Difference between revisions of "Crustacean" - New World Encyclopedia

From New World Encyclopedia
(added approved and images ok tags)
Line 1: Line 1:
{{Contracted}}
+
{{Approved}}{{Images OK}}{{Submitted}}{{Contracted}}
 
{{Taxobox_begin | color = pink | name = Crustaceans}}
 
{{Taxobox_begin | color = pink | name = Crustaceans}}
 
{{Taxobox_image | image = [[Image:Abludomelita obtusata.jpg|200px]]|Abludomelita obtusata | caption = ''Abludomelita obtusata'', an isopod}}
 
{{Taxobox_image | image = [[Image:Abludomelita obtusata.jpg|200px]]|Abludomelita obtusata | caption = ''Abludomelita obtusata'', an isopod}}
Line 48: Line 48:
 
[[Image:Shrimp nauplius.jpg|left|240px|The nauplius larva of a dendrobranchiate]]
 
[[Image:Shrimp nauplius.jpg|left|240px|The nauplius larva of a dendrobranchiate]]
 
Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius.
 
Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius.
In common with other [[arthropod]]s, crustaceans have a stiff exoskeleton which must be shed to allow the animal to grow (ecdysis). Various parts of the exoskeleton may be fused together; this is particularly noticeable in the carapace, the thick dorsal shield seen on many crustaceans. Crustacean appendages are typically ''biramous'', meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is ''uniramous''. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthoropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of [[trilobite]] limbs (Hughes 2003).
+
In common with other [[arthropod]]s, crustaceans have a stiff exoskeleton which must be shed to allow the animal to grow (ecdysis). Various parts of the exoskeleton may be fused together; this is particularly noticeable in the carapace, the thick dorsal shield seen on many crustaceans. Crustacean appendages are typically ''biramous'', meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is ''uniramous''. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of [[trilobite]] limbs (Hughes 2003).
  
 
[[Image:Porcellio scaber.jpg|thumb|240px|''Porcellio scaber'', the common rough woodlouse, a terrestrial crustacean]]
 
[[Image:Porcellio scaber.jpg|thumb|240px|''Porcellio scaber'', the common rough woodlouse, a terrestrial crustacean]]

Revision as of 02:37, 3 December 2006

Crustaceans
Abludomelita obtusata.jpg
Abludomelita obtusata, an isopod
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea

Brünnich, 1772

Classes & Subclasses

Class Branchiopoda

Subclass Phyllopoda
Subclass Sarsostraca

Class Remipedia
Class Cephalocarida
Class Maxillopoda

Subclass Thecostraca
Subclass Tantulocarida
Subclass Branchiura
Subclass Pentastomida
Subclass Mystacocarida
Subclass Copepoda

Class Ostracoda

Subclass Myodocopa
Subclass Podocopa

Class Malacostraca

Subclass Phyllocarida
Subclass Hoplocarida
Subclass Eumalacostraca

Crustaceans (subphylum Crustacea) are the group of arthropods (phylum Arthropoda) that includes the familiar crabs, lobsters, shrimps, barnacles, copepods, krill, water fleas, and crayfish. Crustaceans are characterized by having branched (biramous) appendages, an exoskeleton made up of chitin and calcium, two pairs of antennae that extend in front of the mouth, and paired appendages that act like jaws, with three pairs of biting mouthparts. They share with other arthropods the possession of a segmented body, a pair of jointed appendages on each segment, and a hard exoskeleton that must be periodically shed for growth.

Crustacea is the only subphylum of arthropoda to contain mostly aquatic species (Towle 1989). Most of these are marine, such as shrimps and barnicles. Indeed, crustacea is the largest group of marine arthropods. However, Crustacea also includes many freshwater forms, such as crayfish. Terrestrial forms, including sand fleas, land crabs, woodlice, and sow bugs, generally need humid environments.

Crustaceans offer important economic, ecological, and aesthetic values. Some larger crustaceans are a major food commodity, including shrimp, lobsters, and crabs. Even smaller crustaceans can be integral to food webs, such as the case of krill and copepods, which can make up a considerable portion of plankton, consuming phytoplankton and being consumed by whales and fish. Some crustaceans are detrivores, consuming dead organisms and recycling the nutrients. Beyond these values, crustaceans can provide joy to humans in terms of their diverse colors and forms, and even their taste and texture when prepared as food.

There are approximately 52,000 described species of crustaceans (Martin and Davis 2001). The majority are motile, moving about independently, although a few taxa are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua), and adult barnacles live a sessile life—they are attached head-first to the substrate and cannot move independently.

The scientific study of crustaceans is known as carcinology. Other names for carcinology are malacostracology, crustaceology, and crustalogy, and a scientist who works in carcinology is a carcinologist, crustaceologist, or crustalogist.

