Yoldiella wareni, La Perna, 2004

La Perna, R., 2004, The identity of Yoldia micrometrica Seguenza, 1877 and three new deep-sea protobranchs from the Mediterranean (Bivalvia), Journal of Natural History 38, pp. 1045-1057 : 1049-1052

Yoldiella wareni n. sp.

( figure 3 a–h)

Leda oblonga Jeffreys, 1874: 112 (nomen nudum).

Yoldiella micrometrica auct. non Seguenza, 1877a, 1877b.

Type material. Holotype (left valve, MZB 40661) and 26 paratypes (one sh, MZB 40662; 25 vs, author’s collection).

Type locality. Cruise ‘ Eocumm 95’, southern Tyrrhenian (off northern Sicily), st. 14, 38 ° 20 ∞ 08 ◊ N, 14 ° 10 ∞ 47 ◊ E, 1139 m .

Examined material. Cruise ‘Eocumm95’, southern Tyrrhenian: type materialst. 2B, 38 ° 30 ∞ 27 ◊ N, 14 ° 41 ∞ 42 ◊ E, 1355 m, one v; st. 7, 38 ° 34 ∞ 09 ◊ N, 14 ° 25 ∞ 01 ◊ E, 1197 m, one sh, 22 vs; st. 9, 38 ° 20 ∞ 08 ◊ N, 14 ° 23 ∞ 10 ◊ E, 1511 m, one v; st. 10, 38 ° 13 ∞ 27 ◊ N, 14 ° 24 ∞ 50 ◊ E, 1357 m, eight vs; st. 15, 38 ° 29 ∞ 06 ◊ N, 14 ° 11 ∞ 39 ◊ E, 1521 m, one sh, nine vs; st. 16, 38 ° 33 ∞ 42 ◊ N, 14 ° 13 ∞ 59 ◊ E, 1340 m, 20 vs; st. 26, 38 ° 38 ∞ 30 ◊ N, 14 ° 57 ∞ 53 ◊ E, 945 m, two shs, 22 vs; st. 27B, 38 ° 32 ∞ 26 ◊ N, 15 ° 05 ∞ 02 ◊ E, 1096 m, three vs; st. 31, 38 ° 32 ∞ 03 ◊ N, 15 ° 28 ∞ 18 ◊ E, 1061 m, six vs; st. 32B, 38 ° 33 ∞ 53 ◊ N, 15 ° 05 ∞ 33 ◊ E, 1219 m, two shs, five vs; st. 33, 38 ° 35 ∞ 23 ◊ N, 15 ° 25 ∞ 40 ◊ E, 875 m, 16 vs; st. 34, 38 ° 42 ∞ 36 ◊ N, 15 ° 28 ∞ 06 ◊ E, 1536 m, two vs; Cruise ‘Interreg Italia-Albania’, southern Adriatic: st. 54, 40 ° 03 ∞ 10 ◊ N, 19 ° 21 ∞ 41 ◊ E, 942 m, one v; st. 55, 40 ° 00 ∞ 59 ◊ N, 19 ° 08 ∞ 55 ◊ E, 976 m, 16 shs, 22 vs; st. 57, 40 ° 09 ∞ 28 ◊ N, 18 ° 59 ∞ 13 ◊ E, 910 m, three shs, seven vs; st. 66, 40 ° 50 ∞ 37 ◊ N, 18 ° 29 ∞ 05 ◊ E, 759 m, five shs, five vs; st. 72, 41 ° 27 ∞ 34 ◊ N, 17 ° 25 ∞ 02 ◊ E, 950 m, nine shs, 24 vs.

Description. Shell small, slightly inequilateral, ovate-elongated, not rostrated, moderately inflated, thin-walled, fragile, glossy. Umbo posterior to midline, small,

barely projecting from shell outline, slightly opisthogyrate. Antero-dorsal and postero-dorsal margins convex, making a smooth wide curve. Anterior margin smoothly rounded. Posterior margin a little narrower, obscurely angulated below shell height midline. Ventral margin wide, evenly curved, moderately convex. Surface with fine growth lines, becoming well incised, and irregularly spaced lines near ventral margin. Periostracum thin, shiny, pale. Hinge plate thin, making an angle of about 150 °, anterior row a little longer than posterior one. Dentition taxodont, with slightly chevron-shaped teeth, numbering five anteriorly and four posteriorly. Ligament pit elongated, slit-like, shallow, with a small outward posterior part. Muscle scars roundish, fairly well distinct. Pallial line forming a shallow, hardly distinct posterior sinus. Prodissoconch matt, D-shaped, ca 290 µ m long. Holotype 1.50 mm in length, 0.97 mm in height, 0.31 mm in breadth. Paratypes 1.07–1.51 mm in length.

