Evolution and phylogeny of the deep-sea isopod families Desmosomatidae Sars, 1897 and Nannoniscidae Hansen, 1916 (Isopoda: Asellota)

In the deep sea, the phylogeny and biogeography of only a few taxa have
been well studied. Although more than 200 species in 32 genera have been
described for the asellote isopod families Desmosomatidae Sars, 1897 and
Nannoniscidae Hansen, 1916 from all ocean basins, their phylogenetic
relationships are not completely understood. There is little doubt about
the close relationship of these families, but the taxonomic position of a
number of genera is so far unknown. Based on a combined morphological
phylogeny using the Hennigian method with a dataset of 107 described
species and a molecular phylogeny based on three markers (COI, 16S, and
18S) with 75 species (most new to science), we could separate
Desmosomatidae and Nannoniscidae as separate families. However, we could
not support the concept of the subfamilies Eugerdellatinae Hessler, 1970
and Desmosomatinae Hessler, 1970. Most genera of both families were well
supported, but several genera appear as para- or even polyphyletic. Within
both families, convergent evolution and analogies caused difficulty in
defining apomorphies for phylogenetic reconstructions and this is
reflected in the results of the concatenated molecular tree. There is no
biogeographic pattern in the distribution as the genera occur over the
entire Atlantic and Pacific Ocean, showing no specific phylogeographical
pattern. Poor resolution at deep desmosomatid nodes may reflect the long
evolutionary history of the family and rapid evolutionary radiations.