Metaxia discus, Fernandes, 2024

Fernandes, Maurício Romulo, 2024, Triphoridae (Gastropoda) from Martinique sampled by the MADIBENTHOS expedition, with notes on shallow-water species from Guadeloupe, Zoosystema 46 (18), pp. 457-503 : 472

Metaxia discus n. sp.

( Fig. 9 A-F)

urn:lsid:zoobank.org:act:B613849C-897F-4E0A-BE99-EBD074B89222

TYPE MATERIAL. — Holotype. Martinique. • sh; North of Presqu’Île de la Caravelle , region ‘Nord Atlantique’; MADIBENTHOS expedition; sta. AD245; 14°53’06”N, 60°3’24”W; depth 60 m; MNHN-IM-2000-38601 .

Paratype. Martinique. MADIBENTHOS • 1 sh; type locality; MNHN-IM-2000-38602.

TYPE LOCALITY. — Martinique: North of Presqu’Île de la Caravelle, region ‘Nord Atlantique’; MADIBENTHOS expedition sta. AD245, 14°53’06”N, 60°3’24”W; 60 m.

ETYMOLOGY. — From Latin, discus . The species is named due to the discoid shape of the protoconch.

DIAGNOSIS. — Protoconch with 2.5 much convex whorls, first whorl discoid, broad, with same width than second one, and with two spiral cords of varying expression; shell white.

GEOGRAPHIC DISTRIBUTION. — Martinique (this study).

BATHYMETRIC DISTRIBUTION. — Only known from 60 m.

DESCRIPTION

Shell dextral, conical/fusiform, rectilinear profile, up to 3.2 mm long, 0.8 mm wide, length/width ratio 3.5-4.1. Protoconch paucispiral, of 2.5 much convex whorls, 0.41-0.44 mm long, 0.34-0.37 mm wide; first whorl discoid, broad, with same width than second one, with two spiral cords on the adapical half of the whorl, having varying expression, the abapical cord more prominent and initially undulating (zigzag) to further nearly straight, and the adapical cord undulating (zigzag); on the second whorl, the two spiral cords having nearly the same width and being no longer restricted to the adapical portion of the whorl, situated respectively at c. 39% and c. 71% of whorl height, with small nodules, with incomplete, orthocline to slightly prosocline axial riblets located above adapical cord.Teleoconch with up to seven whorls; four equidistant spiral cords, with the first (adapical) cord being much smaller and the third cord the most prominent, in addition to a much reduced, smooth sutural cord; elliptical nodules of small size; c. 15 weak, orthocline axial ribs on the sixth teleoconch whorl; smooth and narrow subperipheral cord, no distinct basal cord; aperture rounded (but partially broken), with small anterior notch. Shell white.

REMARKS

Metaxia discus n. sp. superficially resembles the morph named Metaxia sp. 1 ( Fig. 9 G-H), from Barbados ( Rolán & Fernández-Garcés 2008). The teleoconch of Metaxia sp. 1 has three main spiral cords, similarly to Metaxia excelsa (Faber & Molenbeek, 1991) or Metaxia taeniolata ( Dall, 1889) , whereas M. discus n. sp. has four main spirals in late whorls (i.e., the upper cord is stronger than that of Metaxia sp. 1 , which seems a weak subsutural cord), similarly to Metaxia rugulosa (C. B. Adams, 1850) . The protoconch of Metaxia sp. 1 is illustrated here ( Fig. 9H ), and, despite slightly worn, is more similar to M. rugulosa than to M. discus n. sp. (see comparisons below). Despite having a nearly identical teleoconch, M. discus n. sp. is differentiated from the widespread West Atlantic species M. rugulosa by the discoid and truncated shape of the protoconch, often with undulating spiral cords ( Fig. 9D ), whereas the protoconch of M. rugulosa is always convex, with a dome-shaped first whorl and straight spiral cords in the second whorl ( Rolán & Redfern 1996; Fernandes & Pimenta 2011). Metaxia discus n. sp. has a protoconch similar to Metaxia quadrata Faber, 2010 , which is known only by the holotype, from Aruba. However, the protoconch of M. quadrata seems to have three spiral cords on the first whorl (vs two cords in M. discus n. sp.), with a more accentuated zigzag pattern than in M. discus n. sp., although the second whorl of protoconch has a nearly rectilinear adapical spiral cord (but more undulating in M. discus n. sp.). In addition, the shell of M. quadrata is yellowish brown and it has a weaker axial sculpture ( Faber 2010) vs entirely white in M. discus n. sp.

Figures

FIG. 9. — A-F, Metaxia discus n. sp.A, B, D, F, MNHN-IM-2000-38601, holotype,3.2 mm;C, E, MNHN-IM-2000-38602, paratype,sta.AD245,2.7 mm;G, H, Metaxia sp. 1 (morph named by Rolán & Fernández-Garcés [2008]), USNM 87301, 4.2 mm (vs 5.0 mm, according to Rolán & Fernández-Garcés [2008]); credits: Yolanda Villacampa (NMNH). Scale bars: A-C, G, 1 mm; D, E, 100 μm; F, H, 200 μm.