Comparative molecular and morphological species delineation of Halammohydra Remane, 1927 (Hydrozoa) – with description of four new species

<p><span>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 <em>Halammohydra</em>, comprising nine nominal species. Species are described with high intraspecific variability in e.g., 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 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 <em>Halammohydra</em>. This, together with the confirmed excessive intraspecific variation in morphological traits, underlines the necessity of molecular sequencing for taxonomy and species identification of <em>Halammohydra</em>. </span></p>