Limatula celtica, 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 : 2611-2614

Limatula celtica sp. nov.

( figures 22–25 , 45d )

Type locality. West-south-west of Ireland, 50 ‡ 43.5 ’ N, 17 ‡ 51.7W, 4632 m.

Holotype. BMNH.

Material examined. Chain sta. 323, 50 ‡ 08.3 ’ N, 13 ‡ 53.7 ’ W, 3356 m, 3 spec; sta. 328, 50 ‡ 04.7 ’ N, 15 ‡ 44.8 ’ W, 4426 m, 7 spec.; sta. 330, 50 ‡ 43.5 ’ N, 17 ‡ 51.7 ’ W, 4632 m, 4 spec.

Atlantis II sta. 155, 00 ‡ 03.0 ’ S, 27 ‡ 48.0 ’ W, 3783 m, 3 spec.

Polygas sta. DS20, 47 ‡ 33.0 ’ N, 09 ‡ 36.7 ’ W, 4226 m, 3 spec.; sta. DS21, 47 ‡ 31.5 ’ N, 09 ‡ 40.7 ’ W, 4190 m, 2 spec.

Biogas III sta. DS45, 47 ‡ 33.9 ’ N, 09 ‡ 38.4 ’ W, 4260 m, 1 spec.; IV sta. DS55, 47 ‡ 34.9 ’ N, 09 ‡ 40.9 ’ W, 4125 m, 2 spec.; sta. DS56, 47 ‡ 32.7 ’ N, 09 ‡ 06.2 ’ W, 4050 m, 2 spec.

Incal sta. DS16, 47 ‡ 29.8 ’ N, 09 ‡ 36.2 ’ W, 4268 m, 5 spec.

Distribution. Present at abyssal depths. Most records are to the west of Biscay and southern Ireland with one much further south off equatorial Africa. This gap in the records possibly relates to the lack of abyssal sampling in the subtropical eastern Atlantic; 3356–4632 m.

Description ( figures 22 , 23 , 45d )). Shell small (max. recorded height 6 mm), height/length ratio 1:0.63, fragile, inflated, wide as long with characteristic quadrilateral outline in dorsal view, equivalve, equilateral, almost symmetrical, white with slight transluscence, ornamented with two fine but prominent central ribs either side of a narrow central furrow and, on either side, extremely fine (sometimes almost invisible to the naked eye) radial ribs (ca 40 in total) that tend to be more prominent laterally and extend dorsally for two-thirds of the anterior and posterior shell margins, apart from two central ribs, ribs end short of the umbonal region, incremental growth lines vary in number; ears and subauricular sinuses absent, curvature of anterior and posterior shell margins similar except anterior limit lies slightly ventral to mid-line while posterior limit lies dorsal to the mid-line, ventral to the mid-line the anterior and posterior shell margins taper to a narrow ventral margin giving a characteristic spindle shape; umbos prominent; dorsal margin of hinge at angle to vertical axis of shell, hinge without teeth, cardinal area flat and lozenge-shaped in dorsal view with lozenge-shaped central ligament. Prodissoconch I length~228 M m; interdissoconch indistinct, ca 2 mm.

Anatomy ( figures 24 , 25 ). Middle sensory mantle fold extremely broad extending inwards across two-thirds of the mantle cavity such that gills extend into pocket formed by fold and mantle epithelium lining the shell, each sensory fold bears three rows of tentacles, innermost largest and outermost the smallest, two inner ventral tentacles are by far the largest with eight evenly spaced smaller tentacles just to outside of these, outer row of 10 very small papillae, long anal tentacle lies immediately ventral to insertion of elongate posterior adductor muscle, adductor small and ovate in cross-section; foot short in preserved specimens with many fine byssus threads; both gill lamellae present and reflected, filaments of outer curve to form cup to hold developing eggs, demibranchs are plicate except for the anterior-most part of inner demibranch which comprises ca nine homorhabdic filaments. In other respects the anatomy is similar to that described for other species.

Remarks. The most remarkable feature of this species is the extent of the mantle folds, so extensive as to form a pair of deep mantle pockets into which the gills hang. The latter are modified to form incubation chambers for the developing eggs as too are the pockets themselves ( figure 24 ). As in the more spectacular species Lima hians , the tentacles are arranged in a number of rows in which the larger tentacles are innermost ( Gilmour, 1963). Unlike Lima hians the tentacles can be contained within the mantle cavity (as they are in all species described here). Although no specimen was taken in which the mantle margin was extended beyond the shell margins, this does not mean to say that this cannot occur. All species have a very long anal tentacle on each mantle edge adjacent to the anus and it is possible that these are used to facilitate the ejection of faeces and pseudofaeces from the mantle cavity.

Although only two specimens were recorded with numerous fine byssal threads, it must be assumed that they were shed in specimens without them. All species examined have well-developed byssal glands and like their larger relative Lima hians may make nests within which they live ( Gilmour, 1963, 1967).

Adult specimens of L. celtica are easily distinguished by their spindle shape and narrow, acutely curved ventral margin, extremely fine lateral radial ribs and inflated valves which peak along the line of the two dominant central ribs. The juveniles are less easy to identify in that they have a broader ventral margin that is similar to that of the most closely related species L. subovata . However, the latter differs by its much coarser and more numerous ribs. It is named after the sea area in which it has been predominantly found.

Figures

FIG. 22. Limatula celtica. Sta. Incal DS16, lateral view of shell from the left side. Scale bar: 1 mm.

FIG. 23. Limatula celtica. Sta. Incal DS16, dorsal view of shell. Scale bar: 1 mm.

FIG. 24. Limatula celtica. Sta. Incal DS16, semi-diagrammatic views of the anatomy: (a) left mantle and viscera; (b) right mantle; (c) excised right demibranch. Scale bar: 1 mm.

FIG. 25. Limatula celtica. Sta. Incal DS16, semi-diagrammatic posterior view of the anatomy of a specimen with developing embryos within the mantle ‘pouches’. Scale bar: 1 mm.

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