Predation by invertebrate predators on the colonial rotifer Sinantherina socialis

Colonies of the freshwater colonial rotifer Sinantherina socialis (Monogononta, Flosculariidae) have been shown to be unpalatable to a variety of small-mouthed, zooplanktivorous fishes. To test whether invertebrate predators ingest the rotifer S. socialis, we conducted two types of experiments: (1) Microcosm experiments-in separate experiments, four invertebrate predators (i.e., dragonfly nymphs, damselfly nymphs, notonectids, and Hydra) were offered prey either singly or in combination. Prey were comprised of S. socialis; Epiphanes senta, a solitary, free-swimming rotifer; and Daphnia magna, a microcrustacean. In each experiment, the percent of prey surviving after 12, 18, and 24 h was recorded. (2) Paired-feeding experiments-in separate experiments, predators were offered prey in a pairwise fashion, in which members of D. magna were alternated with a rotifer, either S. socialis or E. senta. The results of the microcosm experiments showed that, after 24 h, 60-100% prey items of S. socialis survived the predators, but significantly fewer individuals of E. senta (6-89%) and D. magna (< 25%) survived. When offered rotifers and individuals of D. magna simultaneously, predators tested consistently consumed more specimens of Daphnia. However, predators significantly reduced percent survival in E. senta but not in S. socialis. Predators, given a choice between the two rotifer species, all consumed significantly more specimens of E. senta than S. socialis after 24 h. In the paired-feeding experiments, three of the four predators captured members of S. socialis, but these colonies were frequently released rather than ingested, although in some cases colony structure was seriously disrupted. Our results suggest that the unpalatable nature of members of S. socialis to certain fishes extends to several invertebrate predators, but the nature of the putative factor(s) responsible for this remains unknown.