Structure of crustaceans

Crustaceans have three distinct body parts: head, thorax, and abdomen (or pleon), although the head and thorax may fuse to form a cephalothorax. The head bears two pairs of antennae, one pair of compound eyes and three pairs of mouthparts. The thorax and pleon bear a number of lateral appendages, including the gills, and the tail ends with a telson (last division of the body of a crustacean, but not a true segment).

Smaller crustaceans respire through their body surface by diffusion (Pirow et al. 1999), and larger crustaceans respire with gills or, as shown by Birgus latro, with abdominal lungs (Farrelly and Greenaway 2005). Both systems (diffusion and gills) were being used by various crustaceans as early as the Middle Cambrian (Vannier et al. 1997).

The nauplius larva of a dendrobranchiate

Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius. In common with other arthropods, crustaceans have a stiff exoskeleton which must be shed to allow the animal to grow (ecdysis). Various parts of the exoskeleton may be fused together; this is particularly noticeable in the carapace, the thick dorsal shield seen on many crustaceans. Crustacean appendages are typically biramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is uniramous. There is some doubt whether this is an advanced state, as had been traditionally assumed, or whether it may be a primitive state, with the branching of the limbs being lost in all extant arthropod groups except the crustaceans. One piece of evidence supporting the latter view is the biramous nature of trilobite limbs (Hughes 2003).

File:Porcellio scaber.jpg
Porcellio scaber, the common rough woodlouse, a terrestrial crustacean

Although a few are hermaphroditic, most crustaceans have separate sexes, which are 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. Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such as woodlice lay their eggs on land, albeit in damp conditions. In many decapods, the eggs are retained by the females until they hatch into free-swimming larvae.

Taxonomy

Pollicipes polymerus, the gooseneck barnacle

Although the classification of crustaceans has been quite variable, the system used by Martin & Davis 2001) is considered to be authoritative, and largely supersedes earlier works.

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 their own 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.

The exact relationships of the Crustacea to other taxa are not yet entirely clear. Under the Pancrustacea hypothesis (Zrzavý and Štys 1997), Crustacea and Hexapoda (insects and allies) are sister groups. Studies using DNA sequences tend to show a paraphyletic Crustacea, with the insects (but not necessarily other hexapods) nested within that clade.

Crustaceans are sometimes viewed in terms of two groups. One, the entomostracans,includes the smaller species, such as copepods, barnacles, and fairy shrimp. The second group, the malacostaceans, include the larger forms, such as lobsters, crabs, and shrimp.

Geological history

Glyphea pseudastacus, a fossil glypheoid

Those crustaceans that have hard exoskeletons reinforced with calcium carbonate, such as crabs and lobsters, tend to preserve well as fossils, but many crustaceans have only thin exoskeletons. Most of the fossils known are from coral reef or shallow sea-floor environments, but many crustaceans live in open seas, on deep sea-floors, or in burrows. Crustaceans tend, therefore, to be rarer in the fossil record than trilobites. Some crustaceans are reasonably common in Cretaceous and Cenozoic rocks, but barnacles have a particularly poor fossil record, with very few specimens from before the Mesozoic era.

The Late Jurassic lithographic limestone of Solnhofen, Bavaria, which are famous as the home of Archaeopteryx, are relatively rich in decapod crustaceans (five pairs of legs), such as Eryon (an eryonoid), Aeger (a prawn) or Pseudastacus (a lobster). The "lobster bed" of the Greensand formation from the Cretaceous period, which occurs at Atherfield on the Isle of Wight, contains many well preserved examples of the small glypheoid lobster Mecochirus magna. Crabs have been found at a number of sites, such as the Cretacoues Gault clay and the Eocene London clay.

References
ISBN links support NWE through referral fees

  • Farrelly, C. A., and P. Greenway. 2005. The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (Coenobita and Birgus): gills, branchiostegal lungs and abdominal lungs. Arthropod Structure and Development 34(1): 63–87.
  • Hughes, N. C. 2003. Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology 43(1):185-206.
  • Martin, J. W., and G. E. Davis. 2001. An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County.
  • Schram, F. 1986. Crustacea. Oxford University Press. ISBN 0-19-503742-1.
  • Towle, A. 1989. Modern Biology. Austin: Holt, Rinehart and Winston. ISBN 0-03-013919-8.
  • Vannier, J., M. Williams, and D. J. Siveter. 1997. The Cambrian origin of the circulatory system of crustaceans. Lethaia 30(3):169–184.
  • Zrzavý, J. and P. Štys. 1997. The basic body plan of arthropods: insights from evolutionary morphology and developmental biology. Journal of Evolutionary Biology 10:353-367.


Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.