The cyst-motile stage relationships of the dinoflagellatesDiplopelta symmetricaandDiplopsalopsis latipeltata

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Abstract
Resting cysts and motile stages are described for two diplopsalid species: Diplopelta symmetrica, from Norway and Italy, and Diplopsalopsis latipeltata, from Italy. D. symmetrica cysts are spherical, brown, and densely covered by characteristic hair-like processes never before described for dinoflagellate cysts. Its archeopyle is a large zigzag split. D. latipeltata cysts are spherical, brown, and smooth-walled, with a simple split archeopyle. Comparisons between these newly discovered cyst types and the seven types so far described for diplopsalid species raise doubts concerning some previous interpretations of the paratabulation of theropylic archeopyles within the group. Both species studied here are heterotrophic. Both in laboratory cultures and in natural plankton with a great variety of other phytoplankton available, they were observed to feed only on the dinoflagellate Prorocentrum micans. Furthermore, the yearly distribution of both diplopsalid species in the plankton corresponded with that of P. micans, suggesting that this degree of selective feeding is more than just an artefact of culturing.