Hyalokellia tahaia, Raines & Huber, 2012

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 53-55

Hyalokellia tahaia sp. nov.

Figures 27 A–E

Type Material. Holotype: LACM 3152, a 4.7 mm single LV. Paratypes 1–3: LACM 3153, 2 single RV (2.9 & 4.1 mm), and 5.1 mm single LV from the type locality.

Type Locality. Dredged at 30–50 m in fine sand, off the western coastline near Tahai, Easter Island, 27°07’20” S, 109°26’30” W .

Description. A small Hyalokellia species , rarely over 5 mm in length, oval, equally rounded at both ends, produced posteriorly, straight ventrally, thin and fragile; inequilateral; umbones in the anterior third, somewhat erect, small, rounded, slightly above the dorsal line, weakly prosogyrate. Prodissoconch, P1when visible is D-shaped and somewhat smooth, ca. 135 µm length by 120 µm height, P2 suboval, well defined, irregularly growth lines, ca. 311 µm length by 266 µm height. Adult valves weakly inflated, non-gaping; color semi-translucent to opaque white. Moderate variability in shape could be observed during growth, with smaller specimens being less evenly rounded, but slightly less broad on the attenuated posterior side. Surface polished glossy and smooth, weak growth lines marginally, no radial elements. Hinge simple, RV with a single, rather strong and knobby cardinal, situated anteriorly to the subumbonal resilifer pit, no anterior lateral, but a rather long, somewhat oblique posterior lateral, positioned on the lower margin of the hinge line. Above this lateral tooth is a ridge positioned in midst of the hinge line, parallel to the dorsal margin. The LV lateral matches with corresponding ridge and the marginal lateral. LV with a single weak anterior cardinal and a long posterior lateral. Pallial line continuous, muscle scars very weakly impressed, the posterior-one rounded, the anterior-one reniform, both scars are much closer to the dorsal than to the ventral margin. Ligament internal, resilifer obliquely placed in a subumbonal pit. Margins smooth.

Comparative diagnosis. Habe’s Hyalokellia species from Tomioka, Japan and Laseron’s Parvikellia species from southeastern Australia merit close comparison.

Laseron (1956: 9) followed by Chavan in Moore (1969: N522), illustrated the RV of the type species Parvikellia isolata Laseron, 1956 . These conform by having a single cardinal, situated anteriorly, a subumbonal resilifer and a posterior lateral present, but anterior lateral absent. However, apart from the lower and longer posterior lateral in H. tahaia there is a further elongate ridge between lateral and dorsal margin for the reception of the LV lateral. The hinges of the respective left valves are significantly distinct. Assuming Parvikellia depressa Laseron, 1956 , as congeneric, as did Laseron, Chavan, and Lamprell and Healy (1998: sp. 420), then a Parvikellia LV consists of two small diverging cardinals and anterior laterals. In contrast, the hinge of H. tahaia consists of a single anterior cardinal without anterior laterals. In addition, the genus Parvikellia has more rounded and stronger inflated valves and the umbones of P. isolata and P. depressa are acutely pointed and rather orthogyrate.

Very close in fragility, with bean-shaped valves, having small prosogyrate and rounded umbones, by its size, and hinge configuration with a single cardinal in each valve is Hyalokellia polita Habe, 1960 . To date this genus is monospecific. It was based on a Japanese species from Tomioka, Kyushu and in 1981 also recorded from Kii, Honshu. P. Middelfart, (pers. comm., 2010) recognized close affinities to the EI species. Hyalokellia polita was described, but not illustrated by Habe, 1960. The holotypic RV, 5.1 mm, of H. polita is illustrated in Higo et al. (2001: B693). The paratypic LV, 5.8 mm, was described by Habe (1960: 291). Both species share a similar size and a sublittoral habitat (shallower than 50 m). The hinge configuration is identical. However, H. tahaia is more regularly ovate, notably on the dorsal side. The hinge line is also broader and more solid.

Chavan in Moore (1969: N537) placed the then unfigured Hyalokellia under 'genera dubia '. Habe (1960: 290) originally placed Hyalokellia within the invalid family Erycinidae ( Bouchet et al. 2010) . Modern Japanese authors (e.g. Higo et al. 2001) place it near Kellia Turton, 1822 . This view is shared and Hyalokellia tahaia is placed in its vicinity.

Remarks. There is nothing similar known from the Hawaiian Islands, Marquesas Islands nor New Zealand. However, material recently studied from Tahiti by the junior author indicates that an additional undescribed Hyalokellia species lives there, which is larger (8 mm) and very distinct by being more oval and higher in shape.

Distribution. This is an uncommon species from Easter Island and at present is only known from the type locality— E1.

Etymology. Named after the type locality.