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

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., 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 Halammohydra. This, together
with the confirmed excessive intraspecific variation in morphological
traits, underlines the necessity of molecular sequencing for taxonomy and
species identification of Halammohydra.