Toxic effect of dinoflagellate Alexandrium species on Cladocera Moina mongolica Daday

The main objective of this study was to investigate toxic effects of the dinoflagellate Alexandrium species, on cladocera Moina mongolica in the laboratory. Ten strains of Alexandrium species, including Alexandrium tamarense (AT-6, ATHK, ATCI02, ATCI03 and AT5-1), A. catenella (ACDH), A. affine (AC-1 and AS-1), A. lusitanicum and A. minutum were tested. The results showed that A. tamarense (ATHK, ATCI02 and AT5-1) and A. affine (AC-1 and AS-1), but not A. tamarense (AT-6 and ATCI02), A. catenella (ACDH), A. lusitanicum and A. minutum had significantly negative effects on the survival of M. mongolica; when exposed to these ten strains of Alexandrium species in densities of 3 000 cells/cm³ for 7 d, respecyively. M. mongolica could feed on A. tamarense (AT-6, ATHK, ATCI02, ATCI03 and AT5-1), A. catenella, A. lusitanicum and A. minutum, but exhibited little or no grazing on A. affine (AC-1 and AS-1), based on the changes in gut pigment after exposure to alga for 1 and 12 h. A. affine AC-1, which had the strongest toxicity on the survival of M. mongolica, was chosen to further study the negative effects on M. mongolica. The results showed that the effect of A. affine AC-1 on the survival of M. mongolica was density-dependent, and its lethal effects on one-day, two-day and three-day old M. mongolica were exacerbated with increasing age; The whole algal culture, re-suspended algal cells, cell fragments and cell contents all had adverse impacts on the survival of M. mongolica. Moreover, both juvenile numbers and life-span time of M. mongolica were decreased significantly when they were cultured in A. affine AC-1 of 10 and 50 cells/cm³, combined with 3×10⁶ cells/cm³ of Chlorella spp., respectively. As a non-PSP producer, A. affine may produce other toxins, responsible for the strong negative effects on M. mongolica.