Bioacoustics of Bathylepeta linseae

Description . External morphology. Shell ( Fig. 1 A) size of holotype : 26 mm long, 22.7 mm wide, 11.7 mm high, not arched, height/length ratio 0.45, width/length ratio 0.87 ( Table 1 ). Anterior slope straight, posterior slope slightly convex. Aperture broad oval, with posterior widening from midpoint of shell onwards. Shell margin smooth, very thin, sharply edged. Protoconch and teleoconch strongly eroded. Apex (broken during examination) slightly posteriorly directed, situated immediately behind the center. Shell microsculpture smooth; with about 50, randomly arranged, faint off­white radial streaks shining through otherwise translucent blue­grey surface. Interspace of streaks narrower in anterior half. Whole shell with fine concentric growth lines. Soft parts generally cream white, lacking pigmentations. Head about of half of foot length ( Fig. 1 B). In relation to head mouth very large, rounded, anteriorly with large horseshoe­shaped jaw plate showing small beak in center ( Fig. 1 D). Tentacles rather long, eyeless; oral lappets present on both sides of snout ( Figs 1 B, D). Foot fleshy, oval, separated from head by deep furrow. Mantle edge thick, undulated, broad, with darker inner fold ( Fig. 1 B). Mantle cavity shallow around foot, slightly depressed in cephalic region. Gills absent; anus and genital papilla immediately anteriorly of head, on right side ( Figs 1 B, C). Internal anatomy. Gross anatomy is given for externally visible organs only. Shell muscle horseshoes­shaped, even thickness, occupying posterior two thirds of body. Digestive System—Mouth anteriorly with pair of single plates (each approx. 1.3 mm wide), forming jaw plate, centrally with small beak ( Fig. 2 A). Oral cavity nearly completely occupied by pair of elongate oval pharyngeal glands. Odontophore consists of pair of radula cartilages, well­developed, elongate oval, about 3.6 mm long ( Fig. 1 E). Radula ( Figs 1 E–F; 2B–F) docoglossate, formula 2­2­0­ 2 ­2 x 32. Radular ribbon 12.4 mm long. Central element ( Figs 1 F; 2B, C) consisting of fused first pair of lateral teeth sharply pointed, slightly curved and about 365 µm in length, rachidian tooth missing, base up to 140 µm wide. Second pair of lateral teeth sharply pointed, triangular in outline, about 100 x 60 µm ( Fig. 2 C). Basal plates underneath each lateral tooth not overlapping, rectangular, 390 x 295 µm. Two pairs marginal teeth, interpreted as functional uncini ( Lindberg 1998: 644 ); similar in form and size. Each tooth slender, parallel sided, strongly curved, with sharply pointed slightly inward directed tip, edges smooth ( Fig. 2 E); 240 µm in long 47 µm wide. Species L W H WL HL AL Source Iothia 4.9 3.2 1.78* 0.65 0.36 0.12* Dell (1990) Propilidium 4.2 pelseneeri 2.9 1.9 0.69 0.45 0.64* pers. obs. on holotype † 5.8 4.2 2.9 0.72 0.5 0.59 ZSM n = 1 Propilidium 1.7 sp. 1.3 0.7 0.76 0.41 ­ Pelseneer (1903) n = 1 coppingeri n = 1 5.3 4.3 2.5 0.81 0.47 0.14* Smith (1881) holotype Ventrally lying intestine forms three large clockwise arranged loops ( Fig. 1 D). From loop to loop, diameter decreasing. After third loop, intestine bypassing dorsal lying midgut gland ( Fig. 1 C) on right side, before anteriorly bending towards anus, situated anterior of head in so­called nuchal cavity. 2.0–7.7 (4.42) [1.48] 1.1–5.8 (3.14) [1.15] 0.6–3.4 (1.76) [0.73] 0.55–0.77 (0.70) [0.05] 0.29–0.53 (0.39) [0.06] 0.00–0.27 (0.17) [0.06] ZSM (Material and methods) n = 21 Lepeta (P.) antarctica 4 2.75 2 0.69 0.5 0.07* Smith (1907) n = 1 Bathylepeta laevis 30 19.7 23.3 15.1 ­ 6 0.78 0.77 ­ 0.30 ­ 0.56* Moskalev (1977) n = 2 Bathylepeta linseae 26 22.7 11.7 0.87 0.45 0.54 ZSM n = 1 Gonad ventrally situated on left side, partly covered by intestine. Genital papilla 2.3 mm long, with slightly curved tip to right of anus ( Fig. 1 B). Ecological notes. By using data from Sediment Profile Imaging Systems (SPIS), Multi and Large Box corers, and Acoustic Streamer the investigated area is interpreted as a 50 m deep and about 2.5 km wide channel on the sea floor. Its sediment is a greyish brown mud, consisting of angular quartz, lithic and mineral grains. The sediment also contains minor amounts (10%) of diatoms and centrics and radiolarian fragments. Video sequences obtained from SPIS shows a weak bottom current, and a bioturbated seabed with some scattered manganese­coated clasts, and possibly some sponges. A Conductivity­ Temperature­Depth data logger measured at 4731.6 m the following parameter: pressure 483.3 bar, temperature ­0.5°C, salinity 34.64, oxygen­content 5.48 ml/liter.