Motility and size of rotifers as risk factors for being consumed by the passive protistan predator Actinosphaerium sp.

Predation is a well-known factor that structures rotifer communities. However, the role of protists as predators is relatively understudied. Here, we investigated predatory behavior of Actinosphaerium sp., a freshwater heliozoan, on seven rotifer species. Predators and prey were collected from a local playa; except for Brachionus calyciflorus that served as a naive prey control. Prey included large species (>= 175 mu m mean length: Asplanchna sieboldii, B. calyciflorus, Platyias quadricornis, and Lacinularia flosculosa) and small species (< 175 mu m: Cephalodella gibba, Euchlanis dilatata, and Lepadella patella). Four experiments were conducted. (1) Single prey items of varying size and motility. Larger prey types were similar to 1.7 to 3.0 times more likely to be ingested than small prey. No L. flosculosa were ingested, contrary to field observations. No correlation was found between swimming speed and predation risk. (2) Preference tests. Asplanchna sieboldii and B. calyciflorus were favored prey. (3) Growth rate of Actinosphaerium on mixed diets, with and without Asplanchna. Highest population growth of Actinosphaerium was observed in presence of A. sieboldii. (4) Prey defenses. Susceptibility of spined versus unspined B. calyciflorus resulted in no significant difference in predation risk. Thus, size and being mobile (compared to sessility) are the primary risk factors influencing rotifer predation vulnerability.

Automated summary

This study investigated the predation behavior of Actinosphaerium sp., a freshwater heliozoan, on seven rotifer species, and found that larger and more mobile rotifers were more likely to be preyed upon. Preference was shown towards Asplanchna sieboldii and Brachionus calyciflorus, while Lacinularia flosculosa had very low predation probability. Therefore, size and mobility are the primary risk factors influencing rotifer predation vulnerability.