Meeting Abstract
Choanoflagellates are protozoans that can be unicellular or can form multicellular colonies. Because genomic and molecular phylogenetic analyses indicate that animals and choanoflagellates shared a common ancestor, choanoflagellates are used as model organisms to study the evolution of multicellularity. It has been proposed that avoidance of predation by other protozoans might have been an important selective factor favoring the evolution of multicellularity in the ancestors of animals. Protozoans capture prey by a variety of mechanisms, one of which is passive predation (the interception of prey that swim nearby). We studied the effectiveness of a passive protozoan predator, Actinosphaerium nucleofilum, at capturing Salpingoeca helianthica, a choanoflagellate with both unicellular and multicellular forms. A. nucleofilum capture prey on long axopods that radiate from the cell, and transport the prey along the axopods to the cell body, where they are engulfed in vacuoles. Prey can be lost during any of these steps. Frame-by-frame analysis of time-lapse videos taken during the feeding process showed that the feeding efficiency (proportion of prey entering the capture zone of the predator that were eaten) of A. nucleofilum was the same for single cells and colonies. Furthermore, there was no trend in feeding efficiency as a function of colony size. Thus, although some ciliates reject large choanoflagellate colonies but eat single cells, while some raptorial amoeboid protozoans ignore unicellular choanoflagellates but actively capture colonies, predation by passive predators might not have been important in the evolution of multicellularity.
