Tanytarsus heliomesonyctios Langton 1999

Tanytarsus heliomesonyctios Langton, 1999

Material examined: 1 larva, Norway, Svalbard, Nordenskioldland, Kapp Linné, Søndre Borgdam, N78.07055° E13.79345°, 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; AR1.71; 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; S3 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.