Filter-feeding cladocerans can reduce suspended rat lungworm ( Angiostrongylus cantonensis ) larvae without infestation or physiological cost

Planktonic crustaceans provide essential ecosystem functions by filtering water, but they can also serve as hosts of suspended pathogens and parasites. We investigated the interaction mechanisms between a primarily herbivorous filter-feeder, water flea (Daphnia magna) and the larval stage of globally invasive zoonotic nematode rat lungworm (Angiostrongylus cantonensis), focusing on potential benefits (larval removal) and risks (host function). Larval persistence and clearance rates were quantified in controlled microcosm exposures, and water flea individuals were examined for metabolic changes using respirometry, behavioral changes using video tracking analysis, and potential infections via dissections. Water fleas caused a significant reduction in rat lungworm larvae density in the water column during both the first and repeated exposures. In contrast, larvae showed no ability to develop within the host, with no detectable effects on the crustacean’s metabolic rate or movement patterns. This study reveals a previously unknown capacity of herbivorous cladocerans to influence the populations of nematodes relevant to ecosystem and public-health management. These results call for broader, mechanism-focused data on neglected interactions among key taxa in freshwater ecosystems.