Crustacean

Crustaceans
Hyalella azteca
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea
Classes
Remipedia Cephalocarida Branchiopoda Ostracoda Maxillopoda Malacostraca

The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum. They include various familiar animals, such as lobsters, crabs, shrimp and barnacles. They are variously found in marine and freshwater, with a few terrestrial members (such as woodlice).

Structure of Crustaceans

Crustaceans have 3 distinct body parts, head, thorax, and abdomen. They have two pairs of antennae on the head, compound eyes, three pairs of mouthparts and a telson. Smaller crustaceans respire through body surface by diffusion and larger crustaceans respire by gills. Crustaceans typically have a thick carapace the dorsal side of their body. Their appendages are typically biramous, including the second pair of antennae (but not the first).

Reproduction

Most crustaceans have separate sexes and are distinguished by apendages on the abdomen called swimmerets. The first and sometimes the second pair of swimmerets are larger comparatively to the female. Terrestrial crabs mate seasonal and return to the sea to breed. Female crabs eggs are still retained by the females until hatch. When the eggs hatch, they hatch into free-swimming larvae.

Taxonomy

The formal classification of crustaceans varies somewhat. In general, because of the large number of species, taxonomists have made extensive use of subordinate taxonomic categories (suborders, superfamilies and so forth), and the status of different groupings is frequently controversial; this can make taxonomic references hard to follow. Evolutionary relationships between the different groups are not entirely clear, making the exact definition of larger groups difficult.

Some authorities have treated the entire group of crustaceans as a class, in which case the classes escribed in the taxonomic table are treated as subclasses. Other authors omit some or all of the classes listed here, in which case some of the groups given here as subclasses are promoted to full class rank. This listings given at the end of this article and in the summary table at the right are those recommended by ITIS, and as such probably represent a consensus of modern opinion. However good practice in describing crustaceans must clearly be to include descriptions at several taxonomic levels, to ensure that readers can link the information to others' schemes.

Less formally, we can state that the most important groups of crustaceans are barnacles (infraorder Cirripedia), branchiopods, copepods and Malacostraca (crabs, lobsters, shrimps and krill). There are around 1,220 barnacle species, 1,000 branchiopods, 13,000 copepods, and 30,000 Malacostraca.

Classes, subclasses and infraclasses of crustaceans

Subphylum Crustacea

• Class Remipedia
  • Order Enantiopoda
  • Order Nectiopoda
• Class Cephalocarida
  • Order Brachypoda
• Class Branchiopoda
  • Subclass Phyllopoda
  • Subclass Sarsostraca
• Class Ostracoda
  • Order Metacopina
  • Subclass Myodocopa
  • Subclass Podocopa
• Class Maxillopoda
  • Subclass Mystacocarida
  • Subclass Copepoda
  • Subclass Branchiura
  • Subclass Pentastomida
  • Subclass Tantulocarida
  • Subclass Thecostraca
    • Infraclass Cirripedia
• Class Malacostraca
  • Subclass Eumalacostraca
  • Subclass Hoplocarida
  • Subclass Phyllocarida

External links

• ITIS Taxonomy Taxonomic Serial No.: 83677
• Crustacea.net, an online resource on the biology of crustaceans