Morphological delineation and distribution patterns of four newly described species within the <i>Synura petersenii</i> species complex (Chrysophyceae, Stramenopiles)
<div><p>The <i>Synura petersenii</i> species complex represents a common, cosmopolitan and highly diverse taxon of autotrophic freshwater flagellates. In this paper, we describe and characterize four new species (<i>S. borealis</i>, <i>S. heteropora</i>, <i>S. hibernica</i> and <i>S. laticarina</i>) that have been identified during our extensive sampling of freshwater habitats in 15 European countries. Morphometric analyses of siliceous scales led to the significant phenotypic differentiation of all four newly described species, and their separation from other related species of the <i>S. petersenii</i> complex. Two of these newly described species (<i>S. hibernica</i> and <i>S. borealis</i>) can be clearly distinguished by characteristic large colonies consisting of elongated, lanceolate-shaped cells. Development of strongly elongated, narrow cells in <i>S. hibernica</i> could be explained by the adaptation of this species to oligotrophic conditions. Though morphologically distinct, <i>S. borealis</i> possesses an exceptionally high degree of genetic diversity, possibly indicating recent speciation and evolutionary diversification within this taxon. Three of the four newly described species exhibit restricted biogeographic distribution. The evolutionarily related <i>S. borealis</i> and <i>S. laticarina</i> occur only in Northern Europe, and seem to be adapted to colder areas. The most remarkable distribution pattern was observed for <i>S. hibernica</i>, which has a geographic distribution that is restricted to western Ireland.</p></div
