Each with its own idiosyncrasy: a morphological analysis of Pseudostaurosiropsis (Bacillariophyceae) reveals a new species from southern Brazil

A new species, Pseudostaurosiropsis hartziae sp. nov., is described from coastal lakes in Rio Grande do Sul, southern Brazil. This taxon has been consistently identified in previous studies as either related to Pseudostaurosiropsis or other small ‘staurosiroid' diatoms (i.e. Fragilaria sp. or Staurosira sp.). We also provide the first detailed observation on the type material of F. construens var. javanica Hustedt, a morphologically similar taxon from Indonesian tropical coastal lakes of Java, and transfer it to the genus Pseudostaurosiropsis. Both species are classified within Pseudostaurosiropsis based on fine structural characteristics, including stria areola configuration (number per stria), spine positioning along the striae, and the presence of closing plates. Furthermore, we employed statistical morphometric analyses to discriminate between the two species, further supporting their morphological separation. In addition, we describe the ecological preferences of P. hartziae. Given its high abundance and occurrence in the study region, this taxonomic clarification has significant implications for ecological research in southern Brazilian coastal lake systems.