Caecum miliarium, Walter & Renda, 2024

Walter, Angelo Vannozzi & Renda, Walter, 2024, Sampling the depth: New data on the Caecidae (Mollusca, Gastropoda) from northeastern Papua New Guinea, Zoosystema 46 (14), pp. 327-347 : 336

Caecum miliarium n. sp.

( Fig. 5 )

urn:lsid:zoobank.org:act:8FFDAF1B-E8DE-435D-B8AF-4953A5C66CBC

TYPE MATERIAL. — Holotype. Papua New Guinea • sh (length 1.3 mm, Fig. 5 A-D); New Ireland, KAVIENG 2014 Stn CP4418; 2°27’S, 150°40’E; 335-340 m depth; 28.VIII.2014; MNHN-IM-2000-38946 .

Paratypes. Papua New Guinea • 106 sh (10 larval); same data as for holotype; MNHN-IM-2000-38947 .

TYPE LOCALITY. — Pacific Ocean, Papua New Guinea, New Ireland; 2°27’S, 150°40’E; 335-340 m depth.

DISTRIBUTION. — Known only from the type locality.

ETYMOLOGY. — From the Latin adjective miliarius (meaning: of a thousand, containing a thousand) because the sample collected in the type locality contained a great number of specimens of the new species.

DESCRIPTION

Shell small for the genus, smooth, colourless, semitransparent, somewhat iridescent. Protoconch planorbid, multispiral of c. 2 whorls, smooth and shiny, the last portion detached and straight, ending with a simple aperture with slightly reflected lip. Teleoconch tubular, arched, smooth. Early teleoconch subcylindrical, slightly twisted. Adult shell subcylindrical, slightly dextrally twisted in ventral view. Surface covered by very fine wavy longitudinal and discontinuous threads, evenly spaced but becoming more and more numerous and crowded going from the apex to the aperture. Macula none detected. Aperture somewhat inclined, without a clear swelling and ending with a fine reflected lip. Septum low dome shaped without mucro, slightly recessed below the cutting plane. Temporary septum thick, opaque, hemispherical, persistent, lying on the permanent one. Young stage with flatter and more recessed septum. Periostracum none detected. Operculum and soft parts unknown. Holotype: length 1.3 mm, max diam 0.25 mm, min diam 0.20 mm. Protoconch size 395 µm.

REMARKS

Among the characteristics of this species, a rather persistent temporary septum can be highlighted. It is entirely preserved in the holotype and partially preserved in the paratype shown in Figure 5F. The presence of numerous specimens in the type locality, including the complete growth series, allowed a clear and exhaustive knowledge of shell ontogeny of the new species. In fact, except for three juveniles, likely belonging to C. frugi Vannozzi, 2019 , the lot originating from Stn CP4418 contained only specimens C. miliarium n. sp. Furthermore, the presence of all growth stages, all well preserved, suggests that the actual habitat was sampled during the expedition and that this species lives in the aphotic zone, unlike the majority of other members of the family.

The new species can be compared with Caecum frugi , which shows a similar size and septum. However, C. miliarium n. sp. is readily distinguished by the wider tube and the longitudinal microsculpture visible also at low magnification, whereas in C. frugi the microsculpture is denser and visible only at very high magnification. Furthermore, C. frugi shows a different shape of the tube, more cylindrical and with inflated abapical portion. A similar microsculpture is shown also by C. neocaledonicum de Folin, 1868 , described from New Caledonia and occurring in the whole IWP, but this species is larger and otherwise different in several aspects as shown below.

A further comparison can be made with C. suteri Odhner, 1925 , a small species described from New Zealand, currently in the synonymy of Caecum digitulum Hedley, 1904 . In fact, although this species differs by the small finger-like mucro, absent in the new species, it shares a quite similar protoconch showing about the same number of whorls and a prolonged straight portion before the transition with the teleoconch ( Odhner 1924: 29; pl. 1; fig. 19).

Figures

FIG. 5. — Caecum miliarium n. sp.:A-D, holotype MNHN-IM-2000-38946; E-I, paratypes MNHN-IM-2000-38947; E, growth stage; F, specimen with partially worn temporary septum; H-I, postlarval specimen, different views. Scale bar: 1 mm, details not to scale.