pepetj

Most of the time I have a hard time figuring out the species of crustaceans I keep (or dream of keeping). I found an interesting article that clarifies one of the taxonomical steps since it address the classes of crustaceans with detailed info on anatomical features and a bit on their habitat.

Disclousure: The following information on crustaceans was published by The Canadian Encyclopedia, (here modified by me from the original article to include English System measures and avoid references that, in my opinion seem to make sense mostly to Canadians only). Please refer to the original article for clarification since unintentional errors might be present.
(http://www.thecanadianencyclopedia.c...=A1ARTA0002059 accessed October 25, 2009)

Crustaceans
Crustacean, shelled invertebrate with segmented body and limbs at some stage of its life, an exoskeleton and 2 pairs of antennae. The exoskeleton, a protective and supportive framework located outside the body, is periodically molted to allow for growth.

Crustaceans, with over 31,300 living species, comprise a subphylum of the phylum Arthropoda. They occupy terrestrial, fresh, and saltwater habitats, and range in size from the Japanese spider crab, with a 3.65m [12’]claw-span, to microscopic water fleas. The well-known shrimp, crab and lobster belong to the Malacostraca, one of 8 classes.

Various reductions and fusions notwithstanding, the crustacean body is essentially composed of a head with five pairs of appendages, and a trunk with numerous segments and appendages.

The Cephalocarida, first discovered near New York in 1955, are reminiscent of the primitive crustacean condition: many segments and trunk limbs are very similar, one after another (serial homology). Diversification and specialization of the appendages has occurred through evolutionary time: the modern crab bears little superficial resemblance to the barnacle or wood louse.

The filter-feeding habit is common and many bottom-dwelling forms are detritus feeders. Anterior appendages (often 7 pairs) catch and handle food. In some Malacostracans, food maceration is aided by chitinous (horny) teeth in the muscular stomach. Gas exchange takes place in gills attached to the bases of the trunk appendages. A copper-based respiratory pigment in some crustaceans causes blue-tinged blood.

Sexes are separate in some species; others change sexes in midlife, are hermaphroditic, or are born pregnant (making males unnecessary most of the time). Many brood their eggs until larvae emerge. Numerous molts follow while the larva is free-swimming.

The presence of a rigid exoskeleton necessitates a molting process initiated by hormones, following which the naked crustacean rapidly swells with water. The outer cuticle (tough covering) hardens again while the animal remains in hiding.

Ancestral crustaceans were probably swimmers. Through appendage modification, walking, crawling, burrowing, sessile and parasitic lifestyles have also been adopted. The zooplankton link in the ocean's food chain is composed primarily of tiny crustaceans. Many of the great whales (the fin, grey, sei, blue, humpback) feed almost exclusively on these small animals.

Some crustaceans are common pests (eg, terrestrial amphipods in greenhouses, isopods boring into wood and barnacles fouling ships); however, the group makes a major contribution to fisheries. A brief discussion of the 8 crustacean classes follows.

Cephalocarida
Cephalocarida, tiny (2-4 mm [5/64-5/32”] long), hermaphroditic, shrimplike crustaceans; nine species and four genera are known. The body consists of a shield-shaped head, elongated thorax and abdomen. The head has 5 pairs of appendages; the thorax, eight segments, each possessing a similar pair of legs, the last pair reduced or absent; the abdomen, 11 ringlike, limbless body segments (somites) and an anal segment bearing a pair of appendages (uropods). These most primitive living crustaceans inhabit soft, muddy bottoms, silty sands or algal beds and occur in the Atlantic and Pacific oceans between 40° N and S latitudes, usually from intertidal zones to 300 m [984’] depths.
C.T. SHIH

