Material examined: 1 larva, Norway, Svalbard, Nordenskioldland, Kapp Linné, Søndre Borgdam, N 78.07055 ° E 13.79345 °, 35m, 08.viii. 2008, K. Hårsaker, G. Velle.
Diagnostic characters. Tanytarsus heliomesonyctios can be separated from other known Tanytarsus larvae by having the three median teeth on mentum offset and more ventral to rest of mentum; premento-pharyngeal complex normally developed and not heavily sclerotized; mandible with one accessory dorsal teeth and a well developed accessory dorsal plate.
Description (n = 1). Head capsule ( Fig. 7) light yellowish-brown with darker posterior margins, mandibular teeth and mentum. Total length c. 4.2 mm.
Head capsule length c. 225 µm, width c. 200 µm; AR 1.71; AHR 0.34; AAR 1.0; antennal pedestal 76 µm long without spur ( Fig. 27); antenna with segments 1–2 well sclerotized, segments 3–5 hyaline, antennal seta placed at about 1 / 2 length of antennal segment 1; lengths of antennal segments (in µm): 76; 24; 10; 6; 5; Lauterborn organs small, on 35 µm long pedicels, slightly longer than apex of antenna ( Fig. 27); LOR 1.70. SII pectinate; chaetae plumose, pectinate; chaetulae simple; S 3 simple. Pecten epipharyngis consisting of three serrate scales; premandible with 4 narrow teeth, brush well developed ( Fig. 28). Mentum ( Figs 29–31) about 59 µm wide, median tooth without notches closely set with first lateral teeth, together with these offset ventrally to rest of mentum; median tooth about same colour as first lateral teeth, slightly paler than remaining teeth; ventromental plates 75 µm wide, slightly more curved laterally, with obvious striation along whole length; MVR c. 0.79. Mandible ( Figs 31–32) with 3 inner teeth, one apical tooth, two dorsal teeth and one robust dorsal plate; pecten mandibularis slightly convex; postoccipital plate moderately developed, rounded.
Body. Anterior parapods with long, simple spines; second lateral seta of abdominal segments not visible; posterior parapods with about 14 simple hooks in single row ( Fig. 12); anal segment with 4 anal tubules, c. 75 µm long; procercus with one short and one long dorsal seta, with 3 short (c. 125 µm) and 5 long (c. 400 µm) anal setae.