Lucinoma asapheus, 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 : 9-11

Lucinoma asapheus sp. n. Fig. 7

Type material.

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.

Measurements (in mm)

Description.

(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)

Variation.

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).

Molecular data.

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).

Distribution.

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

Etymology.

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

Remarks.

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).