Branchiopoda
Branchiopoda, crustacean class comprising 3 orders: Anostraca (fairy shrimps), Notostraca (tadpole shrimps) and Diplostraca (clam shrimps and water fleas). Branchiopods are nearly all restricted to fresh water. Although most are microscopic, they represent an important element in the planktonic food chain. The name derives from structures on the legs that function as gills; the multiple appendages are used for swimming and filter feeding.
Branchiopods are characteristic of waters that frequently dry or freeze. Many branchiopods are parthenogenetic (males are rare; females are born pregnant). Habitats range from temporary pools in semiarid regions to large lakes and streams including the polar zones, they can quickly take advantage of the reconstitution of their habitat. Most branchiopods are pale and transparent; hemoglobin in the body gives some a rosy colour.
V. TUNNICLIFFE

Ostracoda
Ostracoda, seed shrimp, are microscopic, bean-shaped, bivalved crustaceans living at the bottom of fresh or marine bodies of water. Over 2,000 living species have been described worldwide. Habitats range from temporary pools in semiarid southern Saskatchewan to large lakes and streams transcontinentally, including the Arctic.

Ostracodes molt about eight times before adulthood; shells lack growth lines. Ostracodes serve as food for bottom-dwelling fish. They are important indicators of habitat changes. Their FOSSIL record is continuous and contains the largest number of fossil species (about 10,000) among crustaceans.
L.D. DELORME

Mystacocarida
Mystacocarida, a small class of minute crustaceans, usually less than 0.5 mm [25/65”] long. The elongated, cylindrical body is divided into head, bearing five pairs of appendages, and 11-segmented trunk. Each of the first 5 trunk segments bears a pair of appendages. The telson (last division of the body of a crustacean not considered a true segment which carries no appendages, a forked “tail” called caudal furca is often present) where they possesses a pair of claw-like tail branches. Sexes are separate.
These marine, benthic animals live between fine sand grains in intertidal and subtidal zones of the Atlantic and southwest Indian oceans. There are ten known species in two genera. Inadequate collection probably accounts for the absence of records in some parts of the World.
C.T. SHIH

Copepoda
Copepoda, small crustaceans, from 0.1-300 mm [<1/64-11 13/16”] long (usually 0.5-5.0 mm [1/64-13/64”]), some free-living, others parasitic during part of their lives. The body is divided into cephalothorax, bearing up to 11 pairs of appendages, and limbless abdomen. Parasitic copepods may deviate from the basic form. Copepods inhabit all aquatic environments (including the High Arctic), from deep oceans to temporary freshwater ponds, and may be planktonic, benthic or periphytonic (living attached to underwater surfaces).
Some are symbionts with other marine invertebrates. Parasitic species attach themselves externally or internally to fishes, marine mammals and marine invertebrates. About 8,000 species are known. Common species belong to the following genera: Calanus, Oithona (marine); Diaptomus, Cyclops (freshwater); and Caligus (parasitic).
C.T. SHIH

Branchiura
Branchiura, fish lice, are crustaceans with a depressed body (up to 3 cm [1 11/64”] long) divided into head, four-segmented thorax and un-segmented abdomen. A shield-shaped carapace covers head and part of thorax. The head and its five pairs of appendages are adapted for locomotion. Each thoracic segment bears a pair of legs modified to form a grasping organ in the male. The reduced, fin-shaped abdomen bears a pair of minute tail branches.
Sexes are separate. Fertilized eggs are laid on hard substrata. Young hatch as modified larvae or juveniles and soon attach to a host. Branchiura are external parasites of marine and freshwater fishes, but frequently leave the host to swim or crawl. About 130 species and four genera are known. At least ten species of genus Argulus are recorded from Atlantic and Pacific coastal waters and freshwater lakes.
C.T. SHIH

Cirripedia
Cirripedia, a group of crustaceans with over 900 species, including sessile and parasitic forms. They live from intertidal zones to the deep sea. Familiarly known as barnacles, most cirripedes cement themselves to hard surfaces and secrete protective calcareous plates. A goose barnacle has its plates perched on a flexible stalk.

