Semelidae sp., 1856

Albano, Paolo G., Steger, Jan, Bakker, Piet A. J., Bogi, Cesare, Bosnjak, Marija, Guy-Haim, Tamar, Huseyinoglu, Mehmet Fatih, LaFollette, Patrick I., Lubinevsky, Hadas, Mulas, Martina, Stockinger, Martina, Azzarone, Michele & Sabelli, Bruno, 2021, Numerous new records of tropical non-indigenous species in the Eastern Mediterranean highlight the challenges of their recognition and identification, ZooKeys 1010, pp. 1-95 : 1

Semelidae sp. Figure 44

New records.

Israel • 1 spcm; Palmachim; 31.9737°N, 34.6767°E; depth 35.4 m; 24 May 2017; soft substrate; box-corer; Shafdan project (sample 29(A)); size: L 2.4 mm, H 2.0 mm.

Additional material examined.

Abra alba (W. Wood, 1802): Israel • 1 v; north of Atlit; 32.7422°N, 34.9181°E; depth 30 m; 20 Sep. 2016; sand; grab; HELM project (sample NG30_2M) • 2 vv; Ashqelon; 31.7101°N, 34.5406°E; depth 31 m; 18 Sep. 2016; sand; grab; HELM project (sample SG30_5M).

Remarks.

We were unable to identify this peculiar bivalve beyond family level, despite a thorough search in the literature on Mediterranean and Indo-Pacific mollusks. The shell of the single specimen found is roundly subtrigonal, fragile, with a rounded, steeply sloping anterior and a subacute to subtruncate posterior part; the ventral margin is slightly concave, the umbo submedian. The outer surface is smooth, glossy, and only sculptured by very fine growth lines. Although the small size and outline are reminiscent of certain galeommatoid genera such as Bornia Philippi, 1836, which is also represented in the Mediterranean Sea, the presence of both an external and an internal ligament, the latter situated on a well-developed resilifer, is typical for the family Semelidae ( Oliver 1992; Beesley and Ross 1998; Huber 2010). The hinge of both valves bears two cardinals; the posterior cardinal of the left valve is becoming obsolete by encroachment of the internal ligament portion which also extends vertically beyond the hinge line. Two well-developed laterals are present in the right valve, while in the left valve, only a weak tooth-like ridge is present anteriorly, formed by the dorsal shell margin. Due to the extremely smooth and glossy interior of the valves that renders hardly visible even the adductor muscle scars, it remains unclear whether a deep pallial sinus, another feature typical of semelids, is present in the studied specimen.

Lonoa katoi Habe, 1976, a semelid from Japan, shares with our species the small size and irregular outline with an often concave ventral margin (related to its attached lifestyle); however, the outer shell surface of L. katoi bears rough lamellae and fine radial threads ( Habe 1976; Okutani 2017), while that of the Israeli specimen is almost smooth. The most similar confamilial species from the Red Sea probably is Abra aegyptiaca Oliver and Zuschin, 2000, however, it differs from the specimen described here in shell shape, sculpture, the prosogyrate umbo, and features of the hinge such as the shape of the anterior lateral tooth of the right valve. Similar-sized juveniles of Mediterranean Abra spp., including A. alba , the most common species on the shallow Israeli shelf, have a hinge morphology comparable to our specimen, but differ in their outline and by having more protruding umbos ( Scaperrotta et al. 2013). Abra tenuis is most similar in shape, and a teratological specimen might approach the outline of our shell; however, such a specimen would still differ from Semelidae sp. by the presence of commarginal lines on the early dissoconch ( Scaperrotta et al. 2013; Oliver et al. 2020).

Until the finding of further specimens, it remains open whether the present individual is a juvenile or an adult of a small-sized species. Considering that only a single specimen was found so far, the lack of known native Mediterranean species with a similar morphology, and the geographical proximity of the Israeli coast to the Suez Canal, we suspect that this species might be another Indo-Pacific taxon introduced to the southeastern Mediterranean.

Figures

Figure 44. Semelidae sp., Palmachim, Israel, Shafdan project (sample 29 (A )): left valve outer view (A, B), right valve inner view (D, E), hinge (C, F) and outer view (G, H), left valve inner view (J-K) and hinge (I, L). The pink hue is due to staining with eosin solution. Scale bars: 1 mm (A, B, D, E, G, H, J, K); 0.5 mm (F, L); 0.3 mm (C, I).