SIZE-SELECTIVE PREDATION BY INLAND SILVERSIDES ON AN EXOTIC CLADOCERAN, DAPHNIA LUMHOLTZI

The helmet and large spines of zooplankton inhibit predation by small fishes. Daphnia lumholtzi, a long-spined, helmeted cladoceran, has recently invaded Lake Texoma (Oklahoma–Texas) and much of southeastern North America. To determine if inland silversides (Menidia beryllina) were size-selective predators on D. lumholtzi, we conducted laboratory experiments using 3 size-classes of fish (22 to 35 mm, 46 to 60 mm, and 67 to 80 mm). Inland silversides were allowed to feed for 80 min on D. lumholtzi of various sizes. Prey selection differed among the 3 silverside size-classes. Small inland silversides ate more small D. lumholtzi (total length 0.99 to 1.75 mm) than did large and medium inland silversides. Large inland silversides ate more large (2.62 to 3.44 mm) and very large (3.5 to 5.25 mm) D. lumholtzi than did smaller inland silversides. To examine if these trends occur in nature, we compared the lengths of D. lumholtzi and native Daphnia species (D. parvula and D. galeata) eaten by large and small inland silversides collected from Lake Texoma. As in the laboratory experiment, small inland silversides ate smaller D. lumholtzi than did large inland silversides. Although small inland silversides ate smaller D. lumholtzi, D. lumholtzi might be an important food source for young-of-the-year inland silversides, because D. lumholtzi reaches peak abundances during summer when native daphnid densities are low in Lake Texoma.