Feeding habits of omnivorous Asplanchna: comparison of diet composition among Asplanchna herricki, A. priodonta and A. girodi in pond ecosystems

Spatial distribution and the diet composition of Asplanchna species were studied in 18 water bodies in Matsuyama, Japan. The abundance of Asplanchna and other rotifers, crustaceans, phytoplankton and microbial plankton, together with basic environmental parameters, were determined between October and December 2006, and the distribution and diet composition of Asplanchna species were estimated. Three species of Asplanchna, A. herricki, A. priodonta and A. girodi were found in the present study, but A. herricki was rather less abundant than the other two species. Their diet composition was different among the species, showing that A. herricki consumed only particulate matter while the diet of A. priodonta included mainly phytoplankton, dominated by dinoflagellates. In contrast, A. girodi was rather carnivorous, and included other rotifers in its diet. Their different food habits are not explained by their morphotypes and trophi structures, suggesting this difference might be related to their feeding abilities. For A. girodi, prey selectivity (Chesson's α) for rotifer prey was negative, except for Keratella cochlearis. The amount of rotifers consumed was also low at a mean prey number of less than 3 per A. girodi gut. The result suggests that the predation impact of Asplanchna as a top-down controller of rotifer populations is species-specific and can be apparent only when Asplanchna population reaches high density in these ponds. From the present results, three Asplanchna species were found to belong to basically different feeding groups, A. herricki is detritivore while A. priodonta and A. girodi are omnivores; but A. girodi is more predacious.