Abstract
SYNOPSIS. The family Actiniscaceae Kützing, 1845, (generally placed in the dinoflagellates) includes species derived from Actiniscus pentasterias Schütt, some of which became lacustrine relicts. A. pentasterias v. arcticus. living in Arctic and Altantic waters, differs from the Mediterranean species in having a greater number of siliceous skeletons (pentasters). Advanced atrophy of pentasters in lacustrine A. canadensis, and absence of such structures in Pseudoactiniscus apentasterias, probably results from decreased salinity and an undetermined ecologic factor. Morphogenesis and physiologic transformation of past euryhaline Actiniscaceae into contemporary limnobionts began, presumably, during the prelimnic phase of Great Bear Lake and Keyhole Lake. Arctic Actiniscus which exist under subzero temperatures and endure 10‐month‐long winters, appear to satisfy their energy requirements by heterotrophy and frequent phagocytosis. Survival of lacustrine Actiniscus species in both Arctic lakes seems possible because of lack of competition from co‐existing phytoplankton organisms which can endure extreme dystrophic conditions.
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