The genus Semiorbis (Eunotiaceae, Bacillariophyta) in North America

The genus Semiorbis was erected by R.M.Patrick in 1966 as monotypic based on the relatively rare taxon Semiorbis hemicyclus. Defining characters for Semiorbis include strongly arcuate valves, short eunotioid raphe branches, well-developed external costae with spine-like projections on the virgae, lack of rimoportulae, and a broader dorsal mantle. New populations of Semiorbis were found in the central USA (Wisconsin) on Outer Island, Apostle Islands National Lakeshore, along southeast Lake Superior, from a lake in New Jersey (USA), and from a small arctic lake in Nunavut, Canada. The population in Outer Lagoon, a shallow embayment cut off from Lake Superior by a long-shore bar, provided documentation of living cells and colonies of Semiorbis. We examined the morphology, ecology, and taxonomy of these new populations using light and scanning electron microscopy, traditional morphometrics, and sliding landmarks shape analysis. We compared these populations to European populations of Semiorbis hemicyclus and North American populations of Semiorbis rotundus and Semiorbis catillifera. We determined that the population found in Wisconsin represents a new species herein described as Semiorbis eliasiae Edlund, D.R.L.Burge, N.A.Andresen & VanderMeulen sp. nov., the New Jersey (USA) population is Semiorbis rotundus, and the Nunavut (Canada) population represents a North American population of the generitype Semiorbis hemicyclus.