Abstract
Whereas most cnidarians are macrofaunal, a few microscopic lineages have evolved, and some of them inhabit marine sediments. The meiofaunal genus with the most species is Halammohydra , comprising nine nominal species. Species are described with high intraspecific variability in, e.g., the number of tentacles and statocysts and the shape and length of tentacles and body, complicating morphological identification to species level. Additionally, there is not much molecular data available. This study aims to revise already described species with morphological and molecular methods, as well as, to delineate potential new species answering questions about their geographical distribution. For this, specimens were sampled at 16 locations in the Northwest Atlantic and two localities in the East Atlantic, documented with light microscopy, and fixed individually for sequencing (16S, 18S, and CO1). Herewith, morphological characters were linked to a specific sequence, enabling the testing of character variation within one molecular phylogenetic group. Phylogenetic analyses were conducted (Bayesian Interference and Maximum Likelihood) in combination with species delimitation tests (ABGD, GMYC, and bPTP). Four already described species were identified in the data sets, and all of these were found at multiple localities. Four new species are described. Overall, the combined molecular and morphological data acquisition revealed multiple new species and a high degree of sympatry in Halammohydra . This, together with the confirmed excessive intraspecific variation in morphological traits, underlines the necessity of molecular sequencing for the taxonomy and species identification of Halammohydra .