Cluster dynamics in Maristentor dinoferus,a gregarious benthic ciliate with zooxanthellae and a hypericin-like pigment,in relation to biofilm grazing by the fish Ctenochaetus striatus

The large symbiotic ciliate Maristentor dinoferus (Heterotrichida: Maristentoridae) has an abundant, potentially toxic pigment that makes the cells look black, and it forms clusters making it easy to see in the field. In situ observations of vast Maristentor populations during three bloom years provided insights into the behavioral ecology of the Maristentor holobiont and circumstantial evidence for feeding deterrence. Maristentor migrated onto limestone tiles that were also suitable for development of biofilm, which was consumed by the fish Ctenochaetus striatus. We photodocumented several aspects of Maristentor movement in situ, including a diurnal rhythm of dispersal and re-clustering in the morning, a period of “cocktail party dynamics” that resulted in larger, fewer clusters over the afternoon, and responses to two different disturbances: (1) rapid dispersal–re-clustering when tiles were moved; and (2) swimming off the surface and photoaccumulating on the brightest and highest part of a container when Maristentor was enclosed. Although Maristentor is considered benthic, its abilities to swim rapidly, to photoaccumulate, and to form clusters hanging from a water surface hint that they may also exist in the hyponeuston. Although interaction between Maristentor and Ctenochaetus may not be common, it indicated ways in which the behavior of Maristentor could help defend it from grazing. First, during the morning diaspora and re-clustering, which coincided with a period when Ctenochaetus was not feeding, Maristentor tended to accumulate on thinner biofilm, including edges and previously-grazed areas. Secondly, analysis of bite marks suggested that this fish tended not to select thin biofilm and that, despite rapid feeding, it tended to avoid larger Maristentor clusters.