The Development and Application of a Molecular Assay to Detect In situ Predation of the Pelagic Tunicate, Dolioletta gegenbauri in the South Atlantic Bight (Continental Shelf, USA).

Gelatinous zooplankton are abundant components of marine food webs. However, methodological difficulties associated with observing their trophic interactions (feeding and being fed upon) impede the direct investigation of their trophic roles in situ. Dolioletta gegenbauri is a species of pelagic tunicate that frequently forms large, yet short-lived, blooms within productive subtropical continental shelf environments and presumably impacts pelagic food webs as grazers, prey and prodigious producers of fecal pellets and nutrient rich aggregates. Little is known about the ecological factors that lead to the eventual termination of D. gegenbauri blooms, specifically, the role of direct predation. In this study, we report the development and application of an 18S rRNA-targeted PCR assay to identify predators of D. gegenbauri. The assay was capable of specifically detecting doliolid DNA in the guts of predators. Screening of potential predators captured during 9 of 13 independent cruises on the South Atlantic Bight continental shelf when doliolids were present, identified Geryoniidae hydromedusa as frequent predators of doliolids. During these studies doliolid DNA was detected in association with 18% (6 out of 32) of the hydromedusa collected. Doliolid predation was also detected in a chaetognath. Predation was not detected in ctenophores, pteropods, fish larvae, eel larvae or crustacean larvae, but samples sizes of these predators were small. Residence time of doliolids captured and ingested by the Geryoniidae hydromedusa species Liriope tetraphylla was no more than 45 min as detected by PCR analysis during laboratory feeding experiments. These studies identify hydromedusa as frequent predators of doliolids in subtropical continental shelf environments, but suggest that predation rates are unlikely to be high enough to terminate doliolid blooms.