is used for crushing; the other claw of the pair is much smaller but has sharp edges and is used for seizing and tearing prey. They are scavengers, but also catch fish and break open shelled animals. Crayfish are similar in appearance to lobsters but most live in freshwater whereas lobsters are marine. Also, like lobsters, crayfish are nocturnal and feed on almost any organic matter, living or dead.
Crabs are probably the most successful decapods in that they can live on land as well as in water. They are found at all depths of water, in all parts of the world. They have a wide carapace and, unlike lobsters, a small abdomen which is tucked tightly under the caphalothorax. Crabs range in size from the pea crabs, which live in the tubes constructed by marine annelids, on sea urchins, or in the mantle cavity of gastropods, to the Japanese spider crab whose body measures about 1 inch across and whose legs span is more than 3 feet. Their diversity of form includes the hermit crab which has no shell of its own and which takes over empty gastropod shells for protection.
Crabs are filter-feeders, predators, and scavengers and their method of obtaining food is usually reflected by the shape of their chelipeds.
Shrimps and prawns are much better designed for swimming than the lobster, being laterally compressed with a well-developed abdomen, but most of them bottom-dwellers.
This order contains the only group of truly terrestrial crustaceans - the woodlice - although most species are marine. The shield-shaped body is flat, has no carapace, and has 7 pairs of legs. They have a pair of compound eyes and 2 pairs of antennae. Water loss can be a problem to woodlice as they do not have a waxy cuticle like some other terrestrial arthropods, such as insects. They survive by living in fairly damp habitats.
Crustaceans are the only major traditional arthropod group of which no venomous species were known. Recently, scientists provided the first conclusive evidence that the aquatic, blind, and cave-dwelling remipede crustaceans are venomous, indicating the evolving of venoms in all four major arthropod groups. Analysis of the venom delivery apparatus of the remipede Speleonectes tulumensis showed that remipedes can inject venom in a controlled manner. The anterior trunk of S. tulumensis contains two equally sized venom glands, which connect via ducts to reservoirs located in the terminal segments of a robust pair of legs (maxillules) in the head.1