Crustaceans

Scientific Classification:

Kingdom: Animalia

Phylum: Arthropoda

Subphylum: Crustacea (Brünnich, 1772)

Although the classification of crustaceans has been quite variable, the system used by Martin and Davis ^[1] is the most authoritative, and largely supersedes earlier works. (from Wikipedia)

Six classes of crustaceans are generally recognized:

• Branchiopoda - including brine shrimp (Artemia) and Triops (Notostraca)
• Remipedia - a small class restricted to deep caves connected to salt water, called anchialine caves
• Cephalocarida - horseshoe shrimp
• Maxillopoda - various groups, including barnacles and copepods. It contains Mystacocarida and Branchiura, which are sometimes treated as individual classes.
• Ostracoda - small animals with bivalve shells
• Malacostraca - the largest class, with the largest and most familiar animals, such as crabs, lobsters, shrimp, krill and woodlice.

Crustacea ( from the Latin word “crusta”, meaning crust, and referring to the incrustation of the external skeleton with lime in the larger species), is a large and greatly varied class of animals, of around 52,000 species classed with insects, spiders and myripods ( centipedes and millipedes) in the phylum Arthropoda. The astonishing variety in common characteristics of crustaceans will be recognized when it is seen that the group includes crabs, crayfish, lobsters, barnacles, acorn shells, water-fleas, woodlice, pill-bugs, sand-fleas and shrimps, and is composed of both terrestrial and aquatic animals, the latter being either marine or fresh-water species.

Distribution:

The distribution of Crustaceans is very wide. They frequent the seas at all depths, occur in fresh-water lakes and rivers, and a few dwell in the bark of trees, but a few groups have adapted to terrestrial life, such as terrestrial crabs, terrestrial hermit crabs and woodlice; the extinct species date from the Cambrian period.

Anatomical Characteristics:

The features which are shared by all are a few, and some incredibly simplified parasitic forms occur. Some species are so modified for their mode of life, that they can be classified as crustaceans only by reference to their larval stages.

Nearly all crustaceans have three distinct body parts: head, thorax, and abdomen, breathe by means of gills, and the head has 5 pairs of appendages (namely 2 pairs of antennae and 3 pairs of jaws) and a pair of compound eyes. The thorax bears numerous appendages which are usually biramous (forked) and also the gills, but the smaller species may breathe through their body surface by diffusion. The segmented abdomen of crustaceans also frequently has limbs.

In order to grow, Crustaceans must first shed their hard exoskeleton, which is immediately followed by a rapid growth spur before the new exoskeleton hardens again.

Reproduction:

Reproduction by crustaceans is sexual, the sexes being nearly always distinguished by appendages on the abdomen called swimmerets or, more technically, pleopods. The first (and sometimes the second) pair of pleopods are specialized in the male for sperm transfer. All crustaceans are oviparous (produce eggs which hatch outside the mother’s body), often the female carries her eggs with her under her abdomen or thorax until they hatch, when a series of extraordinary metamorphoses generally takes place.