Adontorhina similis Barry & McCormack, new species

Adontorhina similis, Barry & McCormack, new species

(Figures 4–5)

Mendicula pygmaea Oliver & Killeen (2002, p.56–58, plate 23) Thyasira subtrigona Hartley (1984, p. 192)

Type locality. Porcupine Bank, 53°07.77’N, 13°13.37’W, 252 m Eastern Atlantic.

Holotype. A complete shell, collected by P.J. Barry, NMINH.2006.58. Measurements (Length x height x breadth). 1.14 mm x 0.78 mm x 0.62 mm.

Paratypes. Three specimens, as holotype, NMINH.2006.64.1–2. Measurements 1.25 mm x 0.94 mm x 0.6mm; 1.17 mm x 0.91 mm x 0.6 mm; 0.91 x 0.69 x 0.44 mm.

Etymology. From the Latin similis, ‘similar,’ referring to the high degree of similarity in external appearance to Mendicula pygmaea.

Material examined. CEO3, Station GT, 54° N, 12° 24’W, 320 m 2 spec.; CEO3 Station 9, 52° 30’N, 14°W, 300 m 11 spec.; SFO3 Grab 7, 53°07.77’N, 13°13.37’W 252 m 6 spec.; SFO3 Grab 8, 53°07.77’N, 13°13.37’W 252 m 5 spec.; SFO3 Grab 15, 52°52.42’N, 12°26.52’W 382 m 8 spec.; SFO3 Grab 17, 52°38.77’N, 12°11.08’W 330 m 4 spec.; NMWZ.2001.097, ERT 92/082A 60° 36’N, 01° 39’E 130–145 m 200 spec.;

Distribution. Porcupine Bank, west of Ireland. Depth range 252– 382 m. North Sea oilfields. Depth range 85–161 m (Oliver & Killeen, 2002).

Description. Shell minute (maximum size 2 mm), fragile, moderately compressed, colour white; subovate, longer than high (Figure 4); inequilateral, beaks in posterior; very thin, transparent periostracum; sculpture of weak commarginal striae; sulcus absent, posterior flank flattened; umbones inflated, pronounced, prosogyrate; prodissoconch I approximately 150 µm in diameter; lunule indistinct; escutcheon obscure; anterodorsal margin weakly curved, straight in some specimens, anterior broadly rounded; posterior markedly angulate; posterodorsal margin straight, sloping; hinge plate thin, divided into two sections, anterior section thicker, both with irregular granules (Figure 4 C–D, F–H); small cardinal tubercle in the right valve with a corresponding depression in the left valve.

Internal anatomy. The anterior adductor muscle is larger than the posterior muscle (Figure 5); both muscles are divided into quick and catch areas; anterior muscle is elongate, while the posterior is round. Single point of mantle fusion occurs beneath the gill axis, forming the posterior aperture; mantle folds thin and extended, particularly the middle fold which is filled with glandular tissue; all mantle folds have a small area of concentric muscle within their tips; centre of the mantle edge has a single strand of radial muscle but is otherwise filled with a large blood space; inner mantle is fold compressed, with a poorly defined rejection tract; on the inner surface of the mantle edge between the inner and middle folds, the area has small underdeveloped lobes or folds; periostracal groove deep. There is no region of glandular tissue underneath the anterior adductor muscle. Each gill has a single demibranch comprised of ten to eleven filaments; ascending lamellae three quarters the length of the descending lamellae; filaments thin with well-developed eu-laterofrontal cilia; gill filaments type 2 (Dufour, 2005); where interfilamentar fusion occurs, the abfrontal areas and blood space remains wide and forms a strong connection; filamentar muscles absent. Labial palps relatively large, triangular. Oesophagus thickened, leading into a very large stomach. Hindgut loops above the stomach and descends around the outside of the posterior adductor muscle. Lateral pouches undivided, unlobed and end in a pointed tip ventrally; there are two tubules leading into the pouches. Kidneys paired, small. Foot short with a well developed heel; ventral portion of the heel contains glandular tissue which continues out to the tip of the foot; heel sagittally grooved; tip of the foot is undifferentiated from the heavily ciliated stem.

Differential diagnosis. The sharp angle created by the posterior shell margin in combination with the flattened posterior flank differentiates Adontorhina similis from other Adontorhina species. A. similis is similar to Adontorhina lynnae Valentich Scott, 2000; however, A. lynnae has larger, prominent umbones and a more densely granulated hingeplate. Internally, A. lynnae differs in having almost double the number of gill filaments in each demibranch and the labial palps are far more reduced than those in A. similis. The lateral pouches are larger and develop lobes on the posterior surface while the lateral pouches of A. similis are relatively smooth and simple. Further features which separate A. similis from other species of Adontorhina can be found in Table 1.

Remarks. Oliver & Killeen (2002) were the first to recognise the irregular granules in this species but declined to erect a new species. Specimens of Mendicula pygmaea Verrill & Bush, 1898, from the east coast of America were not available for them to examine. Examination of the holotype of M. pygmaea (Figure 6) and fresh material from the northwest Atlantic for the present study confirmed the lack of teeth in M. pygmaea (Figure 7). However, the European specimens, previously identified as M. pygmaea, have irregular granules on the hinge plate (Figure 4 C–D, F–H), a feature which precludes inclusion in Mendicula. Furthermore, examination of the internal anatomy has shown additional differences between these species. The adductor muscles of M. pygmaea are smaller than those in A. similis as are the lateral pouches. The foot of M. pygmaea does not contain as well-developed a heel as that of A. similis. M. pygmaea from the northwest Atlantic remains a valid species, however, the European form can no longer be recognised as M. pygmaea and is here described as Adontorhina similis.

Oliver & Killeen (2002) reported that specimens which had previously been recorded as Thyasira subtrigona Jeffreys, 1858, by Hartley (1984) were actually specimens of A. similis (although Oliver & Killeen listed them as Mendicula pygmaea). The type specimen of Thyasira subtrigona was destroyed (Jeffreys, 1864) and has been considered a nomen dubium by van Aartsen & Carrozza (1997). Other authors have recognized T. subtrigona as a member of the superfamily Galeommatoidea (Bowden & Heppell, 1968; Oliver & Killeen, 2002).