Fate of resting cysts of Alexandrium spp. ingested by Perinereis nuntia (Polychaeta) and Theola fragilis (Mollusca)

The ingestion of resting cysts of Alexandrium spp. by Perinereis nuntia (Polychaeta) and Theola fragilis (Mollusca) was experimentally examined in the laboratory. P. nuntia and T. fragilis were cultured in bottom sediment containing a high density of Alexandrium cysts under dark conditions. Moreover, to evaluate the degree and consequence of being ingested, the density of cysts in the control sediment (no macrobenthic organisms) and the germination capability of the cysts in the faecal pellets of the two species of macrobenthos were examined.Cysts in the culture sediment were found to be ingested by both P. nuntia and T. fragilis. No difference in the density of cysts between the sediments cultured with and without P. nuntia was observed. However, the density of cysts in the sediments with T. fragilis decreased by 24% compared to the density in the control sediment. It is possible that most of the cysts ingested were digested by T. fragilis. The rate of Alexandrium cyst digestion by this species is estimated 594 cysts/individual/day. It is estimated that 91% of the cysts ingested by T. fragilis were partially or totally digested and only 9% were excreted in a viable state during the experiment. Thus, T. fragilis has a stronger affect on the abundance of Alexandrium cysts compared with P. nuntia.No significant difference was observed between the germination success of the cysts from faecal pellets of P. nuntia and T. fragilis compared to the cysts in the control sediment. If, however, the necessary light for the cysts to germinate is cut off by being enclosed within the faecal pellet, the germination rate of cysts from the faecal pellets may be suppressed.