Crustaceans (Crustacea) form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 12.5 ft (3.8 m) and a mass of 44 lb (20 kg). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by the nauplius form of the larvae.
Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles). The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as Triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as widely fished, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist who works in carcinology is a carcinologist.
From the smallest krill to the largest horseshoe crab, Crustaceans are an integral part to the ecological environment where they live. The subclass copepoda are the most noteworthy of all crustaceans when it comes to ecological importance. There are literally billions upon billions of these exist in all the oceans of the world. They are the basis of marine life as we know it. More fish feed on these copepods than on any other food source in every ocean and most bodies of water. Not only that, but these free swimming copepods are conduits that transform microscopic marine vegetation into a food source that is usable by animals much larger than themselves. Not only are these copepods the bottom link of the aquatic food chain, but uniquely can also be some of the most vicious predators. They have been know to literrally eat a fish from the inside out. Thousands, even millions of them will work together to bring down an organism hundred of thousands of times larger than them.

Other crustaceans also have ecological effects. Decapods have become a food source for both mammals and birds. Not only that, but these larger crustaceans have become predators in their aquatic environments. There are filtering crustaceans, such as barnacles, which are the basis of many food chains, and there are destructive crustaceans, such as those in the subclass cirripedia that burrow and act for both abiotic and biotic destruction. Basically, because of the thousands of species and numerous diverse habitats, crustaceans serve varied, and often crucial, positions in food chains.

First some special biological features of crustaceans are described, including morphology and anatomy (segmentation, extremities, cuticle), respiration, circulation, excretion and osmotic regulation, reproduction and life history strategies (precopula, breeding, larvae, moulting), distribution and abundance, and modes of life (living in the benthos or plankton). This section is followed by a detailed review of results at different methodological levels, particularly focusing on freshwater amphipods, isopods and Cladocerans (e.g. as components of lake zooplankton). One basis for an ecological assessment is knowledge of ecological preferences and distribution of crustaceans under different environmental conditions. Although information on the abundance of species is essential, the relevance of reproduction and population dynamics is emphasized. Subsequently, the development of biotic indices (including the Gammarus/Asellus ratio or the saprobic index involving crustaceans) is discussed. To understand ecological effects of aquatic pollution, information on bioaccumulation of substances is crucial. The applicability of feeding, growth, behaviour (e.g. precopula) and drift as meaningful ecological indicators (or biomarkers) is then discussed at length. Finally, information from field studies on survival and reproduction is presented, regarding aqueous-phase toxicity, sediment toxicity and response and recovery of lake zooplankton communities exposed to acidification and metals. Some details are illustrated in a case study focusing on accumulation strategies, toxicokinetic models and detoxification mechanisms in gammarid amphipods.

Crustaceans are frequently used as bioindicators and biomonitors in various aquatic systems. One reason is that they are a very successful group of animals, distributed in a number of different habitats including marine, terrestrial and freshwater environments. They are thus interesting candidates for comparative investigations.

http://www.personal.psu.edu/jri113/Impacts%20on%20Human%20Society.htm http://en.wikipedia.org/wiki/Crustacean http://www.austincc.edu/sziser/Biol%201413/LectureNotes/lnexamIII/Arthropods-Crustacea.pdf