Echyridella menziesii (Bivalvia: Hyriidae) as a predator of zooplankton of different sizes; are large non-indigenous Daphnia a potential food source?

Interactions between two recent invaders to New Zealand, the cladocerans Daphnia galeata and D. pulex, and native filter-feeding freshwater mussels, are unknown. We examined predation rates of the common native mussel Echyridella menziesii (kākahi, kāeo) on non-native Daphnia (comparatively large zooplankton in New Zealand), relative to two common native species, the smaller cladoceran Bosmina meridionalis and rotifer Brachionus calyciflorus. Controlled laboratory experiments were conducted in which each zooplankton species was exposed to bivalve predation for a two-hour period. Comparing treatments to non-mussel controls, removal rates of non-indigenous D. pulex and native B. calyciflorus were statistically significant. Nevertheless, kākahi removal rates may not be ecologically significant for daphnids (1.7% D. galeata and 7.4% D. pulex). Kākahi removed 8.8% of B. meridionalis and 30.2% of B. calyciflorus, suggesting that small, feeble zooplankton species (particularly rotifers) are most susceptible to predation, and potentially function as an energy source for native freshwater mussels.