Culture of the cladoceran Moina macrocopa: Mortality associated with flagellate infection

Cladocerans are important food animals in aquaculture, key grazers in freshwater ecosystems, and model animals for ecotoxicological investigations. Their epibiont community, extensively studied in Daphnia, includes filamentous bacteria, fungi, algae, peritrich ciliates, and rotifers; although epibionts are usually benign, heavy infections can be detrimental. During our laboratory culture of female Moina macrocopa Straus, we observed a novel flagellate infection associated with mortality. At day 10, all M. macrocopa were alive in uninfected cultures, whereas in untreated infected cultures, the survival was significantly lower: only 26% of cladocerans were alive. In infected cultures treated with humic substances (as 25mgL−1 dissolved organic carbon), mortalities were comparable to those in the untreated infected cultures; in contrast, in the infected cultures treated with 4gL−1 sea salt, mortalities were arrested, and 76% of the M. macrocopa were alive at day 10. Moribund cladocerans were transparent, had empty digestive tracts, and greatly reduced motor activity. Free-swimming flagellates moved forward with a wobbling motion, rotating around their long axis; they also attached to cladoceran tissue, the Petri dish, and the glass slide, by the tip of their posterior flagellum. Flagellates preserved for scanning electron microscopy were 6.9±0.7μm long and 2.1±0.3μm wide, with a short anterior flagellum (6.8±1.1μm) and long posterior flagellum (14.1±1.5μm). Multi-functionality of a flagellum, for locomotion and adhesion, is relatively rare, and previously reported from genera within the Kinetoplastea, suggesting that the flagellate on M. macrocopa may belong to this group. To combat flagellate mass occurrence in Moina cultures, we recommend a treatment with 4gL−1 sea salt. •Mortality of cultured Moina macrocopa was associated with flagellate infection.•After 10 d, 100% uninfected cladocerans survived but only 26% of infected cladocerans.•Moribund cladocerans were transparent, had empty digestive tracts, and moved slowly.•Flagellates possibly kinetoplastids, attach to cladocerans by posterior flagellum tip.•Mortalities arrested in cultures treated with 4g L–1 sea salt.