Solemya borealis Totten, 1834

As given by Taylor et al. 2008. Ligament wholly internal, supported by a buttress and lacking posterior or lateral extensions.

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.

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.

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.

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

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 3A–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.

Solemya johnsoni Dall, 1891

As given by Taylor et al. 2008. Ligament external, as a high arched band.

Holotype: one specimen, TTR12, stn AT391GR, Western Moroccan field, Jesus Baraza MV, 35°35.439'N, 07°12.264'W, 1105m, 09 July 2002, NMWZ.2010.4.3.

Paratypes: one specimen, same data as holotype, DBUA; one shell, TTR 12, stn AT392G, deep-water field, Captain Arutyunov MV, 35°39.658'N, 07°20.018'W, 1320m, 9 July 2002, DBUA; one shell, TTR 16, stn AT607GR, Western Moroccan field, Ginsburg MV, 35°22.677'N, 07°04.979'W, 983m. 29 May 2006, NMWZ.2010.4.4.

Other material examined: one specimen, TTR16, stn AT602GR, El Arraiche field, Pen Duick Escarpment, 35°17.693'N, 06°47.089'W, 556m, 28 May 2006; one specimen, TTR16, stn AT604GR, Western Moroccan field, Yuma MV, 35°25.820'N, 07°06.330'W, 1030m, 29 May 2006; one specimen, TTR16, stn AT605GR, same locality, 35°25.046'N, 07°05.450'W, 975m, 29 May 2006; one specimen, TTR16, stn AT615GR, deep-water field, Carlos Ribeiro MV, 35°47.238'N, 08°25.272'W, 2200m, 31 May 2006; one specimen, TTR16, stn AT617K, same locality, 35°47.246'N, 08°25.303'W, 2230m, 31 May 2006; two specimens, MSM01.03, stn 145, deep-water field, Porto MV, 35°33.773'N, 09°30.416'W, 3902m, 3 June 2006.

(Fig. 5) Calcified shell to 67mm in length, to 85mm including periostracal fringe. Robust. Equivalve. Inequilateral, beaks situated at 1/4 length of shell from posterior margin. Outline subcylindrical, compressed, calcified shell length about 3 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. Including periostracal fringe, anterior appears greatly expanded compared with posterior. Beaks indistinct, umbos sunken. Hinge teeth absent. Ligament external, as a high arched band posterior of the beaks and supported by a thickened shell margin; an oval area of ligament is present immediately behind the beaks and visible internally, anterior of the beaks shell margins fused by periostracal material along entire dorsal margins. Periostracum persistent and extending well beyond the shell margin, initially yellowish brown in colour but darkening with growth to dark brown and black; periostracal frill thickened over ribs but entire. Sculpture of radial ridges, 4 closely spaced over the posterior; median area almost smooth with 2–3 low ribs; anterior with 8–9 deeply cut ribs. Adductor scars impressed, posterior scar subcircular, anterior adductor scar larger, spatulate in outline. Anterior inner shell margin scalloped corresponding to radial ribs.

Posterior siphonal opening surrounded by a series of papillae (Fig. 5E): A single large dorsal papilla (dp) lies above 2–3 pairs of slightly smaller papillae (dmp) on the dorsal margin of the opening, below these surrounding the opening is a series of approximately alternating large and small papillae with those most ventral the largest.

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 is papillate. The anterior dorsal mantle edge is prominently papillate (Fig. 5D). The foot is very large with a broad oval sole, the margin interdigitates between large and small blunt papillae. 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 presence or absence of a gut could not be confirmed.

Acharax gadirae is presently only known from the mud volcano fields in the Gulf of Cadiz, Eastern Atlantic. The specimens have been taken from the Western Moroccan field at Yuma, Ginsburg and Jesus Baraza MVs, and from the deep-water field at Captain Arutyunov, Carlos Ribeiro and Porto MVs at depths between 975 to 3902m. A single specimen was recovered from the shallower El Arraiche field in Pen Duick Escarpment at 556m.

gadirae, from the Phoenician “Gadir” the original name for Cadiz and meaning “walled fortification” and also the root of many Moroccan names such as Agadir. Named to indicate the widespread range across the Moroccan and Iberian margins.

