NITZSCHIA BICAPITATA (BACILLARIOPHYCEAE) AND RELATED TAXA FROM OCEANIC AGGREGATIONS

Abstract

Based upon net collections of various mesh sizes (20–235 μm) from the open ocean, small (mostly 6–31 μm), abundant, bicapitate Nitzschia species have been found to aggregate on substrates. As many investigators have noted from water samples, they are also present as single cells in the plankton, thus filling two different ecological niches at different times. Their reported presence in sediment trap materials indicate that they can be a major food source for some grazers. They have been assumed to belong to a single species, Nitzschia bicapitata Cleve, but several distinctive morphological taxa have been recognized from study in electron microscopes from these and other collections. Samples from the water mass close to the austral type location have been used to emend Cleve's description of N. bicapitata. It is differentiated from a boreal taxon, which is described here as a new variety, N. bicapitata var. faeroensis, from a collection in 1899 close to another of Cleve's collection sites made three years before. In addition to the new variety, two new species, N. leehyi and N. villarealii, are described and compared to selected open-ocean bicapitate taxa, a group that is highly successful in the open ocean and in need of monographic treatment. In general, these species have exhibited considerable overlap in their ranges of valve length and width. In all cases thus far explored, the valve width of a species, as determined by scanning electron microscopy, changes little over cell generations as cells undergo a reduction in length, causing differences in outlines between longer and shorter bicapitate cells within a species. Taking this pattern into consideration, previously noted taxonomic distinctions in shape are supported here. Other distinguishing characters include proximal and distal mantle ornamentation, pervalvar mantle development, and symmetry.