Abstract

The Choanoflagellates are a distinctive group of singled-celled Protozoa universally distributed in aquatic habitats. Their cell morphology resembles that of Sponge choanocytes and for this reason they have traditionally been considered to be the unicellular ancestors of Sponges. Recent molecular phylogenetic studies also show the choanoflagellates to be closely related to Metazoa.

Choanoflagellate biology is paradoxical in that the morphology and function of the cell is uniform and highly conserved whereas the external structures have diversified thereby allowing cells to inhabit an extensive range of aquatic (freshwater and marine) habitats. One group of marine species has developed a ‘basket-like’ exo-skeleton made up of silica ribs. Variations in the size, shape and pattern of this skeleton have been essential in adapting cells to benthic and planktonic environments. During the course of evolution, the mechanism of skeleton production and cell division must have ‘suddenly’ inverted thereby producing a novel and highly successful lineage. Evidence for this conclusion will be presented together with a general review of the group.