LAUSR

Aphelids are a diverse group of intracellular parasitoids of algae and diatoms, and are sister to true fungi. Included in four genera, the 14 described species utilize phagocytosis as their mode of nutrition, and the life cycles of these taxa are remarkably similar. However, their putative specificity of host, morphological and ultrastructural features, and genetic divergence have been considered in taxon delineation. Here, we examine the host specificity, morphology, ultrastructure, and molecular 18S gene sequence of a new species in Aphelida, Aphelidium desmodesmi sp. nov. This taxon is in a well‐supported clade with two other species of Aphelidium, and this lineage is sister to Amoeboaphelidium and Paraphelidium. Of interest, the mitochondrial structure of Aph. desmodesmi is more like that of Paraphelidium than that of Aphelidium aff. melosirae, the only other species of Aphelidium to have been examined ultrastructurally. This research examines and expands our understanding of host range, morphological diversity, and genetic divergence of the aphelids.