Marine hydroids, the polyp stage of hydrozoans, are assumed to be carnivores and play an important role in benthic-pelagic coupling processes by regulating zooplankton populations and transferring energy. The known hydroid diets mainly consist of zooplankton as well as some benthic diatoms and metazoans, almost all of which have smaller dimensions than the polyp. In the present study, a large-scale breeding failure of the early juveniles of the ivory shell Babylonia areolata (Mollusca: Gastropoda: Babyloniidae) was found in a local aquaculture farm in Fujian, China in July 2018. About 640,000 out of 800,000 early juveniles (80%) died due to predation by a millimetre-sized colonial hydroid species that was initially overlooked. The hydrozoan species was identified as Eirene sp. (Cnidaria: Hydrozoa: Eirenidae) via the integration of morphological and molecular data. A laboratory feeding experiment showed that the hydroid predation led to an overall mortality of ivory shell juveniles of up to 92.6% after 48 h. The predation process was observed in the laboratory and a video showing the entire process was prepared. This is the first report of hydroids preying on gastropod juveniles, revealing a previously unknown threat to mollusc aquaculture including ivory shells. Several management practices are suggested to mitigate this newly recognized threat.