Comparative diets, prey selection, and predatory impact of co-occurring larvae of two flounder species

Abstract

The dominant prey of small larvae of two pleuronectid flounder species, greenback flounder Rhombosolea tapirina Günther and long-snouted flounder Ammotretis rostratus Günther, were bivalve veligers, which were also the most abundant zooplankter in the environment. When large larvae of both species were collected together, the dominant prey of Rhombosolea tapirina was the cladoceran Evadne nordmanni Loven, while paracalanid copepodids were the dominant food of Ammotretis rostratus. The relationship between predator and prey size was similar for both species, with size of prey consumed increasing with larval growth. Smallest feeding larvae consumed larger prey ( x ̄ = 100 μm width) than most larvae of marine fish at this stage. Comparison of diets of flounder larvae with abundances of microplankton in the environment revealed strong selection for prey type as well as prey size. Similar distributions of Evadne nordmanni and paracalanid copepodids in microplankton samples indicated that divergence in prey selection by larger larvae was due to innate preference, rather than fine-scale co-occurrence with prey. From a knowledge of feeding period, average gut contents, and gut evacuation time, I estimated that flexion-stage Rhombosolea tapirina would consume ≈ 75 prey per day. I conclude that at levels of larval fish and prey abundance observed in Port Phillip Bay, Australia, flounder larvae would have negligible impact on prey populations.