“Draculamya” uraniae: A New Small-Sized Bivalve from the Mediterranean Sea (Galeommatida, Lasaeidae)

A new Galeommatid bivalve is described for the Mediterranean Sea, tentatively assigned to the elusive genus Draculamya Oliver and Lützen, 2011. “Draculamya” uraniae n. sp is described upon a number of dead but fresh and articulated specimens, plus many loose valves. Its distribution is almost basin-wide in the Mediterranean, and it possibly occurs in the adjacent Gulf of Cadiz. As for many members in Galeommatida, we hypothesize that “Draculamya” uraniae lives as commensal upon a still-unknown host. The possible co-identity of the extant genus Draculamya with the morphologically similar Pliocene Glibertia Van der Meulen, 1951, is discussed, although the lack of anatomical and genetic support leaves the problem open.


Introduction
The order Galeommatida constitutes a species-rich group of small bivalves, most of them commensal with other invertebrates [1]. The phylogeny and systematics within the Galeommatida have been the subject of a variety of taxonomic classifications, e.g., [2][3][4][5][6][7]. Recent work has clarified that Galeommatida are monophyletic, although the placement of the genera is yet far from being adequately assessed [1,[8][9][10]. The Galeommatida are cosmopolitan in distribution, with many species inhabiting European waters [11,12], but our overall knowledge of them remains fragmentary. The first modern revisions were made by van Aartsen [13,14], followed by additional taxonomic descriptions and anatomical remarks [15][16][17][18][19][20][21][22][23][24]. In this context, Oliver and Lützen [25] described the new genus Draculamya, from deep waters in the north-western Atlantic Ocean, characterized by peculiar anatomical features. Here, we describe a new species of Galeommatoidea from the Mediterranean Sea ( Figure 1). Our finding is a further addition to the rich biodiversity of the Mediterranean mollusk fauna [26]. We tentatively place the new species in the genus Draculamya, since it morphologically resembles the type species Draculamya porobranchiata Oliver and Lützen, 2011. This attribution is provisional since the whole premise of the genus Draculamya is mostly based on anatomical traits, rather than on shell characters. Updated taxonomy and nomenclature followed MolluscaBase [27], except where stated otherwise.

Etymology
The species is named to honor the CNR Italian Research Vessel Urania, her captains and crew for their outstanding contribution to the exploration of the Mediterranean Sea. From 1992 until its final decommissioning in 2019, this vessel surveyed many of the sites that provided a substantial part of the material here discussed.

Description (Holotype Measurements in Parentheses)
Shell minute, with L between 0.81 mm and 1.16 mm, seldom exceeding 1.1 mm (1.14 mm). Equivalve, tumid, thin. Slightly inequilateral with beaks behind the midline. Outline subcircular-ovoid, slightly oblique, and expanded anteroventrally. Larger specimens more oblique, possibly suggesting a greater antero-ventral growth. Margins generally rounded, rather subangular in few shells which have a vaguely truncated anterior margin ( Figure 2C,H). Anterior dorsal margin slightly longer than the posterior one. Umbos not prominent, slightly prosogyrate. Sculpture consisting of commarginal lines and growth stops of varying intensity but generally well-marked and step-like ( Figure 2D,E). Periostracum very tenuous, lost in dead shells, consisting of close-set, concentric microscopic ridges, transparent to yellowish in color. Hinge area rather weak. RV ( Figure 2J) with one peg-like C, single AL and PL, AL more close to the hinge area than PL. LV ( Figure 2K) lacking C, with single AL and PL. Lateral teeth conformed as extensions of the shell dorsal edge, the AL suddenly stops near the hinge area. A very small protuberance is often present between the medial end of the AL and the resilifer ( Figure 2L). Ligament internal. Resilifer sunken, posteriorly directed, obliquely set between the cardinal area and the PL. Prodissoconch ( Figure 2I) cap-like, large, 400-430 µm in diameter (415 µm). Prodissoconch I D-shaped, with a weak and irregular microsculpture. Prodissoconch II ovoid and smooth except very fine concentric growth lines. In some shells, it is visible in transparency a radial pattern ( Figure 2G). Muscle scars weak. Adductor scars oval, similar in size; the anterior is slightly larger and situated more closely to the posterior ventral margin than the posterior one. Pallial line hardly visible. Colour whitish to yellowish in color, semitransparent to opaque. Soft parts not studied.

Geographical Distribution
"Draculamya"uraniae n. sp. is recorded thus far from the Alboran Sea, Balearic Islands, Tuscan Archipelago, Strait of Messina, north-eastern and south-western Adriatic Sea, northern Ionian Sea, and Dodecanese islands. Additionally, "D". uraniae records have been published under other names: Arculus sp., off Vallcarca, Spain (p. 86, fig. 107, only the shell on the right [28]); Arculus sp., off Alboran Island, Spain ( fig. 362 [29]); D. porobranchiata from Gazul mud volcano, Gulf of Cádiz, Spain (p. 289, figs. 5C-F [30]). Moreover, Utrilla et al. [30] consider Kelliopsis sp. [29] as conspecific with the material assigned by them to D. porobranchiata. However, Kelliopsis sp. is a different species currently under description [31], possibly belonging to Draculamya as well. The record of D. porobranchiata from Israel [32] does not belong to this species nor to "D". uraniae, and should be assigned to Kelliopsis sp. "D". uraniae has an almost pan-Mediterranean distribution as well as in the adjacent sector of the Atlantic Ocean (Figure 1). Hoffman and Freiwald [33] report empty shells of D. porobranchiata from cold-water corals habitats from off northern Norway, off western Ireland, off north-western Spain, and off Mauritania, at a depth of between 250 m and 1090 m. Based upon a preliminary examination, it seems that two different morphotypes are involved. One of these is more similar to "D". uraniae. This species, therefore, could have a wider distribution in the north-eastern Atlantic Ocean, although further studies are needed to clarify the status of such material [34].

