Ptilomyax hadalis, Oliver, 2012

Oliver, P. Graham, 2012, Taxonomy of some Galeommatoidea (Mollusca, Bivalvia) associated with deep-sea echinoids: A reassessment of the bivalve genera Axinodon Verrill & Bush, 1898 and Kelliola Dall, 1899 with descriptions of new genera Syssitomya gen. nov. and Ptilomyax gen. nov., European Journal of Taxonomy 12, pp. 1-24 : 18-20

Ptilomyax hadalis sp. nov.

Montacutid Mironov, 1978: 722, pl. 1.

Etymology

hadalis — referring to the hadal zone of the deep ocean (deeper than 6000 m).

Material examined

Type series of 18 specimens Banda Trench, RV Vityaz cruise 57, stn 7271, 5°37'S 131°07.5'E, 7340- 7335 m, 21-22 Mar. 1975. Leg. A.N. Mironov.

Holotype

1 specimen, Zoological Museum of Moscow State University , Moscow, ZMMU Ld-3043 .

Paratypes

14 specimens, Zoological Museum of Moscow State University, Moscow, ZMMU Ld-3044. 2 specimens (1 on SEM stub), National Museum Wales, Zoology- NMW.Z.2012.013.

The material has been poorly preserved, thus the shells are considerably softened, and the tissues are rather contracted. No attempt has been made to examine the hinge, as the valves are so soft that they do not survive being separated. However, the distinctive external form of the shell and the anatomy are sufficient to describe it and place it at the family level.

Description

SHELL. ( Fig. 10 A-C) Small, (largest specimens 2.5 mm L x 1.9 mm H). Inequivalve in adult, lateral dorsal margins of right valve elevated and rolled over projecting beyond left valve. Outline slightly inequilateral, beaks slightly prosogyrate, just behind the mid line, anterior more expanded than posterior; dorsal margins more or less straight, those in adult right valve projecting laterally, becoming alate; forming distinct lateral sinuses with adjacent anterior and posterior margins; lateral margins broadly rounded, anterior more so; ventral margin curved. Ligament small, posterior, deeply sunken; dorsal margin with thickened periostracal “ligament”. Hinge plate not examined. Sculpture almost smooth with faint commarginal lines. Prodissoconch I, 105 µm; prodissoconch II, 265 µm, with fine commarginal lines.

ANATOMY. ( Fig. 10D ) Mantle edge mostly unfused with extensive anterior-ventral pedal aperture; fused with terminal of gill axis; exhalant aperture small. Anterior-ventral inner margin raised into series of prominent evenly spaced transverse ridges. Anterior and posterior adductor muscles of similar size, oval in section. Posterior pedal retractor and anterior pedal muscles present. Ctenidium of a single demibranch, ascending and descending lamellae of almost equal size. Labial palps small. Foot with well developed toe, sole flattened and apparently with tissue projections (these may be an artefact of preservation); heel distinct, byssus functional producing numerous thin filaments.

Differential diagnosis

The projecting and rolled over dorsal margins of the right valve are reminiscent of pteriids but Ptilomyax gen. nov. is clearly allied with the galeommatids as evidenced by the anatomy. Ptilomyax gen. nov. shares many common features with other montacutids associated with echinoids including: a large anterior pedal aperture, mantle fusion limited to junction with gill axis, ctenidium with a single demibranch, foot with prominent toe with a sole and with a functional byssus, ligament internal and weak hinge with poorly developed teeth ( Oldfield 1961; Ockelmann 1965; Ponder 1968; Jespersen et al. 2004). The Galeommatoidea exhibit an extraordinary range of shell morphologies as can be seen in Chavan (1969) but none have projecting dorsal margins. This unique feature is sufficient to warrant the creation of a new genus and new species.

Figures

Fig. 10. Ptilomyax hadalis sp. nov. A. Holotype attached to echinoid spines. B. Paratype, left side. C. Juvenile paratype lacking “wings”. D. Anatomy of a paratype as viewed from the left side.