Fine setae (bristles) filter out food particles from water currents generated by leg movements. Parasitic barnacles can be found on organisms ranging from jellyfish to whales. One species grows to 15 cm [2 29/32”]. V. TUNNICLIFFE

Malacostraca
Ancestral malacostracans lived in the oceans some 200 million years before dinosaurs roamed the earth. Today, they constitute nearly 75% of all crustacean species and include lobsters, crabs, crayfish and shrimp, some of the largest and most economically valuable invertebrate species. Most are still found in the marine environment, although a significant number of species have become established in fresh water, and the pill bugs (wood lice) are a fairly large terrestrial group.

A typical malacostracan has stalked eyes, 19 segments plus a telson (18 according to some experts, who do not consider the antennules to be associated with a true segment), a cylindrical body in which the thorax is partially fused with the head and covered with a carapace, and one or more pairs of anterior thoracic appendages modified for handling food. However, the malacostracans are a diverse group, with as many exceptions to this basic plan as followers of it.

In two of the largest orders, the Isopoda and Amphipoda, there is no carapace and the eyes are sessile (immovable) rather than stalked. In the isopods the body is generally flattened from back to belly (dorsoventrally); in the amphipods it is more commonly compressed laterally. Variations on the generalized plan seem limited only by the number of species and the variety of ecological niches occupied.

The various classification systems divide Malacostraca into 10-12 orders, of which only four have freshwater representatives in North America. Two-thirds of all malacostracans are in three major orders: Isopoda, Amphipoda and Decapoda. The remainder are mainly small, little-known species, but many are important parts of aquatic food chains for economically valuable animals. Mysids (opossum shrimp) are almost exclusively marine, but in the Great Lakes a freshwater mysid occasionally constitutes 80% of the diet of trout.

Another lesser-known group, the pelagic euphausiids (krill) often occur in large numbers in the marine environment; some species form a major part of the diet of large whales. It has been estimated that a blue whale can consume 2-3 tons of euphausiids in a single feeding. The euphausiids are also unusual in being bioluminescent, ie, capable of producing their own light from organs (photophores) located at several points on the body.

The order Isopoda includes some 4,000 species, most of which are marine and have dorsoventrally flattened bodies, sessile eyes and no carapace. The order also includes parasitic forms, freshwater species and the only large group of terrestrial crustaceans, the pill bugs, sow bugs or wood lice. The name pill bug comes from the animal's tendency to roll into a pill-shaped sphere for protection from physical danger or dehydration. The various species of pill bugs can be found beneath stones, matted vegetation and fallen trees, where they feed on algae, moss, bark and decaying organic matter.

The order Amphipoda comprises about 3,600 species, typically with sessile eyes, no carapace and a humpbacked, laterally compressed body. Most are marine but there are a significant number of freshwater species, and even a few that might be considered terrestrial.

Although some live in the ocean depths, amphipods are most common in shallow water and in the intertidal zone, where they are known as sand-hoppers or beach fleas. In the freshwater habitat they serve as transmitters of acanthocephalan parasites (intestinal worms) of ducks, and as an important food for aquatic birds and fish.

The order Decapoda contains 8,500 species, including the shrimp, crayfish, crabs and lobsters, and is by far the largest crustacean group. Most decapods are marine, but crayfish, some shrimp and a few other species are found in fresh water. In decapods, the first three pairs of thoracic appendages are modified as maxillipeds for the handling of food, leaving five pairs as walking legs (hence the name Decapoda, "ten feet").

Decapoda are divided into two major groups: the Natantia (swimmers), encompassing the shrimp, and the Reptantia (crawlers), including lobsters, crayfish and crabs. Decapods are among the most interesting invertebrates and, as a group, are certainly the most economically valuable.
D.E. AIKEN

Authors contributing to this article:
Author D.E. AIKEN, V. TUNNICLIFFE, C.T. SHIH, L.D. DELORME

Pepetj
Santo Domingo