The genus Hymeraphia currently comprises three species and all records of the genus are from the northeast Atlantic and the Mediterranean Sea. The present paper describes a new species, H. vaceleti sp. nov. from a deep-water canyon, off southwest Ireland. H. vaceleti sp. nov. is morphologically distinct from other Hymeraphia species and 28S rRNA sequences show substantial differences between our new species and other Hymeraphia spp. We present data on the prevalence of sponge species established on the basis of single specimens (singletons) or uniques (species only known from a single locality). We argue for the recognition of singletons for species that are rare or from habitats that are very difficult to survey or sample, such as the deep sea.
Keywords: Porifera, Sponges, systematics, taxonomy, Raspailiidae, singletons, uniques, monophyletic, 28S rRNA, R.V. Celtic Explorer.