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caused by eating shellfish, and ciguatera by eating tropical fish. The toxins involved evoke a variety ofgastrointestinal and neurological symptoms in humans (Table l), but rarely affect the nervous systemsof fish or shellfish. Another group of toxins, called ichthyotoxins, selectively kill fish by inhibitingtheir respiration. On a global scale, close to 2000 cases of human poisoning through fish or shellfishconsumption occur each year, and economic damage through reduced local consumption and reducedexport of seafood products can be considerable.Generally speaking, the numbers and intensity of algal blooms seem to be on the rise and theirgeographic extent seems to be spreading. While toxic plankton blooms appear regularly on a seasonalbasis in temperate waters of Europe, North America and Japan (TAYLOR and SELIGER, 1979), theyare still relatively rare in tropical regions. As a result, the Indo-West Pacific countries have littleknowledge of how to deal with this environmental and economic threat. The present paper illustratesand discusses several potentially hazardous organisms that have been recognized in these tropicalwaters. Clinical symptoms of various types of fish and shellfish poisoning are summarized to assisttheir diagnosis by medical practitioners, and illustrations of the plankton organisms are provided toimprove identification by local plankton workers. Blooms of the toxic species, in particular, need tobe carefully monitored, and fish and shellfish products from affected areas should be tested for toxinsby public health officials who should, if necessary, issue warnings promptly and effectively.DESCRIPTION OF RED-TIDE ORGANISMSNowhere is there a more important need for correct taxonomic identification of planktonorganisms than in the study of the toxic species. Red tides are often monospecific blooms, and understandingthe autecology of the constituent species thus becomes crucial not only to understanding thebloom event but also when deciding on possible measures for its control. Some dinoflagellates (e.g.Protogonyaulax) produce benthic cysts that can seed further blooms, and the monitoring for thesespecies must also take into account benthic cyst populations and sedimentary processes. Threecategories of ‘red tide organisms’ are distinguished: species that produce mostly harmless waterdiscolourations; species non-toxic to man but harmful to fish and invertebrates by damaging orclogging their gills; and species which produce potent toxins that can find their way through the foodchain to man.HARMLESS WATER DISCOLOURATIONSThe most common red tide organisms in the Indian Ocean, the blue-green alga Trichodesmiumand the dinoflagellate Noctiluca, produce mostly harmless water discolourations. Only in exceptionalcases do such plankton blooms cause fish kills in sheltered bays due to the generation of anoxicconditions.Trichodesmium: This tropical blue-green alga produces seasonal (February- April) water blooms inthe Andaman, Arabian, Java, Banda, Arafura and Coral Seas. These appear as yellow-grey (earlybloom) or reddish-brown (late bloom) coloured windrows, occupying up to 40,000 square kilometers.The long filaments mass together to form raft-like (T. erythraeum EHR., Fig. la,b) or radiating orbundle-like aggregations (T. thiebautii GOMONT, Fig. lc,d). At the start of the bloom the filamentsusually appear throughout the water column, but during late bloom stages the strong gas vacuolescause a massive rise of the algae to the surface layers. Differentiated cells within the centre of thecolony are capable of fixing atmospheric nitrogen, which allows the algae to thrive undernutrient-impoverished oceanic conditions where they readily outcompete other phytoplankton. Waveaction can break up the bundles and inactivate the central nitrogenase enzyme (CARPENTER andPRICE, 1976), which is why calm seas are a prerequisite for Trichodesmium blooms. The alga canbe a nuisance to swimmers, but harmful effects on fish are seldom observed (DEVASSY et al., 1978)except in sheltered bays where the decaying bloom may generate anoxic conditions that can causeindiscriminate kills of fish and other marine fauna (e.g. CHACKO, 1942). An unusual mass death ofcorals caused by the decomposition of masses of Trichodesmium driven ashore by the wind has beenrecorded from New Caledonia (BAAS BECKING, 1951).Trichodesmium red tides (“sea sawdust”)106