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The effect was studied of the red tide dinoflagellate Alexandrium tamarense (= Gonyaulax excavata, G. tamarensis) on the behaviour and growth of the tintinnid ciliate Favella ehrenbergii. Six clones of A. tamarense with different levels of PSP toxin content were used. A. tamarense produces substances lethal to F. ehrenbergjj. The algae are ingested by the ciliate, which is only affected by exudates in the medium. The exudate acts on the cell membrane and induces ciliary reversals resulting in continuous backwards swimming. After some time, ciliates swell and subsequently lyse. The exudate is very labile and the toxic effect of filtrates disappears within a few hours. In batch cultures of A. tamarense, the toxicity of the medium depends on the growth phase. The highest level of toxicity is found in the period following exponential growth; this suggests that cells especially exudate toxins when growth becomes limited. Indirect evidence suggests that the toxic effect on ciliates is caused by PSP toxins. The growth response of F. ehrenbergii on A. tamarense is clone-specific. The clliate will grow when fed low concentrations of the Wh 7 clone of A. tamarense at generation times of 26 to 32 h. Similar rate constants are obtained with similar concentrations (in terms of volumes) of the non-toxic dinoflagellate Heterocapsa triquetra. Favella ehrenbergii will not grow when fed the Ply 173a clone of A. tamarense at any concentration.

The red tide dinoflagellate Alexandrmm tamarense: effects on behaviour and growth of a tintinnid ciliate