Hippomedon tracatrix J. L. Barnard 1971

Hippomedon tracatrix

DIAGNOSIS.—Head rather small but lateral cephalic lobe quite long, strongly projecting, well delineated dorsally, narrow, acute; eyes absent; article 1 of antenna 1 barrel shaped, lacking a distal tooth, or dorsal crest, article 1 of flagellum about two-thirds as long as article 1 of peduncle; epistome and upper lip flat anteriorly, separated by a deep incision; article 3 of mandibular palp about 65 percent as long as article 2, of intermediate stoutness, medial edge setose along three-fourths of its length; mandibular molar small for the genus, poorly projecting, teeth and ridges numerous but not strongly developed, molar bearing a plumose seta; coxa 1 slightly shorter than 2, slightly tapering distally, essentially the taper occurring as posterior bevelment, distal end rounded; article 6 of gnathopod 1 slightly longer than 5, palm oblique but well defined and shorter than posterior margin of article 6; gnathopod 2 subchelate, dactyl of intermediate length; coxa 4 very poorly developed for the genus, posterior edge concave but posteroventral lobe scarcely defined; posterior lobe of article 2 of pereopod 5 scarcely tapering distally, broad ventrally, posterior serrations large and numerous for the genus; pleonal epimeron 1 with anteroventral quadrate lobe, posteriorly rounded, epimeron 2 rounded anteroventrally, with rudimentary posteroventral tooth, epimeron 3 rounded anteroventrally, with intermediate-size posteroventral tooth slightly upturned; pleonite 4 with large, erect, triangular crest, acute; inner ramus of uropod 3 as long as article 1 of outer ramus, article 2 of outer ramus about 35 percent as long as article 1; telson short for the genus, thus rather broad, cleft about five-eights its length, each apex rather broad and armed with a spine and a setule; body with faint striation-sculpture.

HOLOTYPE.—USNM 127135, female, 7.4 mm. Unique.

TYPE-LOCALITY.—Station 81, 44°38.1′N, 125° 35.0′W, 2,800 m, 11 August 1964.

RELATIONSHIP.—Because of the cutoff condition of coxa 1, this species resembles Hippomedon tasmanicus J. L. Barnard (1961b) more than any of the other known species. In its gnathopod 2, palm of gnathopod 1, and the third pleonal epimeron it also resembles H. tasmanicus, but it differs by the more clearly defined lateral cephalic lobe, the erect, acute dorsal crest of pleonite 4, the inverse relationship of the lengths of articles 5 and 6 of gnathopod 1, the slightly different shapes of the second articles of pereopods 3–5, the shorter telson, and the pattern of the chitin surface.

This species is outside of the mainstream of the genus Hippomedon in the slightly tapered coxa 1 and the development of a dorsal crest on pleonite 4 and in the very short article 5 of gnathopod 1 and the smallness of that gnathopod; the upper lip does not project in front of the epistome. But H. tracatrix resembles Uristes typhlops mediator J. L. Barnard (1962d) from the South Atlantic Ocean and differs from that subspecies in the thinner tooth of epimeron 3, the sharper ocular lobe, and the slightly shorter article 5 of gnathopod 1. Hippomedon tracatrix differs from Uristes t. typhlops in the very small gnathopod 1 with short article 5, the slightly elongate inner plate of maxilla 2, and the distally narrowed coxa 1. Coxa 1 is not certainly known in U. t. typhlops; and U. t. mediator may actually represent a distinct species in a distinct subgenus (or genus) from U. t. typhlops.

Hippomedon tracatrix also resembles Paracentromedon carabicus J. L. Barnard (1964e) but has a longer article 3 on the mandibular palp, a stronger mandibular molar, a slightly narrowed coxa 1, stronger anteroventral protrusion on epimeron 1, a thinner tooth on epimeron 3 and lacks serrations on that ventral margin, and finally has a much shorter article 5 of gnathopod 1.

Elimedon cristatus J. L. Barnard (1962d) is another species having resemblance to H. tracatrix, but the latter differs from the former in the longer article 3 of the mandibular palp, the shorter article 5 of gnathopod 1, the tapered coxa 1, and a shorter telson.

From Hippomedon concolor J. L. Barnard (1961b), H. tracatrix differs in the longer and sharper ocular lobe, the thinner tooth on epimeron 3, the tapered coxa 1, and the longer, sharper ocular lobe.

The species carabicus, cristatus, and concolor all have the sharp dorsal crest of pleonite 4 and thus resemble H. tracatrix, whereas most other species of Hippomedon lack the crest and/or have an untapered coxa 1.

DISTRIBUTION.—Oregon, 2,800 m.