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The centroheliozoon Chlamydaster fimbriatus n.sp. has been studied in culture by light and electron microscopy. The cell body is surrounded by a fimbriated mucus envelope through which axopods extend. Each cell contains two or three contractile vacuoles and an eccentrically located nucleus with a single nucleolus. The cells have typical centroheliozoan characteristics such as a centroplast with a trilamellate disc, mitochondria with ribbon-like cristae, ball-and-cone kinetocysts, and a peripheral lacunar system. The axopodial axonemes possess a single hexagon of six microtubules. The genus is defined to include only centroheliozoa with a mucus coat. C. fimbriatus differs from the only other species recognized in this genus (C. laciniatus and C. sterni) in its size and number of arms. This is the first ultrastructural account of a species of the genus Chlamydaster.

The selective retention of functionally intact chloroplasts by the algivorous centroheliozoa (protists) Acanthocystis serrata, Raphidocystis tubifera and Chlamydaster fimbriatus is documented by ultrastructural accounts of individual cells from natural habitats. The plastids are derived from different algae. The 'plastidoplasm' may be bounded by two or three membranes, in the latter case the outer membrane having been provided by the centroheliozoon. Such symbioses only involve certain species of centroheliozoa, and are short-lived. These appear to be examples of fortuitous symbioses and their study may provide clues as to the mechanisms by which stable symbioses are established., (Copyright © 1987 Gustav Fischer Verlag · Stuttgart · New York. Published by Elsevier GmbH.. All rights reserved.)