Algae. 2000; 15(4): 299-306.

Effects of Planktivorous Fish and Zooplankton on the Morphology of the Small Cryptomonad Plagioselmis prolonga var. nordica (Cryptophyceae)

Baik Ho Kim, Min Kyu Choi, Noriko Takamura

Two morphs of the small cryptomonad Plagioselmis prolonga var. nordica with a posterior tail, were observed during summer and fall in Lake Kasumigaura, Japan. The tail shortened in mesocosms stocked with planktivorous silver carp (Hypophthalmichthys molitrix Val.) and elongated by more than 50% in mesocosms from which silver carp were removed. The density of Plagioselmis cells increased significantly upon fish stocking and decreased upon fish removal. The tail length was negatively correlated with algal abundance and positively correlated with crustacean densities, but there was no correlation with nutrient levels or physical environment parameters in the mesocosms. The variation in tail length was induced by the presence/absence of fish, but was not related to their density. However, silver carp manipulation strongly affected the density of the majority of zooplankters and, interestingly, there was a strong correlation between zooplankton density and tail-length change in Plagioselmis. We propose an possible herbivore-induced defense mechanism triggered by the top predator, silver carp. Further research is necessary to better understand the underlying mechanisms. Most importantly, the effect of silver carp on Plagioselmis morphology needs to be differentiated from those of zooplankton and Plagioselmis densities.