The illusion of rarity in an epibenthic jellyfish: facts and artefacts in the distribution of Tesserogastria musculosa (Hydrozoa, Ptychogastriidae)

Epibenthic and benthopelagic medusae are rarely collected by standard benthic or pelagic sampling methods, and many species are considered uncommon and geographically restricted. Peer-reviewed scientific literature contains only two records of medusae belonging to the monotypic genus Tesserogastria Beyer, 1958 since their original description, both from the vicinity of the type locality in Oslofjord, contributing to an illusion of extreme rarity and restricted distribution. Our analysis of fresh samples and a thorough evaluation of all previous records of this taxon from both peer-reviewed scientific sources and “gray” literature show that the species is both more common and widespread than suggested by the scant records in primary scientific literature, and represents an example of an overlooked taxon in the epibenthos. High numbers of medusae of Tesserogastria musculosa Beyer, 1958 were collected at Raunefjord in western Norway. New data, together with validated observations from fjords in western and eastern Norway as well as western Sweden, demonstrate that the species is much more common than is evident from published records. Data on the mitochondrial 16S ribosomal RNA and cytochrome oxidase I molecular markers for the species are provided for the first time, as well as new observations on the morphology of living animals. Tesserogastria musculosa constitutes an example of a hydrozoan species with a misleading reported distribution, a situation likely to occur in all members of family Ptychogastriidae and other delicate epibenthic invertebrates. Sampling techniques specifically targeting the epibenthos and careful processing of the samples are essential for correctly assessing the presence of the species, suggesting that the lack of records for this and other epibenthic medusae may in part be an artefact of the commonly used sampling methods. A comparison of molecular data for species and genus delimitation in Ptychogastriidae, presented here for the first time, highlights the need for a thorough taxonomic revision of the family.