to be in any danger of extinction, since they generally have wide distributions and deep, inaccessible populations which can act as reservoirs. However, several of man's activities are injurious to cnidarians. Certain species have shown a decline in U.K. waters in recent years, including the sea anemones Adamsia carciniopados and Aiptasia mutabilis the jellyfish Aurelia aurita some hydroids, gorgonian sea fans and cup corals. In many cases it is difficult to know if such species are truly declining and, if so, what the causal factors are (2). Amphianthus dohrnii, a small anemone which attaches itself to gorgonians and hydroids, was formerly common in the English Channel and Mediterranean, but now seems to be absent from the latter and very scarce in the former (2,5). In British waters it used to occur on the sea fan Eunicella verrucosa which itself has undergone a decline in abundance in some areas recently (see review). The high value of precious and semi-precious corals has led to over-exploitation in many areas (see reviews). The scouring of the sea bed by trawls, oyster dredges and other fishing gear probably affects populations of sea fans, sea pens, alcyonarians and sea anemones, but few data are available on this subject. A number of cnidarians have restricted distributions and may be vulnerable if they occur in intertidal, marsh or lagoon habitats. The sea anemone Nematostella vectensis is one example (see review). Another is Edwardsia ivelli known only from its type locality in Sussex, U.K, a brackish lagoon which could be affected by development. Further research may reveal such species to be more common subtidally, but until this is known care must be taken to protect known littoral populations (5). Freshwater cnidarians are likely to be vulnerable to pollution. In the U.S.A, both thermal and chemical pollution have been shown to have dramatic effects on the freshwater hydrozoan Cordylophora lacustris, although there is no evidence that this species is currently declining (18). The greatest documented threat to cnidarians is the destruction of coral reefs, which is occurring worldwide on an unprecedented scale. The literature on this subject is considerable and no attempt has been made to summarize it here. The LUCN Coral Reef Specialist Group has been reviewing the overall situation and several countries have documented the status of their reefs at the national level (60,61,62). The greatest destroyers of stony corals may well be natural catastrophes, such as storms, hurricanes and rapid drops in temperature, and it has been shown that given time, reefs have the ability to recover from either natural or human-induced damage (8,26,33,36). Several studies have shown coral reefs to have a natural ecological residence (31,32), and that moderate storms actually contribute to the high species diversity of corals on a reef (3). Regeneration rates for reefs and growth rates for coral appear to be highly variable and probably depend on a wide range of factors. It was originally thought that corals had very slow growth rates and this may be true of some species. Studies of dynamite damaged reefs in the Philippines indicated a recovery time of 38 years to 50 per cent area coral cover (16). Pontes compressa was shown to have a growth rate of 2 mm a year in the field (71) although rates of up to 20 mm a year have been recorded in the laboratory (70). A growth rate of 6 mm a year has been recorded for Pocillopora damicornis in the laboratory (70) although rates of 2-4 cm a year have been estimated for P. meandrina (69). Other species grow quite rapidly, and damaged reefs have been recolonized within two to three years (53). In some of the faster-growing species, such as Acropora cervicornis which break easily, the fragments re-anchor and may have growth rates of up to 3-5 cm or even 12 cm a year (9,21,26). Over the past decade much publicity has been given to the problem of Acanthaster r planci the Crown-of-thorns Starfish, (see Echinodermata). The extent to which outbreaks of this species have damaged coral reefs has been extensively debated. Certain areas of the Great Barrier Reef are being subjected to renewed 23