Isorropodon megadesmus, 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 : 13-15

Isorropodon megadesmus sp. n. Figs 910 C–D

Material examined.

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.

Measurements (in mm)

Description.

(Figs 9, 10 C–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.

Distribution.

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

Etymology.

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

Remarks.

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).