A new deep-sea species of epibenthic acorn worm (Hemichordata, Enteropneusta)

Individuals of an enteropneust, Tergivelum baldwinae n. gen., n. sp. were videotaped at a depth of about 4 km in the eastern Pacific and collected by a remotely operated vehicle. The living worms range in length from 9 to 28 cm and are dark brown anteriorly and beige posteriorly. The proboscis is shaped like a shallow dome, indented on either side by a laterodorsal fossa housing a prominent proboscis nerve. The collar comprises a thin transverse crest dorsally and two laterally projecting lips on either side of the mouth ventrally. The mouth is oriented parallel to the substratum and is flanked by large left and right buccal muscles (contrasting with the rudimentary musculature elsewhere in the body). The respiratory pharynx of the trunk extends far anteriorly so that much of it lies dorsal to the mouth opening. The gill bars are not joined by synapticles. The laterodorsal body wall at the anterior extremity of the trunk extends as two conspicuous flaps (back veils) that run posteriorly as unattached coverings over the anterior 30-50% of the trunk. On either side of the midline, the body wall of the trunk is extended as a narrow lateroventral fold. Within the trunk runs the intestine, which lacks hepatic sacculations and opens at an anus at the posterior end of the body. Frame analysis of videotapes suggests that the worm can secrete a mass of mucus around the body to facilitate demersal drifting from one epibenthic foraging site to the next. We include a preliminary phylogenetic analysis based on rDNA sequences from T. baldwinae n. gen., n. sp. and additional deep-sea enteropneusts not yet formally described taxonomically (sequence data place them unexpectedly close to ptychoderids). Until more is known about the group as a whole, it is prudent to leave family level classification of T. baldwinae n. gen., n. sp. as incertae sedis.