The spines of the Slate-pencil Urchin are used in the sheilcraft trade in large quantities particularly in the Philippines (18). Sea urchins are the main grazing element in shallow waters and play an important role in controlling algal growth. Off Plymouth, U.K, at least one third of the rock is cleared of its seaweeds and encrusting algae each year by Echinus esculentus (see data sheet). Under certain conditions, urchin populations may reach pest proportions and cause overgrazing of kelp beds. This has occurred in some areas of Canada, possibly as a result of the decline in abundance of the lobster, Homarus americanus which is a known predator of the urchin Strongylocentrotus droebachiensis (32). The starfish Asterias of northern temperate seas may be a pest of commercial shellfish beds. The Crown-of-thorns Starfish Acanthaster planci of the Indo-west Pacific has gained a certain notoriety on account of the damage it can cause to coral reefs. It feeds on living corals, and in some areas where major population outbreaks have occurred up to 95 per cent destruction has been recorded. There has been considerable controversy over the causes of such plagues. It is not clear to what extent this species may have a natural fluctuating population, or whether the current outbreaks are due to human activities (see section on Cnidaria). No satisfactory methods have been found to control the outbreaks (22). THREATS TO SURVIVAL 1. Pollution Echinoderms are nearly all sensitive indicators of clean water and may be killed in large numbers by toxic oil. It has been found that a 001 per cent oil emulsion inactivated the tube-feet of urchins and was lethal within one hour of exposure (2). The developing eggs of Strongylocentrotus purpuratus have been found to be very sensitive to oil pollutants, particularly crude and heavy bunker oils (3). In a study of urchins on the coast of Italy it was found that there was a much lower density of Paracentrotus lividus and Arbacia lixula near Naples than further south in unpolluted areas, and that these two species were absent at Sorrento and Naples wherever the sewage loads were very high. Furthermore, the dry weight of urchins was found to be much smaller in the Bay of Naples than further south (). However, a study on the French coast near Marseilles revealed that domestic pollution could lead to huge population oscillations in sea urchins with densities as high as 'FOO per 10 m being reached (5). The same study illustrated the sensitivity of three species ( Paracentrotus lividus Arbacia lixula Sphaerechinus granularis ) to industrial pollution, which caused a reduction in the size of individuals as well as a decrease in numbers. The sea cucumbers Holothuria tubulosa and H. forskali were found to be even more sensitive and disappeared in areas of highpollution. This is probably due to their feeding method since they ingest sediment and pollutants tend to become concentrated in this part of the benthos (5). Two populations of highly deformed sea urchins of the species Tripneustes cf. gratilla were found in the vicinity of a combined power and desalination plant in the Gulf of Eilat (Aqaba), Red Sea, an area which is highly polluted by thermohaline and heavy metal ion effluents. More than 60 per cent of the urchins showed irregular bulging of the aboral half of the test and under unpolluted conditions in aquaria the growth rate of such urchins was very slow (14). A large concentration of Lytechinus variegatus urchins were found in Cartagena Bay, Colombia with skeletal deformations which were probably caused by oil and waste water pollution (15). Studies following the wrecking of the 'Torrey Canyon' tanker showed that oil had affected a number of echinoderms including the starfish Marthasterias glacialis the burrowing heart urchin Echinocardium cordatum and Echinus esculentus although some species (eg. the starfish Asterias rubens and the brittle star Acrocnida brachiata ) appeared to be little affected (23). The effects of pollution could be particularly serious in places such as the Red Sea where echinoderm endemicity has been estimated at as high as 12 per cent (2). 2. Exploitation Although small scale artisan fisheries probably have little effect 541