CRYOTHECOMONAS AESTIVALIS SP-NOV, A COLORLESS NANOFLAGELLATE FEEDING ON THE MARINE CENTRIC DIATOM GUINARDIA-DELICATULA (CLEVE) HASLE

The vegetative life cycle, host specificity, morphology, and ultrastru cture of a new phagotrophic nanoflagellate are described: Cryothecomon as aestivalis Drebes, Kuhn & Schnepf sp. nov. During summer and autumn it is frequently found in the North Sea phytoplankton feeding on the centric diatom Guinardia delicatula. The flagellate penetrates the dia tom cell and phagocytizes the host cytoplasm by means of a pseudopodiu m that emerges from the posterior cell pole. The mature trophont gives rise to eight or more biflagellate swarmers which leave the emptied d iatom frustule. Transmission electron microscopy revealed a delicate t heca surrounding the swarmers. The pseudopodium protrudes through a ga p in the theca. The cytostome consists of a membranous labyrinth. The mitochondria are of the tubular type. The two apically inserted flagel la are heterodynamic and of unequal length. They are smooth, and their basal bodies are surrounded by satellites and fibrous strands (''tran sitional fibres'' sensu Thomsen et al., 1990). In the trophonts and di viding flagellates the transition region between the flagellum and the basal body ends apically with a massive transitional cylinder instead of distinct microtubules, and is surrounded by a funnel of the theca. The nuclear envelope disintegrates during mitosis. Due to the fine st ructural details the new flagellate is placed in the genus Cryothecomo nas Thomsen et al., a genus of still uncertain position.