The genus Acharax is recognizable from the large external ligament and the generic placement of Acharax gadirae is confirmed.

The genus is rare in the Atlantic Ocean unlike the situation in the Pacific where species of Acharax are frequently recorded from chemosynthetic settings (Neulinger et al. 2006). Only two species are known from the Atlantic. Acharax grandis (Verrill and Bush 1898) is known only from the original material collected from depths between 548 and 2926m in the region of the New York Bight. Acharax caribbaea (Vokes 1970) again is only recorded from the original material collected from a depth of 350m off Colombia in the Caribbean Sea. However, the genus is recorded in recent studies from both the Gulf of Mexico (Carney 1994) and the Barbados prism (Olu et al. 1997) but the species are not identified.

Acharax grandis differs from both Acharax gadirae and Acharax caribbaea in being less inequilateral with the beaks distinctly more towards the mid-line. Acharax caribbaea differs from both Acharax gadirae and Acharax grandis in having very few (4) anterior ribs compared with the 6–8 on Acharax grandis and 8–9 on Acharax gadirae of similar size. Unfortunately, there are no anatomical data for either Acharax grandis or Acharax caribbaea, making a thorough comparison impractical.

There are no given ecological data for either Acharax grandis or Acharax caribbaea. The type locality for Acharax grandis, which is the region around the Hudson Shelf and Canyon, has no recorded seep or vent activity. In contrast the region around the type locality of Acharax caribbaea is known for a variety of chemosynthetic settings (Carney 1994).

The bathymetric range of Acharax in the Gulf of Cadiz is large, 556–3902m and specimens have been taken at many mud volcanoes raising the possibility that more than one species is involved. Unfortunately the specimens from the abyssal sites are all small about 10mm or less making comparison with the large specimens from the bathyal sites inconclusive. The specimens from Carlos Ribeiro MV (2200m) (Fig. 6C–D) are prominently wedge shaped in outline compared with the specimen from Porto MV (3902m) (Fig. 6B). The latter is not dissimilar to those from Capt. Arutyunov MV (1325m) (Fig. 6E) with the specimen from Pen Duick Escarpment (556m) (Fig. 6A) somewhat more elongate but not as wedge shaped as those from Carlos Ribeiro MV.

The specimens from the Capt Arutyunov MV are most problematic in that the siphonal opening appears to be devoid of any surrounding tentacles or papillae (Fig. 6H). This does not appear to be a function of size as similar specimens from other sites have siphonal papillae. Should this observation be confirmed in further material it would be appropriate to describe this as a separate species. Comparing the siphonal papillae of specimens from Pen Duick Escarpment (Fig. 6F) and Carlos Ribeiro MV (Fig. 6G) indicates a more complex arrangement in the latter but, with so few specimens, this is inconclusive.

Lucina filosa Stimpson, 1851

As given by Oliver and Holmes (2006a)

Holotype; one complete specimen, live collected, TTR 15, stn AT569GR, El Arraiche field, Mercator MV. 35°17.917'N, 06°38.717'W, 358m, 25 July 2005, NMWZ.2010.4.5.

Paratypes; five specimens, as holotype, NMWZ.2010.4.6.

(Fig. 7). Shell to 34 mm in length. Solid. Equivalve. Equilateral. Tumidity variable (Fig. 7G, H) but mostly rather compressed. Umbos low, beaks pointing forward. Outline lenticular; posterior dorsal margin almost straight, sloping gently; posterior margin curved but less so than anterior; anterior dorsal margin short, a little concave. Escutcheon narrow, edges slightly raised, extending the length of the posterior dorsal margin; three-quarters filled by ligament, remainder smooth. Lunule distinct, width dependant on tumidity of shell; edges raised, sharp. Sculpture of numerous, low but erect, thin, concentric lamellae; between lamellae are weak concentric lines. Ligament external as a prominent, raised, arched band. Set on a narrow nymph. Hinge weak; two small cardinal teeth in each valve, RV anterior and LV posterior weakly bifid; anterior lateral protuberance distinct to obscure. Pallial line entire. Anterior adductor scar greatly elongate, approximately 3/4 free from pallial line. Shell white, periostracum thin but persistent, straw coloured (all material collected has been stained in Rose Bengal, thus the pink tinge).

