Microalgae are the foundation of aquatic food webs. Their ability to defend against grazers is paramount to their survival, and modulates their ecological functions. Here we report a novel anti-grazer strategy in the common green alga Chlorella vulgaris against two grazers, Daphnia magna and Simocephalus sp. The algal cells entered the brood chamber of both grazers, presumably using the brood current generated by the grazer’s abdominal appendages. Once inside, the alga densely colonised the eggs, significantly reducing reproductive success. The effect was apparent under continuous light or higher light intensity. The algal cells remained viable following removal from the brood chamber, continuing to grow when inoculated in fresh medium. No brood chamber colonisation was found when the grazers were fed the reference diet Raphidocelis subcapitata under the same experimental conditions, despite the fact that both algal species were readily ingested by the grazers and were small enough to enter their brood chambers. These observations suggest that C. vulgaris can directly inflict harm on the grazers’ reproductive structure. There is no known prior example of brood chamber colonisation by a microalgal prey; our results point to a new type of grazer-algae interaction in the plankton that fundamentally differs from other antagonistic ecological interactions.

Genetically identical Daphnia magna and Simocephalus sp. (Cladocera) used in this study were grown at 20±1°C, under an 18L:6D photoperiod and a light intensity of 70 µmol photons m^-2s^-1.  The cladocerans was fed ad libitum daily with an equal mixture of C. vulgaris and R. subcapitata.