Molecular Phylogeny of Pacific Archigregarines (Apicomplexa), Including Descriptions of Veloxidium leptosynaptae n. gen., n. sp., from the Sea Cucumber Leptosynapta clarki (Echinodermata), and Two New Species of Selenidium

Although archigregarines are poorly understood intestinal parasites of marine invertebrates, they are critical for understanding the earliest stages in the evolution of the A picomplexa. Previous studies suggest that archigregarines are a paraphyletic stem group from which other lineages of gregarines, and possibly all other groups of apicomplexans, evolved. However, substantiating this inference is difficult because molecular phylogenetic data from archigregarines, in particular, and other gregarines, in general, are severely limited. In an attempt to help fill gaps in our knowledge of archigregarine diversity and phylogeny, we set out to discover and characterize novel lineages of archigregarines with high‐resolution light and scanning electron microscopy and analyses of small subunit ( SSU ) rDNA sequences derived from single‐cell ( SC ) PCR techniques. Here, we describe two novel species of Selenidium , namely S elenidium idanthyrsae n. sp. and S . boccardiellae n. sp., and demonstrate the surface morphology and molecular phylogenetic position of the previously reported species S . cf. mesnili . We also describe a novel genus of archigregarine, V eloxidium leptosynaptae n. gen., n. sp., which branches with an environmental sequence and, together, forms the nearest sister lineage to a diverse clade of marine eugregarines (i.e. lecudinids and urosporids). This molecular phylogenetic result is consistent with the hypothesis that archigregarines are deeply paraphyletic within apicomplexans, and suggests that convergent evolution played an important role in shaping the diversity of eugregarine trophozoites.