Abstract :
[en] Using a specific and quantitative enzymatic-identification procedure, the polysaccharide chitin has been found in a wide range of animal species. It is used by protozoans, mainly ciliates, to build up cyst walls. It also constitutes the bulk of the stalks or stems of most hydrozoan colonies, but it is rarely produced by Scyphozoa (jellyfishes) and Anthozoa, and is absent in sponges. Chitin is the main structural polysaccharide of most invertebrates belonging to the Protostomia. Arthropods are the best known and most important chitin-producing animals ; the dry organic matter of their cuticles can contain up to 80% chitin. Besides the arthropods, relatively large amounts of chitin may be found in the setae of annelids (from 20 to 38% of dry weight), in the skeleton of the colonies of Bryozoa and in the shells and other structures (jaws, radulae, gastric shield) of many mollusc species (up to 7% of the dry organic matter in gastropods and bivalve shells, and up to 26% in cephalopods). Chitin is only absent in free and parasitic flatworms (Platyhelminthes), nemerteans, sipunculids and leeches. In some other groups, such as nematodes and rotifers, chitin is present only in the egg envelopes. Chitin synthesis has never been observed in echinoderms or vertebrates, but the tubes of some Pogonophora contain 33% chitin, while tunicates secrete a chitinous peritrophic membrane. From an ecological point of view, besides crustaceans and molluscs, marine benthic animals are a rich source of chitin. Despite their small size, bryozoan and hydrozoan colonies yield a large biomass with relatively substantial amounts of chitin. Some bryozoans play a role in the epuration of fresh water and produce a considerable biomass of chitin-containing colonies as well. The proportion of "free" chitin, i.e., not bound to other organic molecules, is generally low (less than 10%), although it can account for 80% of the total chitin in mollusc shells.
