Thermiphione risensis (Pettibone 1986)

Iphionella risensis

MATERIAL EXAMINED (East Pacific Rise).—Holotype: USNM 81967, 20°50′N, 109°06′W, Alvin dive 1226-7, 10 May 1982, 2616 m, Rifta, Calyptogena, and Alvinella wash, coarse fraction.

Paratypes: USNM 81969, 20°50°N, 109°06′W, Alvin dive 1222-5b, 6 May 1982, 2614 m, rubble sample from Calyptogena residue, young of 11 segments. USNM 81970, 20°50′N, 109°06′W, dive 1223-17, 7 May 1982, 2616 m, rubble, 1 paratype. USNM 81971, same coordinates and dive, 2 paratypes, young of 24 and 18 segments. USNM 81968, 97401, same coordinates, Alvin dive 1226-7, 10 May 1982, Riftia, Calyptogena, and Alvinella wash, coarse fraction, 2 paratypes, young of 11 segments.

DESCRIPTION.—Length of holotype 10 mm, width 6 mm with setae, segments 28 (incomplete); lengths of two complete paratypes 9 and 11 mm, widths 5 and 8 mm, segments 29. Young of 24 segments, 4.5 × 3 mm; young of 18 segments, 3 × 2 mm; two young of 11 segments, 1.2 and 1.0 × 1.0 mm.

The body is ovate, greatly flattened, tapered slightly anteriorly and more so posteriorly, with the parapodia very close and crowded. Thirteen pairs of reddish elytra (mostly missing) and large elytrophores are found on segments 2, 4, 5, 7, on alternate segments to 23, and on 26, with dorsal cirri and large dorsal tubercles on segments 3, 6, 8, alternate segments to 24, and on 25, 27, 28, and 29 (Figure 12A,C,D). The elytra are oval and reniform (Figures 12F, 14A,B,E,F). The elytral surface is nearly covered with hexagonal or polygonal areas enclosing small areolae, with scattered lateral papillae. Along the lateral borders some of the areolae are raised, forming conical microtubercles.

The elytrophores are large and bulbous, their place of attachment oval with a thin-walled medioposterior extension (Figures 12A,C,D; 13B,D). The dorsal tubercles are thin-walled and somewhat ruffled, continuing laterally with a large curved glandular area on the posterior basal part of the long cirrophores of the dorsal cirri; the styles are papillate, tapered and extend beyond the tips of the setae Figures 12C; 13C,E).

The prostomium and first or tentacular segment are partially fused and withdrawn in segments 2 and 3 (Figure 12A,B). The prostomium is bilobed, forming two separate rounded lobes, with anterolateral bulbous extensions fused to the medial facial tubercle, which is continuous with the upper lip of the anterior mouth, and enclosed by the stout, smooth, tapered palps. Antennae and eyes are lacking. The first segment is not visible dorsally; long cylindrical tentaculophores emerge lateral to the prostomium and palps, each with a stout aciculum, a few capillary setae on the inner side and distally with a pair of short, papillate dorsal and ventral tentacular cirri (Figures 12A,B, 13A).

The second segment has a small medial nodule overlapping the prostomium, large elytrophores bearing the first pair of elytra and biramous parapodia directed anteriorly and enclosing the prostomium and tentaculophores, and ventrally, with stout buccal cirri similar to the tentacular cirri (Figures 12A,B, 13B). The large ventral lip is enclosed in the biramous parapodia of segments 2 and 3. Due to the anterior position of the mouth, the anterior, lateral and posterior lips of the mouth are visible dorsally, anterior to the prostomium.

The distal rim of the eversible stout muscular pharynx has 9 pairs of soft, transparent dorsal and ventral papillae and 2 pairs of stout hooked jaws (Figure 12E).

Segment 3 is indistinct dorsally, the dorsal cirri and parapodia are wedged between the large elytrophores and parapodia of segments 2 and 4. Small oval medial nodules are found on segments 4 and 5 (Figure 12A). Most of the segments have slightly raised transverse dorsal ridges, 2 per segment, with deep rounded depressed areas medial to the dorsal tubercles and elytrophores (Figure 12C,D). The small conical pygidium and dorsal anus are enclosed in the small posterior segments (28, 29, plus remnants of additional minute parapodia on one of the paratypes; Figure 12D).

The rami of the biramous parapodia are closely allied, with conical notopodia on the anterodorsal sides of the neuropodia (Figure 13B–E). The very numerous notosetae are straw-colored, delicate, capillary, and bipinnate feathered, with a slender axis and close-set lateral spines emerging on the same level (similar to the notosetae of Iphione; Figure 13F). The presetal lobes of the larger neuropodia are conical with a projecting acicular process, the postsetal lobes are shorter and rounded, together enclosing very numerous amber-colored neurosetae. The upper group of neurosetae are feathered, similar to the notosetae except that they are stouter, with shorter bare pointed tips (Figure 13G). The rest of the neurosetae are stouter, longer (upper ones) to shorter (lower), with close-set spinous rows on the basal enlarged part and with rather long bare slightly hooked tips (Figure 13H). The neurosetae of segments 2 and 3 are of the slender feathered type, similar to the upper neurosetae of the following segments. The ventral cirri are short, tapering and papillate (Figure 13C–E) The ventral nephridial papillae are small, beginning about segment 8.

A minute young paratype, 1 mm in length, possessed 11 segments, with indication of developing twelfth parapodia (Figure 14C–F). The prostomium is squarish, showing slight indications of developing anterolateral extensions; some pharyneal papillae protrude from the mouth. Two elytra remained: a large reddish one on segment 5 and a small transparent developing one on segment 11 (Figure 14E,F).

BIOLOGY.—I. risensis was collected in rubble, residue, and washes associated with vestimentiferans, Riftia pachytila Jones, giant clams, Calyptogena magnifica Boss and Turner, and ampharetid polychaetes, Alvinella pompejana Desbruyères and Laubier.

DISTRIBUTION.—Off western Mexico, in 2614–2616 meters.

ETYMOLOGY.—The species is named risensis based on its association with the hydrothermal rift-area on the East Pacific Rise.