The anatomy is essentially that described for Lucinoma borealis by Allen (1958) and for Lucinoma kazani by Salas and Woodside (2002). The inhalant siphon is surrounded by numerous short tentacles and papillae (Fig. 7I)

The shell can be rather compressed (Fig. 7H) or tumid (Fig. 7G) and this may be related to age rather than size as suggested by Oliver and Holmes (2006a) for Lucinoma gagei from the Arabian Sea. Some shells also show distortion with radial depressions developing abruptly (Fig. 7E).

Tissues were sent to Dr. John Taylor (NHM, London) for inclusion in his survey of Lucinidae and the 16S and CO1 genes were compared with those of Lucinoma borealis. The results although not entirely conclusive indicate that the two populations are not conspecific. More recently, John Taylor’s group has demonstrated that Lucinoma kazani and Lucinoma borealis are distinct (J. Taylor pers. comm).

Only found live at Mercator MV in the Gulf of Cadiz (358m).

asapheus from asaphes Greek: meaning “indistinct” and “baffling”, referring to the lack of distinctive morphological characters and the consequent unsettling taxonomic issues.

A morphometric analysis was done comparing the Gulf of Cadiz shells with those of Lucinoma borealis from numerous localities from around the British Isles. This analysis could not demonstrate any statistically valid differences in the outline, the relative size of the anterior adductor scar (aa l on Fig. 7B) or the angle of divergence of this scar from the pallial line (aa d on Fig. 7B). It should be noted that the Cadiz sample size was small and that conclusive probability results were unlikely. However, the variation in tumidity and irregularity of some of the Cadiz shells is not found in samples of Lucinoma borealis. Anatomically Lucinoma asapheus and Lucinoma borealis are alike including the papillae that surround the inhalant aperture. Further evidence for the species level distinction between Lucinoma asapheus and Lucinoma borealis comes from the molecular data but here again the few specimens available curtails the analysis. Ecologically one might expect mud volcanoes and near shore sulphide enriched sediments to support different species. This argument was used by Salas and Woodside (2002) to support the distinction between Lucinoma kazani and Lucinoma borealis, but they also listed some morphological differences and this has been supported by molecular data (J. Taylor pers comm). Some of these, namely the tumidity of the valves, the width of the lunule and the expression of the lateral teeth are found here to be variable and therefore not conclusive. Similar variability was recorded for Lucinoma gagei (Oliver and Holmes 2006a) and Lucinoma myriamae (Cosel 2006) suggesting that small morphological differences in Lucinoma shells, especially if observed between small samples, may not be reliable taxonomic characters. The papillation of the inhalant siphon does appear to be much less developed in Lucinoma kazani compared with that in Lucinoma borealis and Lucinoma asapheus. The angle of divergence of the anterior adductor scar also shows a difference with that in Lucinoma kazani having a mean value of ca. 25° and both Lucinoma borealis and Lucinoma asapheus a mean value of ca. 15°. Given that Lucinoma kazani and Lucinoma asapheus both inhabit mud volcano settings one might expect them to be conspecific. However, accepting the morphological differences given by Salas & Woodside between Lucinoma kazani and Lucinoma borealis and that these also hold true for Lucinoma asapheus then the two must be considered distinct. This may be supported by the wide difference in depth range with Lucinoma asapheus coming from 358m in contrast to Lucinoma kazani from 1700–2030m.

