An intracellular microsporidian parasite was first observed within oocytes of Maculaura alaskensis, a small pilidiophoran nemertean, commonly found on sandflats along the Pacific coast of North America. Infected oocytes have large vesicles containing dozens to hundreds of diplokaryotic, ellipsoid spores measuring 1.3 by 2.3 μm. A partial small subunit nuclear ribosomal 18S gene sequence isolated from the microsporidian does not match any known microsporidian sequences in the public databases. Phylogenetic analysis groups it with Hepatospora eriocheir in a sister clade to the Enterocytozoonidae. All the known life stages of this parasite are contained within a membranous envelope. This microsporidian was identified in M. alaskensis, Maculaura aquilonia, Maculaura oregonensis, and Maculaura cerebrosa in Coos Bay, Oregon, in M. alaskensis from Newport, Oregon, and in M. aquilonia collected in Juneau, Alaska. This is, to our knowledge, the first species of microsporidia found to directly infect nemertean host cells.
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.
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