Solemya (Petrasma) elarraichensis, Olive, Graham, Rodrigues, Clara F. & Cunha, Marina R., 2011

Olive, Graham, Rodrigues, Clara F. & Cunha, Marina R., 2011, Chemosymbiotic bivalves from the mud volcanoes of the Gulf of Cadiz, NE Atlantic, with descriptions of new species of Solemyidae, Lucinidae and Vesicomyidae, ZooKeys 113, pp. 1-38 : 4-6

Solemya (Petrasma) elarraichensis sp. n. Figs 23 A–D 4

Material examined.

Holotype: one specimen, TTR14, stn AT528GR, El Arraiche field, Kidd MV, 35°25.304'N, 06°43.972'W, 489m, 03 August 2004, NMWZ.2010.4.1

Paratypes: ten specimens, same data as holotype, NMWZ.2010.4.2; seven specimens, TTR15, stn AT569GR, El Arraiche field, Mercator MV, 35°17.917'N, 06°38.717'W, 358m, 25 July 2007, DBUA.

Other material examined: eight juveniles specimens, same data as holotype; two specimens, TTR12, stn AT407GR, El Arraiche field, Pen Duick Escarpment, 35°17.695'N, 06°47.082'W, 560m, 15 July 2002; three specimens, TTR14, stn AT560B, El Arraiche field, Kidd MV, 35°25.306'N, 06°43.976'W, 498m, 8 August 2004; one specimen, TTR15, stn AT586GR, Western Moroccan field, Meknès MV, 34°59.146'N, 07°04.380'W, 701m, 28 July 2005; four specimens, TTR16, stn AT604GR, Western Moroccan field, Yuma MV, 35°25.820'N, 07°06.330'W, 1030m, 29 May 2006; two specimens, TTR16, stn AT607GR, Western Moroccan field, Ginsburg MV, 35°22.677'N, 07°04.979'W, 983m, 29 May 2006.

Measurements (in mm)

Description.

Shell (Figs 2, 3): to 35mm in length. Fragile. Equivalve. Inequilateral, beaks situated at 1/4 length of shell from posterior margin. Outline subcylindrical, compressed, length about 2.6 times height, slightly deeper towards the anterior, dorsal and ventral margins subparallel, anterior margin more broadly rounded than anterior, posterior dorsal margin projecting a little. Beaks indistinct, umbos sunken. Hinge teeth absent. Ligament primarily internal, supported by a prominent chondrophore that extends only slightly as a chondrophore ridge around the posterior adductor, lacking posterior and anterior extensions but a small roughly heart shaped area is present in front of the chondrophore and this is also visible externally just behind the beaks. Periostracum persistent and extending well beyond the shell margin, initially yellowish brown in colour but darkening with growth to a dark chestnut brown. Sculpture of weak radial ridges, 5-6 over the posterior and 10-12 over median and anterior. Adductor scars impressed, dorsal part of posterior scar angulate where bounded by chondrophore ridge, anterior adductor scar larger, spatulate in outline.

Anatomy (Figs 3G, 4): The posterior siphonal opening is surrounded by a series of papillae: A single large dorsal papilla (dp) lies above two smaller but still large papillae (dmp) on the dorsal margin of the opening, below these is a short smooth section (sa) followed by a series of papillae increasing in size towards the ventral margin, there are 6 primary papillae (psp) on either side and a single ventral median papillae, between these on the inner side are smaller papillae (ssp); a pair of subsiphonal ridges (ssr) are present below the siphonal crown.

The mantle edge is fused from the posterior siphon for half the length of the ventral margin where there is a large anterior pedal gape. The mantle edge surrounding the rear of the foot bears a few tabulate papillae (pgp). The anterior dorsal mantle edge is prominently papillate (dap) and there is a single papilla on the junction of the mantle edge anterior of the anterior adductor muscle (admp). The foot is very large with a broad oval sole, this fringed by large papillae, all equal in size. The ctenidium is large with numerous laminar filaments attached to a prominent gill axis. The palps are short, twisted and flattened with cup shaped terminations. The gut is present but difficult to examine due to its small dimensions but the hind gut and rectum were easily visible.

Distribution.

Solemya (Petrasma) elarraichensis is presently only known from the mud volcano fields in the Gulf of Cadiz, Eastern Atlantic. The majority of specimens have been taken from the El Arraiche field off the coast of Morocco in Kidd, Fíuza and Mercator MVs and the Pen Duick Escarpment at depths between 358-560m. A few specimens have been taken from the Western Moroccan field at the Meknès, Yuma, Ginsburg and Darwin MVs at the slightly deeper range of 700-1115m.

