Acharax gadirae, 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 : 6-9

Acharax gadirae sp. n. Figs 5-6

Type material.

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.

Measurements (in mm)

Description.

(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.

Distribution.

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.

Etymology.

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.

Remarks.

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. 6 C–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.