With over 30,000 species including the familiar shrimps, crabs, crayfish and barnacles as well as many smaller and less well-known animals, crustaceans are such a large and varied group that it is often hard to see how all the animals are related.
Plankton is the drifting life of the oceans. Most planktonic animals are very small but extremely numerous and form an extremely important part of marine food webs. The most common planktonic animals are the small crustaceans known as copepods. They are probably the most numerous animal group in the world. Together with copepods, other small crustaceans such as water fleas combine to make this varied group a very important part of the animal plankton.
Blue-spot Rock Lobster / Panulirus femoristriga / Family Palinuridae
Soldier crabs / Mictyris sp. / Family Mictyridae
Hingebeak Shrimp / Rhynchocinetes uritai / Family Rhynchocinetidae
All crustaceans have a body covered with a protective shell composed of a horny substance called chiton. The outer skeleton is not continuous but made up of divided sections called somites. Crustaceans have a number of jointed legs, two pairs of antennae and sometimes a pair of 'nippers'. The body is divided into three parts: a head, a middle region (thorax) and a tail region (abdomen). Often the head and thorax are joined together and covered by a single shell called a carapace.
Crustaceans vary in the habitat in which they live. Some live in the ocean, some in fresh water and some on the land. Many crustaceans are nocturnal and spend the day hidden in a burrow, buried in the sand, or resting in a crevice.
Crustaceans produce from eggs, which have been fertilized by sperm in much the same manner as other animals. The eggs are produced in the ovaries in the female and passed to the outside through oviducts. The sperms are produced in tubular testes in the male. After the eggs have been fertilized, they begin development and then hatch.
When the eggs hatch, this can take several days to several weeks depending on the species, the young larvae are detached. From this point on they are on their own and must fee, grow, swim and survive. After a series of transformations, the larvae becomes a miniature adult.
Crustaceans cannot grow as many other animals do because of their outer skeleton. Instead they periodically shed the outer skeleton, grow rapidly for a short time, and then form another hard skeleton. While this process is taking place they hid in an isolated place.
Another remarkable ability the crustacean has is to be able to break off or to drop their appendages. This is called autotomy. They have special breaking-off points near the body. If caught they can quickly break-off this appendages to get away. A new appendage is more easily grown.
Crustacean's food varies, some eat plants, some eat flesh, and some feed from the bottom of the ocean on anything they can find. The way in which they feed themselves also varies. Barnacles and anemone crabs use fine hairs on their appendages to filter food while most of the larger crustaceans are scavengers. The cleaner shrimp feeds on the mucus and parasites covering the skin and gills of fish.