freshwater and terrestrial isopods have evolved directly from marine ancestors and their current distributions provide useful data on early geological events. The suborder Phreatoicidea, for example, is a very ancient group, fossil phreatoicids from Middle Pennsylvanian deposits in Illinois dating back some 120 million years (5). The phreatoicid fauna of Great Lake in Tasmania is also of interest with two endemic genera ( Mesacanthotelson and Onchotelson ) and nine endemic species (f). The Ostracoda have the most extensive fossil record of any crustacean group, stretching back over 300 million years. Certain oil-bearing strata are rich in certain ostracod fossils which can be used as indicator species when searching for new oilfields (1). The main value of crustaceans to man is as a food resource and they are a major element in both commercial and subsistence fisheries. Of greatest economic importance are the prawn and shrimp fisheries, the world catch of which totalled over 15 million tonnes in 1980 (12,13). FAO has produced a catalogue of almost 350 species of shrimps and prawns that are used by man or are considered to be of potential commercial value throughout the world (58). Over 20 major types of crab fisheries are listed by FAO, and the 1980 world production totalled over 800 000 tonnes (12). The lobster is probably the most valuable and popular crustacean and the world lobster catch (including both clawed and spiny lobsters) totalled over 150 000 tonnes in 1980 (12). Spiny lobsters of the family Palinuridae are found in all major tropical and temperate oceans and support subsistence, large-scale commercial or recreational fisheries in most areas. The 1980 catch totalled nearly 70 000 tonnes, 70 per cent coming from the Caribbean, eastern Indian Ocean and south-east Atlantic (12,15). The recreational harvest of spiny lobster used to be insignificant but, with the growth in popularity of SCUBA and skin diving, has become important in several areas (17). Although lobsters are now considered a luxury food in most countries, in the past 'they were taken for use as fertilizer, bait and as a subsistence food (16). The clawed lobsters, Homarus americanus in the west Atlantic and H. gammarus and Nephrops norvegicus from the eastern Atlantic and Mediterranean, have been exploited intensively for centuries (16). A wide range of other marine crustaceans are used as food by man, including barnacles which are eaten in some countries including the Philippines k9) and Spain (50). The potential value of the krill stocks of the Southern Oceans has recently become a major issue. Krill fishing is still considered a pilot-scale activity, but the swarms that appear regularly in the Antarctic summer make this species easy to catch. Krill catches in 1980 totalled over it20 000 tonnes, of which the U.S.S.R. was responsible for 388 000 tonnes (12). Estimates of sustainable yield are extremely variable, ranging from a few million to tens of millions of tonnes. Krill is now considered unlikely to have any major value as a human food resource, but krill protein concentrate could have potential as animal feedstock although other species currently provide a more economical source of fish meal (10,13,67). Freshwater crustaceans accounted for 82 787 tonnes of the 1980 world crustacean catch (12). In Europe crayfish have been consumed in large quantities since Roman times. They are now considered a luxury food in most of the western world but in regions such as New Guinea and Madagascar they are an important source of quality protein (2). The U.S.A. is probably the major crayfish producer. The industry is centred on southern Louisiana where in recent years 30-60 million pounds (13-27 thousand tonnes) of Procambarus spp. (mainly P. clarkii) have been taken annually from natural swamps and marshes and about 50 000 acres (20 235 ha) of culture ponds. The wild harvest accounts for about 60 per cent of the total, and varies with the amount of sustained spring flooding (2,18,19). Pacifastacus leniusculus is exploited commercially in California, Oregon and Washington and Orconectes rusticus and O. virilis in Wisconsin. Sport fisheries for food crayfish 263