Other Eastern Atlantic species are Lucinoma vestita (Dautzenberg and Fischer 1906) from Cape Verde at 600m, Lucinoma atalantae Cosel, 2006 from Mauritania at c.2000m and Lucinoma myriamae Cosel, 2006 from the Angola margin at c.360m. Comparisons with Lucinoma asapheus are as follows. Lucinoma vestita is a smaller species not exceeding 16mm in length, more rounded in outline and with a weak sculpture of poorly developed (often absent) comarginal lamellae. Lucinoma atalantae has a distinctly longer and more steeply sloping anterior dorsal margin, a more angular posterior profile and irregular sculpture. Lucinoma myriamae is much larger reaching over 50mm in length and has a distinct angular anterior profile; in this respect, it resembles Lucinoma saldanhae Barnard, 1964 a species not considered by Cosel (2006).

Spinaxinus sentosus Oliver & Holmes, 2006b

As given by Oliver and Holmes (2006b)

One live collected specimen, MSM01.03, Stn 190, deep-water field, Captain Arutyunov MV, 35°39.665'N, 07°19.970'W, 1322m, 28 April 2006, NMWZ.2010.4.7

(Fig. 8A–D). This specimen measures only 2.3 mm in length and is damaged. The outline agrees with that of Spinaxinus sentosus in being extended anteriorly with a long lunule depression and in the presence of a long but shallow posterior sulcus. These features are in contrast to the juveniles of Thyasira vulcolutre (Rodrigues and Oliver 2008), which also occurs at similar depths and settings.

The periostracal spines are typical of Spinaxinus but are not seen in this specimen. The periostracum is coarse and the vestiges of lamellae and projections can be seen on the extreme edges of the shell especially on the ventral margin.

Although the identification is not conclusive the likelihood that this shell is a juvenile Spinaxinus is high and as such represents the first finding of this species in a non-anthropogenic setting. The proximity of the type locality (off northern Portugal) to the Cadiz mud volcanoes makes this supposition more reasonable.

Thyasira vulcolutre belongs to a group of thyasirids with relatively large shells with weakly defined posterior sulci. It was concluded that it was most similar to Thyasira southwardae (Oliver and Holmes 2006b) from the Anyas Garden site. At that time no thyasirid material had been found at cold seep sites along the West African margin. Consequently, unlike the situation for Lucinoma (Cosel 2006) and Isorropodon (Cosel and Salas 2001, Cosel and Olu 2008) no further biogeographic comparisons could be made. Recently we were able to examine a thyasirid (Fig. 8E–F) from the REGAB site (courtesy of Karine Olu) and although it superficially resembles Thyasira vulcolutre it significantly differs in having a minutely spicate periostracum (Oliver in prep).

Thyasira striata Sturany, 1896 has long been known from deep water in the eastern Mediterranean but was recently re-discovered at cold seep sites (Olu-Le Roy et al. 2004). Small specimens resemble Thyasira flexuosa but larger examples (Fig. 8G–H) are very tumid with prominent lunule and deep posterior sulci quite unlike Thyasira vulcolutre.

These new data suggest that those thyasirids closely associated with active cold seeps have restricted ranges within the eastern Atlantic/Mediterranean region.

Isorropodon perplexum Sturany, 1896

As given by Cosel and Salas (2001)

Holotype: one complete specimen, live collected, MSM01.03, stn 218, deep-water field, Captain Arutyunov MV. 35°39.642'N, 07°20.049'W, 1321m, 30 April 2006, NMWZ.2010.4.8.

Paratypes: ten specimens, four shells and one valve, same data as holotype, NMWZ.2010.4.9.

Other material examined: over thirty decalcified juvenile specimens, MSM01.03, stn 218, deep-water field, Captain Arutyunov MV. 35°39.642'N, 07°20.049'W, 1321m, 30 April 2006; one specimen, MSM01.03, stn 225, same locality, 35°39.707'N, 07°20.020'W, 1322m, 4 May 2006.

