Limatula smithi, 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 : 2607-2611

Limatula smithi sp. nov.

( figures 17–21 , 45c )

Type locality. Atlantis sta. 191, 23 ‡ 05.0 ’ S, 12 ‡ 31.5 ’ W, 1546 m.

Holotype. BMNH.

Material examined. Atlantis II sta. 186, 22 ‡ 57.0 ’ S, 13 ‡ 05.0 ’ W, 481 m, 3 v; sta. 189, 23 ‡ 00.0 ’ S, 12 ‡ 45.0 ’ W, 1007 m, 45 spec.; sta. 191, 23 ‡ 05.0 ’ S, 12 ‡ 31.5 ’ W, 1546 m, 11 spec. z2 v; sta. 198, 10 ‡ 29.0 ’ S, 09 ‡ 04.0 ’ W, 4559 m, 1 spec.

Also examined: USNM No. 149297, Valorous sta. 11, 57 ‡ 50.0 ’ N, 44 ‡ 52.0 ’ W, 3385 m, 1 shz6 v.

Distribution. West European to Angola Basins and central Atlantic from upper to lower slope depths ranging from 481 to 3385 m.

Description ( figures 17–19 , 45c ). Shell moderately small (maximum recorded height 11.7 mm), height/length ratio 1:0.59, moderately fragile, inflated, length/width ratio 1:0.90, equivalve, very slightly ineqilateral with posterior margin slightly more narrowly curved than anterior, transluscent white, ornamented by straight radial ribs crossed by variously spaced concentric incremental lines, radial ribs number 40–48, except antero- and postero-dorsally close to ears ribs extend over entire shell, lateral ribs progressively less well-defined, small sharply pointed reticulations occur where ribs and concentric lines cross, two central ribs larger than remainder, internally shell centrally deeply channelled, ears relatively small, subauricular sinuses present, anterior the more pronounced; umbos prominent, beak pointed, dorsal hinge margin very slightly concave on either side of mid-point; hinge plate with central grooves on either side of resilium into which fit extended ligament, resilium and central ligament broadly oval. Prodissoconch I length~ ca 110 M m; interdissoconch indistinct, ca 0.7 mm.

Anatomy ( figures 20 , 21 ). The anatomy is similar to that described for L. subovata and L. margaretae . Mantle margin less broad than other species described here, each sensory fold bears 14–15 tentacles, small papillate ones alternate with larger finger-like tentacles of which latter three ventral-most ones are largest, postero-dorsal anal tentacle on each side separate from other tentacles, gills extensive, both demibranchs with ascending and descending lamellae.

Remarks. The specimens from Valorous sta. 11, which are part of the Jeffreys collection in the USNM are labelled Limatula confusa but are the same species as described here. The label is old but probably not the original. Curiously, Jeffreys (1876) does not report on specimens from this station. What he did describe were specimens from the adjacent Valorous sta. 12. These he identified as Lima (~ Limatula ) ovata S. V. Wood. This latter was an identification that Smith (1885) disagreed with and Smith included the specimens from Valorous sta. 12 with Lima (Limatula) confusa which he described from various Challenger stations (see Appendix). Presumably the specific name refers in part to Jeffreys’ misidentification and partly to the fact that Jeffreys (1876) reported that there were about 50 radial ribs whereas Smith (1885) reported that there were 30 in his specimens and those of Jeffreys (1876) from Valorous sta. 12. It would appear then that Jeffreys probably referred specimens to the wrong station.

Apart from the fact that L. smithi has a greater number of ribs than L. confusa , it also possesses an internal central groove, which L. confusa is said not to have ( Jeffreys, 1876). Furthermore, L. smithi has a less oblique outline, is less broad and the posterior margin is shallowly curved giving the shell, particularly of moderate- and larger-sized specimens, a characteristic outline, in which the anterior and posterior margins appear almost parallel. Limatula hyperborea , a northern shallow-water species, has a similar outline, although the latter has fewer ribs that do not extend as far dorsally as those of L. smithi . Limatula ovata like L. confusa has a much broader shell.

In this species there is no evidence that the gills are used as brood pouches nor that the eggs are retained within the mantle cavity. This species has the smallest prodissoconch of all the species described here and it may well be that there is a lecithotrophic planktonic phase. The species is named after A. E. Smith who described the bivalves of the Challenger Expedition.

Figures

FIG. 17. Limatula smithi. Sta. 191, lateral view of shell from right side. Scale bar: 1 mm.

FIG. 18. Limatula smithi. Sta. 189, dorsal view of shell. Scale bar: 1 mm.

FIG. 19. Limatula smithi. Sta. Valorous 11, internal view of hinge of left valve. Scale bar: 1 mm.

FIG. 20. Limatula smithi. Sta. 189, semi-diagrammatic view of the anatomy from the right side. Scale bar: 1 mm.

FIG. 21. Limatula smithi. Sta. 191, inner view of left mantle margin to show disposition of the tentacles and papillae. Scale bar: 1 mm.

FIG. 45. SEM photographs of species of Limatula. (a) L. louiseae; (b) L. margaretae; (c) L. smithi; (d) L. celtica..