Hemichordate molecular phylogeny reveals a novel cold-water clade of harrimaniid acorn worms

Hemichordates are instrumental to understanding early deuterostome and chordate evolution, yet diversity and relationships within the group have been understudied. Recently, there has been renewed interest in hemichordate diversity and taxonomy, although current findings suggest that much hemichordate diversity remains to be discovered. Herein, we present a molecular phylogenetic study based on nuclear 18S rDNA sequence data, which includes 35 previously unsampled taxa and represents all recognized hemichordate families. We include mitochondrial 16S rDNA data from 66 enteropneust taxa and three pterobranch Rhabdopleura species, and recover colonial pterobranchs and solitary enteropneusts as reciprocally monophyletic taxa. Our phylogenetic results also reveal a previously unknown clade of at least four species of harrimaniid enteropneusts from cold waters, including Antarctica, the North Atlantic around Iceland and Norway, and the deep sea off Oregon. These small worms (1-5 mm in length), occur from 130 to 2950 m and are not closely related to other deep-sea harrimaniids, indicating that diversity of enteropneusts within the deep sea is broader than previously described in the literature. Discovery of this clade, as well as larger torquaratorids from Antarctica, strengthens hypotheses of close associations between Antarctic and deep-sea fauna.