Ecology
The type material of "D". uraniae n. sp. consists of dead but fresh articulated shells, with the periostracum still intact. We hypothesize that they were substantially in situ, suggesting that this new species occurs in circalittoral to upper bathyal habitats between 48-180 m. The Messina material originates from the bottom of semi-dark caves/ravines rich in cnidarians, while the Tuscan material came from bioclastic substrates with an abundance of cnidarian and siliceous sponges in the mesophotic/rariphotic zone [35,36]. The other material consists of loose valves deprived of periostracum, from a depth range of 45-795 m. We cannot conclude whether "D". uraniae is actually a eurybathic species, extending its range down to ca. 800 m., as the deeper records refer to long-dead valves only.
Reworked and/or subfossil dead molluscan assemblages have been frequently reported for the Mediterranean [37][38][39]. In general, we hypothesize that "D". uraniae lives at depths shallower than D. porobranchiata (1100 m to 1350 m). Regarding its trophic habit, the host relationships of "D". uraniae cannot be established at present as it has not been collected alive.

Generic Placement
The genus Draculamya is chiefly diagnosed on the peculiar anatomical features of the type species, D. porobranchiata. In particular, the septibranch-like ctenidium, the suctorial oesophagus and the unique puncturing organ, suggesting an ectoparasitic feeding mode [25]. The fossil D. pumila as well as "D". uraniae n. sp. are assigned to the genus based upon shell morphology (but see below). Anatomical and molecular information are needed to confirm the generic placement of "D."uraniae.

Draculamya vs. Glibertia
Van der Meulen [40] described a minute bivalve as Glibertia prosperi gen. et sp. nov. from Pliocene deposits in The Netherlands. He gave a detailed description and depicted the shell by drawings, and assigned it to the Condylocardiidae Bernard, 1896, although its hinge structure differs from the condylocardiid-type [41][42][43]. Since its description, G. prosperi has been seldom mentioned in the literature [44][45][46][47][48]. Recently, Wesselingh and Moerdijk [49] redescribed this species and synonymized it with Kellia pumila J. de C. Sowerby, 1844, relying on the comparison of the only good SEM photos of Kellia pumila at that time [15], with SEM photos of topotypical G. prosperi shells [50]. These authors recognized Glibertia as distinct from Kellia Turton, 1822 or Lasaea Brown, 1827, hence the new combination Glibertia pumila (Sowerby, 1844). Shortly after, Oliver and Lützen [25] included K. pumila in their new genus Draculamya. MolluscaBase [27] accepts the combination "Draculamya pumila", but the binomen Glibertia pumila is widely used in the paleontological context. Although the settlement of this taxonomic issue is beyond the scope of this contribution, we provide some information in order to offer a better understanding of these taxa. A preliminary examination of topotypical G. prosperi shells through SEM images shows strong similarities with K. pumila; however, the shell morphology of this species appears quite variable, with some specimens approaching the "Draculamya" hinge pattern while others to Mancikellia divae van Aartsen and Carrozza, 1997 [25], pers. obs. Therefore, G. prosperi is provisionally retained as a distinct species, pending the examination of further material of both taxa. Glibertia and Draculamya could be reasonably assumed to be congeneric if based on conchological grounds only, as minor differences of the hinge pattern are present. In such a case, Glibertia has priority over Draculamya. On the other hand, an "identical" shell morphology does not imply identical anatomy, as observed in other galeommatid bivalves. In the lack of anatomical and molecular data for the fossil Glibertia, the two genera are therefore kept distinct.

Material and Methods
The largest part of the study material derived from dredge and trawl bottom sampling during cruises SETE06, SASSI08, MARCOS, CORSARO, COBAS, GECO of R/V Urania. The study of sediment residues hosted at the Ismar-CNR Bologna facility was entrusted by one of the Authors (MT) to the 'Gruppo malacologico livornese' (GML, Leghorn's malacological group-Livorno). The material considered comprised shells sorted by GML members: Attilio Pagli (Empoli); Alessandro Raveggi (Firenze); Cesare Bogi (Livorno); Carlo Sbrana (Livorno); Luigi Romani (Capannori); Stefano and Maria Bartolini (Firenze). In addition, articulated shells and several loose valves of the new species were collected from sediment samples obtained by SCUBA diving or through the analysis of commercial trawling by-catch. The materials examined are listed in Table 1 and the localities in Figure 1. Shells were examined through Lomo MBC-10 and Konus Crystal-45 stereomicroscopes, photographed with Canon EOS 400D and Nikon Coolpix 990 cameras, while measurements were carried out by means of an eyepiece micrometer. Selected shells and details were examined with: Quanta200 SEM in the Centro de Apoyo Científico y Tecnológico a la Investigación (CACTI)-University of Vigo; JEOL JCM 5000 Neoscope SEM in the Department of Natural Sciences, NMW Cardiff; Jeol, JSM-5400 SEM at Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Bologna University, after air drying and mounting on SEM stubs. Comparisons were conducted with material figured in the literature. The study material was preserved in institutional (MNHN, MZB, NBC, NMW, SMF, MLK) and private (AR, CB, LR, SB) collections. This published work and the nomenclatural acts it contains have been registered in ZooBank. The ZooBank LSID ((Life Science Identifiers) for this publication is: urn:lsid:zoobank.org:pub:892D49EC-B26B-40A8-B218-42200939992A. The associated information can be viewed at http://zoobank.org/References/892D49EC-B26B-40A8-B218-42200939992A (accessed on 1 Jun 2021).