Morphological variation in cysts of the gymnodinialean dinoflagellate Polykrikos

摘要

On the basis of both a literature survey and incubation experiments, the cyst-motile form relationship in the two Polykrikos species (Dinophyceae), P. schwartzii and P. kofoidii, must be reassessed. Surface ornamentation of the cysts of P. schwartzii and P. kofoidii has been considered the most important morphological feature differentiating these species. The cyst of P. schwartzii has been considered to be characterized by reticulate ornament, and that of P. kofoidii by separate, rod-like processes. In our incubation experiments, P. kofoidii produced a cyst covered with complete reticulate ornament; this species also germinated from a cyst with incomplete reticulate ornament. We found four morphological types of Polykrikos cysts in the surface sediments of Omura Bay. The ornament varied from rod-like elements (Type 1), to separate rows of lumina (Type 2), shelf-like ornament with incomplete reticulum (Type 3), to a complete reticulum (Type 4). Our observations show that cysts of P. kofoidii have not only variants with rod-like processes, but also forms with a reticulate network, and that intermediate forms sometimes occur. In particular, the presence of intermediate forms strongly suggests that a separator based on the surface ornament of cysts is not effective for differentiating these two species. Therefore, the taxonomic criterion that cysts with reticulate ornament are identical to P. schwartzii and those with rod-like form to P. kofoidii, is considered untenable.