Loricate choanoflagellates of the Southern Ocean with new observations on cell division in Bicosta spinifera (Throndsen, 1970) from Antarctica and Saroeca attanuata Thomsen, 1979, from the Baltic Sea

The loricate choanoflagellate Bicosta spinifera was observed frequently in Antarctic samples collected along Scotia/Weddell Sea transects. The entire population showed a conspicuous bimodal size distribution. Large forms were predominant in Scotia Sea samples, while only small specimens were found in the Confluence area. Prior to cell division Bicosta spinifera produces a complete set of costal strips in a temporary "tail"-like protrusion. Small specimens sometimes possess a "tail" similar to that found in large specimens. The fact that B. spinifera may increase considerably in size following cell division, in connection with the finding of aberrant minute forms, have prompted us to hypothesize a polymorphic life history in B. spinifera. The term "caudiform" division is introduced to describe division in B. spinifera. Saroeca attenuata also produces a "tail" prior to cell division, but is otherwise shown to undergo a mixed caudiform/tectiform type of division.