How do algae form multicellular groups?

Background: Theory suggests that how groups are formed can be a major influence on the evolution of cooperation, and whether cooperative groups make the major transition to a higher level individual. The formation of clonal groups, by remaining with parents (subsocial group formation) leads to a greater kin selected benefit of cooperation, compared with formation of groups by aggregating, with potential non- relatives (semisocial). Freshwater algae form multicellular groups in response to the presence of predators, but it is not clear whether they form groups by remaining together (subsocial) or by aggregation (semisocial). Organisms: The freshwater algae Chlorella sorokiniana, Chlorella vulgaris and Scenedesmus obliquus, and the freshwater crustacean predator Daphnia magna. Results: Fluorescence microscopy and time-lapse photography revealed that, in response to predator supernatant/live predators, these algae form groups both subsocially and semisocially. Additionally, different algal species form mixed-species multicellular groups in response to predation. Conclusion: The observation of semisocial, and even between species, group formation in these facultatively multicellular algae: (i) emphasises the direct fitness benefits of forming groups to avoid predation; and (ii) strengthens the across species correlation between the method of group formation and whether multicellularity is facultative or obligate.