Etymology. Named in honour of Anders Warén, eminent Swedish malacologist.

Distribution. Although erroneously identified as Y. micrometrica , Y. wareni n. sp. has been not infrequently recorded from the Mediterranean, both from the Eastern Basin ( Janssen, 1989) and the Western ( Di Geronimo and Bellagamba, 1986; Bonfitto et al., 1994; Di Geronimo et al., 2001). It is also one of the most common deep-sea protobranchs, mainly ranging from 500 m down to 2000 m, or possibly more ( Di Geronimo et al., 2001). It was not reported either from the Alboran Sea, or from the Ibero-Moroccan Gulf by Salas (1996) and this may be the definitive answer about its questioned occurrence in the North-East Atlantic, from where there is a single record by Jeffreys (1879), most probably based on a different species (see below). ‘ Y. micrometrica ’ was also reported from the Pleistocene of southern Italy by La Perna (in press). On closer examination, these records turned out to be based on the true Y. micrometrica and on a form very similar to Y. wareni n. sp. ( figure 3g, h). The available material is too scarce and poorly preserved to decide if these valves, a little larger, more elongated and inequilateral than the Recent ones, can be referred to Y. wareni n. sp., and thus it is provisionally reported as Y. aff. wareni .

Remarks. In spite of the long history of misidentification of Y. wareni n. sp. as Y. micrometrica , these two species are not particularly similar, the former being markedly ‘bean-shaped’, thinner and smaller. Similar differences also occur between Y. ovulum n. sp. and Y. wareni n. sp. There is a sole living species which may be worth being compared with Y. wareni n. sp., i.e. the Atlantic Yoldiella dissimilis Verrill and Bush, 1898 , with bean-shaped valves. However, it is larger (up to ca 4 mm in length), more solid and with an evenly ridged surface. Also Yoldiella fabula Allen, Sanders and Hannah, 1995 was described as ‘bean-shaped’ ( Allen et al., 1995) and is very similar to, or possibly a synonym of, Y. dissimilis .

The tradition of applying Seguenza’s name to this species was started by Jeffreys (1874), who reported Leda oblonga from off Algeria as a nomen nudum. L. oblonga was then included in the check-list of Mediterranean species by Monterosato (1875 a, 1878) and also reported by the same author (1875b) from the Sicily Channel. After the description of Leda micrometrica by Seguenza (1877a, 1877b), Jeffreys (1879) definitively adopted this name (‘I had previously given to this minute but distinct species the name oblonga , by which Monterosato called it’). While the identity of the species reported as Leda oblonga from the Mediterranean is undoubted, it is difficult to understand the identity of the species reported under this name from off western Ireland by Jeffreys (1879). Actually, he did not take very much notice of Monterosato’s work, but he corresponded and exchanged specimens with Seguenza ( Warén, 1980). It may also be noted that he never believed in Darwinian evolution and seems to have had an overemphasized idea that all Tertiary and Quaternary molluscs still existed. Moreover, the small size of Y. wareni n. sp. must have caused misidentifications with other small species or juveniles, like the case of Tindaria sericea (Jeffreys, 1876) reported by Warén (1989).

Yoldiella wareni n. sp. is one of the smallest European protobranchs. Miniaturization is sometimes associated with paedomorphic features (e.g. Hanken and Wake, 1993; Gofas and Salas, 1996), i.e. retained juvenile or even larval features, and indeed the present species shows juvenile ligament features. The ligament pit is elongated, slit-like and with a small outward posterior part, very similar to that of juvenile nuculanids, as well illustrated by Ockelmann and Warén (1998). With growth, the ligament pit becomes roundish or somewhat triangular and fully internal.