(Figs 9, 10C–D). To 15mm in length. Thin. Equivalve. Inequilateral, beaks in front of the midline. Compressed, length to tumidity ratio 2.3 to 2.5. Outline subovate, anterior rounded, posterior a little obliquely truncated; ventral curvature at its maximum well to the posterior of the mid line. Lunule indistinct, not depressed. Escutcheon narrow, deeply excavated but entirely occupied by ligament. Sculpture of dense concentric lines and irregular growth stops or wrinkles. Hinge plate prominent dominated by a long nymph supporting a very large external ligament; ligament rises well above the dorsal margin of the shell and extends posteriorly beyond the nymph to fill the escutcheon. Hinge teeth complex; RV with a single prominent anterior lateral tooth situated in front of the beak in the form of a narrow projecting peg with a flat or slightly excavated dorsal surface; below the beak is an arched laminar tooth its anterior end overlapping the lateral tooth, its posterior slopes steeply and ventrally and merges with a second ridge only noticeable by a weak notch mid way on this combined ridge. LV with a thin laminar posterior cardinal angled obliquely plus two combined cardinals in a horizontal orientation the posterior part larger than the anterior with a distinct notch between the two parts. Pallial line entire with a very small straightened section below the posterior adductor scar; adductor scars of about equal size; anterior pedal retractor scar deeply impressed, situated immediately in front of the hinge plate. Periostracum thin, persistent, glossy. Shell white.

Mantle thin, mantle edge unfused except for short inhalant and exhalant siphonal apertures; inhalant aperture with many papillae increasing in size dorsally, exhalant with papillae of equal size. Foot with a distinct finger-like toe and poorly developed heel, pedal retractors prominent, the anterior attached in a deep impression close to the hinge. Anterior adductor muscle oval in cross-section, posterior adductor muscle subcircular, smaller than the anterior one. Ctenidia of a large, single (inner) demibranch, ascending part approximately one half the height of the outer, filaments fine tightly connected.

Isorropodon megadesmus is restricted to Captain Arutyunov MV (1321–1322m).

megadesmus from the Greek mega meaning large and desma meaning bond; referring to the external ligament.

The taxonomy of Isorropodon in the Atlantic and Mediterranean is complex and potentially confused (Cosel and Salas 2001, Cosel and Olu 2009). Cosel and Salas (2001) described two new species from the Eastern Atlantic, namely Isorropodon bigoti and Isorropodon curtum. They transferred a third from Kelliella, namely Isorropodon elongatum (Allen 2001). In discussing, the Mediterranean, Isorropodon perplexum Cosel and Salas (2001) stated that Isorropodon species are variable with regard to outline, tumidity and development of hinge teeth and this is illustrated in their figures 36–47 for Isorropodon perplexum. They noted similarities in shell morphology between the Eastern Mediterranean species Isorropodon perplexum and the West African Isorropodon bigoti but suggested that these taxa were isolated geographically and doubtfully could have gene flow between them. They further supported this argument by stating that Isorropodon perplexum had not been found in the Western Mediterranean or Ibero-Moroccan Gulf. In 2009, Cosel and Olu described another Isorropodon from West Africa (Isorropodon atalantae) and placed another vesicomyid in this genus (Isorropodon striatum Thiele and Jaeckel 1931). Therefore, before the discovery of Isorropodon in the Gulf of Cadiz there were already five west African species and one from the eastern Mediterranean. With the discovery of Isorropodon at the Capt. Arutyunov MV the assertion made by Cosel and Salas (2001) on genetic isolation can be questioned, as there is the possibility of gene flow between the seeps in the Mediterranean and around the east African coast.

In contrast to the variability given by Cosel and Salas (2001) for Isorropodon perplexum, all of the shells from station 218 examined here are constant with regard to features of outline, tumidity and hinge teeth. However, a single shell from station 180 is distinct, being inflated, having a distinct lunule, having a much smaller ligament and in the ventral margin being more convex. The shells from station 218 are distinct from all the shells of Isorropodon perplexum figured by Cosel and Salas (2001) in having a much longer nymph with the posterior teeth reaching only about one third of the ligament as opposed to the half distance given for Isorropodon perplexum by Cosel and Salas (2001). Furthermore, most of the shells illustrated by them have a more convex ventral margin than the shells from station 218. The single shell from station 180 shares more features with the Mediterranean shells in having the small ligament, convex ventral margin and being more inflated.

The outline of Isorropodon bigoti differs from all of the above in the narrower anterior and distinct angulation of the ventral curve, but is has a short nymph similar to Isorropodon perplexum and the shell from station 180.

