Zooplanktophagy in the natural diet and selectivity of the invasive mollusk Limnoperna fortunei

Since the 1990s the Rio de La Plata basin has been widely colonized by the invader mollusk Limnoperna fortunei. In spite of a relatively well-developed knowledge of the biology and ecology of this mollusk, not much is known about its natural diet and possible food selectivity. Stomach contents of 120 specimens attached to the roots of Eichhornia crassipes, and available planktonic food sources, taken from seven representative environments of the Parana River floodplain, were analyzed. The diet consisted of 156 taxa with a wide size range (2-1,178 mu m) and representing a great variety of organisms. Phytoplankton was dominant in abundance (98.6% of the total diet), while zooplankton, mainly small cladocerans, was dominant in volume (67% of the total diet). L. fortunei showed a positive food selectivity for organisms with limited escape ability and low to moderate size (Euglenophyta, Rotifera, Chydoridae and Bosminidae), and a negative selectivity for many algae. Therefore, the plankton community structure would be changed due to this differential predation pressure. These are the first observations about the feeding of L. fortunei upon zooplankton. The results of the present study agree, in general, with those of descriptive field studies and laboratory experiments on the effect of Dreissena polymorpha on zooplankton, but differ in some aspects. The differences suggest the possibility of different impacts on the ecosystem, and a probable complementary coexistence in potentially 'invadable' environments. Ecological interaction between L. fortunei and the invasive hydrophyte Eichhornia crassipes suggests a possible synergistic effect on the distributional efficiency of the mollusk.