Limatula thalassae, Allen, 2010

Allen, J. A., 2010, The Recent species of the genera Limatula and Limea (Bivalvia, Limacea) present in the Atlantic, with particular reference to those in deep water, Journal of Natural History 38 (20), pp. 2591-2653 : 2614-2616

Limatula thalassae sp. nov.

( figures 26–28 , 46a )

Type locality. Thalassa sta. Z397, 47 ‡ 33.8 ’ N, 07 ‡ 12.6 ’ W, 511 m.

Holotype. MHNP.

Material examined. Thalassa sta. Z397, 47 ‡ 33.8 ’ N, 07 ‡ 12.6 ’ W, 511 m, 2 spec.; sta. Z402, 47 ‡ 39.5 ’ N, 07 ‡ 28.5 ’ W, 450 m, 1 spec.; sta. Z416, 48 ‡ 13.5 ’ N, 09 ‡ 07.5 ’ W, 480 m, 2 spec.

Distribution. All specimens were taken at slope depths in the Bay of Biscay, WSW of Brest from muddy sands with an admixture of stones, rocks and calcareous maerl; 450– 511 m.

Description ( figures 26 , 27 , 46a ). Shell small (max. recorded height 5.3 mm), slender, height/length ratio 1:0.56, fragile, inflated, almost as wide as long, equivalve, almost equilateral, white, ornamented with ca 38 ribs of which 16 central ribs are prominent (of these the two most central are no more prominent), lateral ribs less prominent, growth lines strongly marked and widely separate, sharp-pointed reticulations where growth lines cross ribs, ears slightly produced with relatively deep posterior auricular sinus and much smaller and shallower anterior sinus, curvature of both posterior and anterior shell margins shallow giving shell typical elongate parallel appearance, ventral margin smoothly curved; umbos prominent; dorsal margin of hinge sharp and more or less straight at very slight angle to vertical axis, hinge plate moderately strong, without teeth; ligament central and triangular. Prodissoconch ca 420 M m; interdissoconch ca 0.85 mm.

Anatomy ( figure 28 ). Mantle margin broad, composed largely of middle sensory fold and which bears ca 18 tentacles in a specimen 1.5 mm in height, these include two large annular ventral tentacles and one large annular anal tentacle on each lobe together with smaller tentacles and papillae, the latter comprise two small tentacles and three papillae on each posterior sensory fold, one small tentacle between the two large ventral tentacles and two small tentacles and four papillae on each anterior sensory fold, posterior adductor relatively small and broadly ovate in cross-section; outer demibranch more lobate and posterior in position than inner. In other respects the anatomy is similar to that of other species described here.

Remarks. Limatula thalassae is characterized by its slender, elongate shape and almost straight and parallel anterior and posterior shell margins. It most resembles the high arctic species L. hyperborea in number and disposition of the radial ribs and lack of a marked median rib, however, the latter species has slightly more rounded anterior and posterior shell margins and does not possess such spinose reticulations.

Limatula thalassae is named after the Campagne de la ‘Thalassa’ which took place from 21 to 30 October 1973 and which first obtained the species.

Figures

FIG. 26. Limatula thalassae. Sta. z387, lateral view of shell from left side. Scale bar: 1 mm.

FIG. 27. Limatula thalassae. Sta. z416, internal view of the hinge of the right valve. Scale bar: 1 mm.

FIG. 28. Limatula thalassae. Sta. z397, semi-diagrammatic views of the left and right mantle margins to show disposition of the tentacles. Scale bar: 1 mm.

FIG. 46. SEM photographs of species of Limatula and Limea. (a) Limatula thalassae; (b) Limatula laminifera; (c) Limea lirata; (d) Limea sarsi.