Isorropodon atalantae has a more sunken lunule and more angular posterior profile than either of the Gulf of Cadiz taxa. Isorropodon curtum Cosel and Salas, 2001, from off Mauritania, is more circular in outline and I striatum Thiele and Jaeckel, 1931 from off Angola, is a much larger and more elongate form.

Cosel and Salas (2001) reassigned Kelliella elongata Allen, 2001 to the genus Isorropodon. Following examination of the type material in the Natural History Museum, London (BMNH 1998180) we conclude that it is not conspecific with any of the taxa discussed here. It is a small species not exceeding 2mm in any of the over 300 specimens listed by Allen (2001). It is inflated with a distinct lunule but the demarcating line illustrated by Allen (2001) is not so apparent. The hinge of the right valve has three distinct teeth including a small posterior tooth (4b in Allen 2001), which is not present in either of the species from the Cadiz mud volcanoes. Furthermore, the ligament is small and does not project as in Isorropodon megadesmus. From the ctenidial anatomy there is no indication that this species is chemosymbiotic. In addition to the morphological differences, Isorropodon elongatum has been collected from a wide geographical range, wide bathymetric range and associated with the typical oligotrophic deep-sea bivalve assemblage (derived from Allen 2008). It would appear that Isorropodon elongatum, if a chemosymbiotic species is not confined to seep/vent settings but as stated by Allen (2001) it is absent from the European basin and it was not present in the samples taken in the Gulf of Cadiz away from the vicinity of the mud volcanoes (Rodrigues 2009).

One complete specimen, live collected, MSM01.03, stn 180, deep-water field, Captain Arutyunov MV. 35°39.740'N, 07°19.960'W, 1323m, 27 April 2006, NMWZ.2010.4.10.

6.4mm (L) × 5.3mm (H) × 3.6mm (T)

(Figs 10A–B, 11). 6.4 mm in length. Thin. Equivalve. Inequilateral, beaks in front of the midline. Inflated, length to tumidity ratio = 1.8. Outline subovate, anterior bluntly rounded, posterior a little obliquely truncated; ventral curvature at its maximum more or less at the midline. Lunule indistinct, slightly depressed. Escutcheon narrow, deeply excavated. Sculpture of dense, concentric, fine lines and few irregular growth stops. Hinge plate narrow, nymph supporting an external ligament; ligament scarcely rises above the dorsal margin of the shell and extends posteriorly to half the length of the escutcheon. Hinge teeth complex; RV with a single prominent anterior lateral tooth situated in front of the beak in the form of a narrow projecting peg with a flat or slightly excavated dorsal surface; below the beak is weakly arched laminar tooth its anterior end overlapping the lateral tooth, its posterior slopes steeply and ventrally and merges with a second ridge only noticeable by a weak notch mid way on this combined ridge. LV with a thin laminar posterior cardinal angled obliquely plus two combined cardinals in a horizontal orientation the posterior part only slightly larger than the anterior with a distinct notch between the two parts. Pallial line entire with a very small straightened section below the posterior adductor scar; adductor scars of about equal size; anterior pedal retractor scar deeply impressed, situated immediately in front of the hinge plate. Periostracum thin, persistent, glossy. Shell white

Mantle thin, mantle edge unfused except for short inhalant and exhalant siphonal apertures; inhalant aperture with few papillae increasing in size dorsally, the latter as short tentacles, exhalant with papillae of equal size. Foot with a blunt finger-like toe and poorly developed heel, pedal retractors prominent, the anterior attached in a deep impression close to the hinge. Anterior adductor muscle pyriform in cross-section, posterior adductor muscle subcircular, smaller than the anterior one. Ctenidia of a large, single (inner) demibranch, filling the majority of the mantle cavity, ascending part approximately one half the height of the outer, filaments fine tightly connected.

The numerous crystalline growths seen on and between the filaments are believed to be natural and not an artifact of preservation.

Isorropodon sp indetis restricted to Captain Arutyunov MV (1323m).

As discussed above for Isorropodon megadesmus.