Pelagic larval polyclads that practice macrophagous carnivory

Abstract

We report evidence that hatchling polyclads of several genera feed in the plankton on large prey. These ciliated swimmers, despite apparently lacking means to concentrate food or even detect it at a distance, subdue and consume fast-moving active-swimming plankters such as crustacean larvae and copepods, or molluskan veligers. We describe feeding events in captivity using videomicroscopy, and identify several wild-caught predatory pelagic polyclad larvae to genus or species level by DNA barcoding. Remarkably, one of these types is identified unambiguously with a species previously observed as Müller's larvae, which live as conventional planktotrophs on an inferred diet of small phytoflagellates. Therefore we conclude first that while so-called "direct-developing" polyclad flatworms may hatch with juvenile-like morphology, at least some of these are functionally larvae. Second, that some species of polyclad have at least a triphasic life cycle, with a first larval stage living in the plankton on primary producers followed by a second larval stage living in the plankton by macrophagous carnivory, before presumably settling to the benthos for adult life.