Cryothecomonas aestivalis sp. nov., a colourless nanoflagellate feeding on the marine centric diatom Guinardia delicatula (Cleve) Hasle

Abstract

The vegetative life cycle, host specificity, morphology, and ultrastructure of a new phagotrophic nanoflagellate are described: Cryothecomonas aestivailis Drebes, Kühn & 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 diatom cell and phagocytizes the host cytoplasm by means of a pseudopodium that emerges from the posterior cell pole. The mature trophont gives rise to eight or more biflagellate swarmers which leave the emptied diatom frustule. Transmission electron microscopy revealed a delicate theca surrounding the swarmers. The pseudopodium protrudes through a gap in the theca. The cytostome consists of a membranous labyrinth. The mitochondria are of the tubular type. The two apically inserted flagella are heterodynamic and of unequal length. They are smooth, and their basal bodies are surrounded by satellites and fibrous strands ("transitional fibres" sensu Thomsen et al., 1990). In the trophonts and dividing 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 structural details the new flagellate is placed in the genus Cryothecomonas Thomsen et al., a genus of still uncertain position.