Etymology.

elarraichensis, denoting the geographic origin of the type locality; the El Arraiche field.

Remarks.

The form of the ligament, which is primarily internal, supported by a chondrophore and lacks any lateral or anterior extensions, confirms the placement of Solemya (Petrasma) elarraichensis in the subgenus Petrasma Dall, 1908 ( Taylor et al. 2008). Some of the specimens carry an initial identification of "cf. Solemya togata " as might be expected from the proximity to the Mediterranean but the ligament of Solemya s.s. has prominent anterior extensions (Fig. 3E).

The subgenus Petrasma is not known from the North-East Atlantic but is represented in the Western Atlantic by three species. Two species are known from near shore waters off the northeast coast of the USA: Solemya (Petrasma) velum (Say, 1822) and Solemya (Petrasma) borealis (Totten, 1834). The third, Solemya (Petrasma) occidentalis ( Deshayes 1857) is known from the warm waters of Florida, Caribbean and S. America ( Mikkelsen and Bieler 2008).

We note that the curvature of the chondrophore and chondrophore ridge is circular in Solemya (Petrasma) velum (Fig. 3F) but angular in Solemya (Petrasma) elarraichensis (Figs 3 A–D). Furthermore, the siphonal papillae of Solemya (Petrasma) velum are by comparison less in number and reduced in development (Morse 1913, Taylor et al. 2008).

Abbott (1974) following Morse (1913) noted that the siphon of Solemya (Petrasma) borealis differed markedly from that of Solemya (Petrasma) velum , confirming that siphonal characters were important for distinguishing species. In Solemya (Petrasma) borealis , the ventral-most dorsal marginal papillae are very large, and as big as the dorsal papilla, and much larger than any of the ventral papillae. This contrasts with the condition in Solemya (Petrasma) elarraichensis where the ventral-most dorsal marginal papillae are smaller than the dorsal papilla and where the ventral papillae are fewer in number and distinctly increasing in size ventrally, with the ventral-most papillae equal in size to the dorsal marginal papillae. Conway et al. (1992), following Barnard (in Reid 1980) suggested that Solemya (Petrasma) borealis lacked a gut and this would be in contrast with Solemya (Petrasma) velum and Solemya (Petrasma) elarraichensis .

The character of the ligament and chondrophore are rather similar in Solemya (Petrasma) elarraichensis and Solemya (Petrasma) borealis .

Abbott (1974) and Mikkelsen and Bieler (2008) note that Solemya (Petrasma) occidentalis lacks any perceptible chondrophore ridge and give this as the main characteristic separating Solemya (Petrasma) occidentalis from Solemya (Petrasma) velum and, therefore, also from Solemya (Petrasma) elarraichensis .

Ecologically Solemya (Petrasma) velum and Solemya (Petrasma) borealis are very different from Solemya (Petrasma) elarraichensis in that they are not associated with deep-water methane seeps. In contrast they are found in sublittoral or shallow shelf settings with high organic enrichment (Morse 1913 in Conway et al. 1992). Mikkelsen and Bieler (2008) give a similar habitat for Solemya (Petrasma) occidentalis (Deshayes, 1857) noting its occurrence in mangrove channels and around sewage outfalls.

Given the above differences in habitat and form we conclude that none of the Atlantic species is amphi-Atlantic, unlike Solemya (Petrasma) pervernicosa Kuroda, 1948, which is considered to be amphi-Pacific by Kamenev (2009). If any of the Western Atlantic species were amphi-Atlantic it is unclear why, in the Eastern Atlantic, they should be absent from their typical settings (which are plentiful) and found only in deep water methane seeps.

Other North Atlantic species referred to as Solemya , Solemya grandis Verrill and Bush, 1898 and Solemya caribbaea Vokes, 1970 are excluded here because both belong to the genus Acharax ( Abbott 1974).

A solemyid living at a pockmark, at a depth of 1607m, has been reported from the Eastern Mediterranean ( Rodrigues et al. 2011). Unfortunately the small, single specimen was damaged and its taxonomic affinities remain unclear.

In conclusion, there are sufficient morphological and ecological grounds for considering the Gulf of Cadiz species of Petrasma to be new to science.