Rimoportula Distribution, Heterovalvy and Heteropolarity in Hyalosira (Bacillariophyta: Rhabdonematales), with Revision of H. hesperia and Two New Species

The genus Hyalosira Kützing was recently split and emended based on morphology and molecular phylogeny but many uniseriate taxa could not be resolved. All populations examined in that study had one rimoportula on each valve (i.e., 1 + 1 in the cell). Recent collections from Turkey, Australia, and Micronesia had uniseriate taxa with different numbers of rimoportulae and other new features distinguishing them from all other uniseriate taxa. Two species had two rimoportulae per valve (i.e., 2 + 2) and deep septa, of which one is shown to be Hyalosira hesperia Álvarez-Blanco & S.Blanco, for which we provide a revised description, and the other, from Melbourne and Yap, is proposed as H. pacifica, sp. nov. They are separated by stria density and copula areola density. A third species, from the Great Barrier Reef, H. flexa, sp. nov., is the first Hyalosira found with heterovalvy—including a rimoportula on only one valve (i.e., 1 + 0)—and heteropolarity, including three variously reduced pore fields and the fourth, with the rimoportula, always involved in attachment, essentially a basal pole. A corrected diagnosis of the genus is provided and implications for exploring the functions of rimoportulae are discussed.