“Plawenia gen. nov.
Type species. Simrothiella schizoradulata Salvini-Plawen, 1978.
Known distribution. West European Basin and Bay of Biscay; Argentine Basin; off South Shetland Islands; off Peru; off San Diego, California (undescribed species); Japan Trench (undescribed species); and off La Réunion Island, Indian Ocean (undescribed species); from 2000 to nearly 6000 m.
Diagnosis. Body rotund, small, spiny; with thickset, hollow, upright epidermal spicules, skeletal spicules lacking; cuticle and epidermis thick; radular teeth with a strong, lateral buttress beneath large, lateral denticles; long, spiral anteroventral radular pockets retaining initial, triangular teeth; with two pairs of pharyngeal salivary glands, the dorsalmost acinar; seminal receptacles paired, bilobed; lower gametoducts first uniting and then becoming bilobed before emptying into mantle cavity through a papilla; copulatory spicules single, paired; a dorsoterminal sense organ present.
The following applies to all three species herein described: Epidermal spicules long, thin-walled (2 μm or less), gently curved or straight above an often bent and recurved base (Fig. 16); hollow internal space running from close to base nearly to tip in most spicules; proximal ends flat in widest spicules, flat to rounded in narrower spicules.
Paired anteroventral radular pockets remarkable, with numerous contained rows of teeth spiraled into flat, plate-like structures positioned perpendicular to dorsoventral axis of the body; left pocket spiraled counterclockwise, right one clockwise in ventral view (Fig. 19E, F). Teeth with distinctive parts (Fig. 17C, D): (1) a bar thickened laterally into a thick buttress; (2) a serrated or scalloped distal margin of the denticle-bearing membrane; (3) a pair of large, lateral denticles flanking two smaller ones above the buttress; and (4) a medial and distal hemispherical extension of the bar. Original, triangular tooth retained in anteroventral radular pocket (Fig. 17H–K).
Copulatory spicules long, with four distinct sections: (1) a terminal, thickened, conical, curved tip (Fig. 18A–C, F, H); (2) a long, distal, thin-walled, twisted hemispherical channel (Fig. 18F, H) followed by (3) a short, narrow, solid, twisted and furrowed section (Fig. 18E, G, H); and (4) a broad, hollow basal portion (Fig. 18D, E, G). Paired bundles of curved, hollow, thick-walled, accessory copulatory spicules bearing distal protuberances (Fig. 18J, K) present amongst thickly set epidermal spicules adjacent to mantle-cavity opening.
Remarks. No entire, fully developed copulatory spicule was recovered from 10 dissected Plawenia individuals. In two small individuals (1.5 mm in P. sphaera, 1.4 mm in P. argentinensis) the spicules were composed only of distal sections 1 and 2 from just above the solid, yet-to-be-formed midsection (Fig. 18F). In five of six larger individuals, the distal sections of the spicules were missing and the spicules broken, either through the solid midsection (Fig. 18D, G), or somewhat above it (Fig. 18E). In one individual of intermediate size, the basal section was beginning to form while the tip was retained (Fig. 18H). Such a constancy between body size and absence of the distal section perhaps indicates that the distal end is deciduous rather than missing as an artifact of dissection.
The biogeographic questions raised by the distributions of the three Plawenia species herein described are provocative. P. schizoradulata lives at 4758 m on the Pacific side of the great eastward-bending parabolic arc that stretches from the southern tip of South America through the Scotia Ridge to the islands lying northeast of the Antarctic Peninsula; the benthos is therefore part of the Pacific fauna. The species is also represented by a single individual from the Atacama Trench off Peru at 5821 m (Salvini-Plawen, 1978; specimen not examined by authors). The Scotia Ridge effectively cuts off the deep stenobathyal bottom fauna of the Pacific from the Atlantic. That P. argentinensis from the Atlantic Argentine Basin at >4000 m depth shows morphological differences from P. schizoradulata is therefore not surprising; what is puzzling is the close similarity between P. schizoradulata and P. sphaera from only 2091 m in the West European Basin in the northeast Atlantic.”
(Scheltema & Schander, 2000: 138-139)
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