When red tide occurrs, various marine animals are affected by the red tide organism. In last three decades, Chattonella, Gymnodinium and Heterosigma were representative nuisance red tide genera off the Japanese coast and these organisms have frequently caused serious damages mainly to cultured finfish (Honjo 1994). These harmful genera damage the epithelial tissues of the fish gills, subsequently depress the gas exchange rate at gill surface. In H. circularisquama red tide, however, any negative responses and dying of wild and farmed finfish has never recorded. In some cases, caged fishes had normally swimming in the assemblages of H. circularisquama cells (>10⁷ cells/L).
H. circularisquama red tide kills mainly bivalve molluscs; pearl oyster Pinctada fucata, manila clam Ruditapes philippinarum, pacific oyster Crassostrea gigas, blue mussel Mytilus galloprovincialis etc (see below).

Dead pearl oysters due to Heterocapsa red tide(Ago Bay, 1992)
 

Dead pacific oysters due to Heterocapsa red tide(Hiroshima Bay, 1995)

Dead blue mussel due to Heterocapsa red tide (Hiroshima Bay, 1995)

Farming ground of manila clam destroyed by red tide(Hiroshima Bay, 1998)

Blue crab and grass puffer in red tide area (Hiroshima Bay, 1998)

    Further, several commercially important gastropods: abalones Hariotis discus, top shell Turbo also affected by the H. circularisquama red tide. However, other marine animals i.e. crustacean, vertebrates are not affected by the H. circularisquama red tide. As the results, harmful dinoflagellate which kills only bivalve and gastropod has not been reported.
 

2) Effects of Heterocapsa circularisquama on bivalve physiology

    Laboratory experiment (Nagai et al. 1996) demonstrated that pearl oysters spat exposed to >10⁶ H. circularisquama cells/L contracted their mantles markedly, closed their valves, and paralyzed, in some cases, the heartbeat of the juveniles completely ceased, and then they died. These negative symptoms were closely related to those of previous field observations (Matsuyama et al. 1996). Mortality increased markedly with the cell density of up to 2×10⁷ H. circularisquama cells/L. Heart beat arrest or termination were occurred in both juvenile and adult pearl oysters above 10⁶ H. circularisquama cells/L exposure within 30 minutes (Kiyohito NAGAI, per. comm.). In addition to lethal effect, the spat of pearl oyster significantly reduced their filtration rate when exposed to 5×10⁴ H. circularisquama cells/L, and markedly decreased with increasing H. circularisquama cell densities (Matsuyama et al. 1997). This findings implicated that bivalves considerably ceased filtering activities at only one percent of red tide cell density.

Relationships between H. circularisquama cell densities and clearance rates of the several bivalve molluscs
 

3) Toxic nature of Heterocapsa circularisquama

    In the past, harmful red tide organisms have frequently caused serious damages mainly to cultured finfish, with bivalves to be a minor groups of damaged animals. Previous mortalities of bivalves have been recorded only after the red tides caused by oxygen depletion in association with sulfide production as a result of disintegration of red tides. In the previous surveys, major death of bivalve due to H. circularisquama was observed early to middle period of the red tide outbreaks when dissolved oxygen was still near saturated (Matsuyama et al. 1996). It was also observed that mortality of wild and cultured fish was not the case during the red tide. From these facts, H. circularisquama has a toxicity specific for bivalves molluscs.
    According to previous study, the mussel M. galloprovincialis significantly reduced filtration rate at 5×10⁴H. circularisquama cells/L presence, accompanied with contraction of mantle edges and exhalant siphons (Matsuyama et al. 1997). However, no inhibitory effects on the filtration rate of mussels were observed with the filtrate of a dense H. circularisquama culture (10⁸ cells/L). Therefore, it seems likely that H. circularisquama cells do not produce extracellular metabolites which repress bivalve clearance rates apart from in their cell walls. Furthermore, some physical treatment on intact cell of H. circularisquama markedly inactivated the toxicity on filtration rate. Supplemental experiment demonstrated that toxic nature of H. circularisquama localized in cell surface (Matsuyama et al. 1997). The results of some treatment on H. circularisquama cells by surfacant and enzyme indicated that labile protein-like agent is probably a toxic nature of H. circularisquama (Matsuyama et al. 1997) which causes sustained disturbance of homeostasis and consequent death of shellfish life.

Effect of physical treatment on the toxicity of H. circularisquama
 

Effect of chemical treatment on the toxicity of H. circularisquama

    Otherwise, there is a possible concern to the human illness responsible for the consumption of bivalve which accumulated the H. circularisquama cells. However, No human poisoning and shellfish poisoning have been reported during the bloom of H. circularisquama. Direct HPLC analysis showed that neither PSP toxins nor DSP toxins were detected in the cells of H. circularisquama (S. Sakamoto and T. Suzuki, pers. comm.). Mouse bioassay on the oysters collected from red tide area showed no toxicity. On the basis of previous studies obtained until now, it is concluded that there are no affair of human illness involving with H. circularisquama bloom. Threfore, toxic mechanism of H. circularisquama is greatly different from those of shellfish poisoning causing genella Alexandrium, Dinophysis and Pseudo-nitzschia spp..
 

4) Possible ecological meaning of toxicity on animals

    In general, some phytoplankton produce "phycotoxins" to prevent from predation and exclude the competitor in the ecosystems. Protozoa, zooplankton, nekton (jellyfish), and filter-feeders (bivalve, ascidian) are regarded as the major active predator on dinoflagellate H. circularisquama. Recent studies revealed that H. circularisquama inhibit not only filter-feeder physiology but also their active predator and competitor such as protozoa (Kamiyama and Arima 1997) and mixotrophic dinoflagellate (Uchida et al. 1995). Thus, the inhibitory effect of H. circularisquama upon their active predator and competitor might, itself, be of great advantage for the development of red tide　
 

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