An Externally Brooding Acorn Worm (Hemichordata, Enteropneusta, Torquaratoridae) from the Russian Arctic

Abstract

Abstract.A single specimen of a previously undescribed acorn worm in the family Torquaratoridae was trawled from a bottom depth of about 350 m in the Kara Sea (Russian Arctic). The new species is the shallowest of the exclusively deep-sea torquaratorids found to date, possibly an example of high-latitude emergence. On the basis of ribosomal DNA sequences and morphology, the worm is described here as the holotype of Coleodesmium karaensis n. gen., n. sp. It is most similar in overall body shape to the previously described enteropneust genus Allapasus, but is uniquely characterized by a tubular component of the proboscis skeleton ensheathing the collar nerve cord. Additionally, within the proboscis, the sparseness of the musculature of C. karaensis clearly distinguishes it from the much more muscular members of Allapasus. The holotype is a female bearing about a dozen embryos on the surface of her pharyngeal region, each recessed within a shallow depression in the dorsal epidermis. The embryos, ranging from late gastrula to an early stage of coelom formation, are a little more than 1 mm in diameter and surrounded by a thin membrane. Each embryo comprises an external ectoderm of monociliated cells (not arranged in obvious ciliated bands) and an internal endo-mesoderm; the blastopore is closed. In the most advanced embryos, the anterior coelom is starting to constrict off from the archenteron. Coleodesmium karaensis is the first enteropneust (and indeed the first hemichordate) found brooding embryos on the surface of the mother's body.

Hemichordate models

“…However, the question of whether colonial tube-dwelling pterobranchs are sister to vermiform enteropneusts or whether they should be nested within them has been much debated [21][22][23][24]. Also, enteropneust phylogeny requires revision in light of recent investigations of deep-sea acorn worms [25][26][27]. Phylogenetic analysis based on morphology split the solitary enteropneusts into four families: the direct developing Harrimaniidae, two groups of indirect developers, the Spengelidae and Ptychoderidae, and the deep-sea spaghetti worms of the Family Saxipendiidae [28].…”

Biogeography and adaptations of torquaratorid acorn worms (Hemichordata: Enteropneusta) including two new species from the Canadian Arctic

“…This video survey recorded 92 individual Tergivelum cinnabarinum and found that it was most abundant in the northern section, 273 individuals of Yoda purpurata that had a higher density in the southern section, and one Allapasus isidis in the southwest (Jones et al 2013). More polar species include Coleodesmium karaensis (Osborn et al 2013) from the Kara Sea in the Russian Arctic. Others not described, but identified based on DNA sequences of worm fragments include one from Iceland, and two from the Ross Shelf Antarctica, collected between 732 and 531 meters depth (Cannon et al 2013).…”

“…The torquaratorid Allapasus aurantiacus is unusual in that it burrows beneath the surface (Holland et al 2012;Osborn et al 2013), whereas the remaining torquaratorids are instead epibenthic, surface deposit feeders. Their presence is sometimes made apparent by the presence of fecal casts that they deposit on the sea-bed, and that form distinct clockwise or counter-clockwise shaped spirals (Smith et al 2005;Jones et al 2013).…”

“…The least understood of these is the Torquaratoridae, which was established following the remarkable discovery of Torquarator bullocki (Holland et al 2005), collected from the deep-sea off the coast of Oregon. Since that time an additional six species have been described from five genera: Allapasus (Holland et al 2012), Coleodesmium (Osborn et al 2013), Tergivelum (Holland et al 2009), Torquarator (Holland et al 2005), and Yoda (Priede et al 2012), all from deep waters.…”

“…Yoda purpurata is a hermaphrodite (Priede et al 2012), a common trait in deep-sea animals where conspecifics can be uncommon. Coleodesmium karaensis externally broods its embryos in small sacs that connect to the trunk by a stalk (Osborn et al 2013).…”