Newnesia abyssalis, Moles & Avila & Malaquias, 2018

NEWNESIA ABYSSALIS View in CoL SP. NOV.

( FIG. 3A–I)

urn:lsid:zoobank.org:act:D0D2FCFF-9B8F-4451-A40B-825C19ACBA9E

Type locality: Drake Passage, 60°39 ′ 11 ″ S, 53°56 ′ 51 ″ W, Western Antarctica GoogleMaps ; 2325–2893 m depth.

Diagnosis: Shell external, globose, rather thick; apical area flattened, with large aperture. Broad cephalic shield, posterolateral cephalic lobes present. Tentacular processes absent. External sperm groove present, running laterally on right side of body from gonopore to penial aperture. Parapodia absent. Pedal gland furrow absent. Radular formula: 2.1.2. Sharp unicuspidated rachidian teeth with five to eight denticles along sides. Jaws and gizzard plates absent. Cuticularized and spinous gizzard. Male reproductive system tubular; prostate narrower, curled. Penial papilla short, unilobed. Two gills one lying on roof and one on floor of mantle cavity. Two repugnatorial glands present, one placed on left anterolateral side, the other on right posterolateral side right after mantle cavity (infrapallial lobe).

Material examined ( Fig. 1): North of the South Scotia Ridge, 60°39 ′ 11 ″ S, 53°56 ′ 51 ″ W, 30 January 2002, 2889– 2893 m depth, 1 spc. sequenced, ZSM Moll 20021145, H = 21 mm (holotype). Drake Passage , north of King George Island , South Shetland Islands, 58°6 ′ 41 ″ S, 61°17 ′ 21 ″ W, 21 December 2015, 2325 m depth, 1 spc. dissected and sequenced, ZMBN 116994 View Materials , H = 22 mm (paratype) GoogleMaps .

Etymology: The name stems from the fact that this species was found in the upper abyssal zone.

Shell ( Fig. 3A–D): Maximum H = 22 mm. External; thick, white, globose; aperture wide, oblique to shell axis; covering whole viscera; protoconch not-protruding; spire short; sculpture consisting of transverse lines of squared pits ( Fig. 3D); umbilicus absent; thin, lip unornamented, parietal callus absent; thin, translucent periostracum.

External morphology ( Fig. 3A–C): Animal beige to whitish when alive and preserved. Cephalic shield broad, thickened, trapezoidal; mouth opening lying ventrally; two large, folded, posterolateral orientated velar lobes displaying ciliated grooves; penial opening placed in right anterior notch under cephalic lobe. External sperm groove present, running laterally on right side of body from gonopore to penial pore. Tentacular processes and parapodia absent. Mantle cavity placed on right side, partially covered by shell; prominent, plicate, primary gill inside; accessory gill smaller, placed directly underneath; anus opening posteriorly on right side of body, close to edge of mantle cavity. Two repugnatorial glands present, one placed on left anterolateral side, the other on right posterolateral side right after mantle cavity (infrapallial lobe). Foot broad, protruding posteriorly body perimeter; propodium squared and slightly lobulated; metapodium oval. Pedal gland not observed.

Radula ( Fig.3E): Radular formula 28 × 2.1.2. Rachidian large, triangular; tip pointed with sharp central cusp; base large, hollow; five to eight small sharp denticles on each side curved inwards. Lateral teeth flat, thin, triangular-hook shaped with convex anterior margin; well separated from rachidian teeth.

Digestive tract ( Fig. 3F): Pharynx surrounded by thick, non-striated muscle layers. Jaws absent. Salivary glands sausage shaped with narrow section, lying close to oesophagus and gizzard. Oesophagus elongate cylindrical. Gizzard plates absent; pointed, chitinous gizzard spines present. Digestive gland occupying most of visceral whorl.

Male reproductive system ( Fig. 3G, H): Tubular with narrower and slightly curled prostate. Penial region elongated, broad. Penial papilla short, unilobed, unarmed, wrinkled.

Ecology ( Fig. 3I): Known only from depths between 2325 and 2893 m (this study). Diatoms observed inside gizzard.

Distribution ( Fig. 1): The Drake Passage, at the north of King George Island and the Scotia Ridge.

Remarks: Despite the overall similarities in shell shape between N. antarctica and N. abyssalis sp. nov., the latter has a less developed periostracum and a distinct microsculpture made of transverse lines with welldefined square pits ( Figs 2D, 3D). Morphologically, contrary to N. antarctica , we could not observe a distinct furrow on the pedal gland of N. abyssalis sp. nov. ( Fig. 3B), but this needs to be further investigated with histological studies. The radula of N. abyssalis sp. nov. is more complex with higher number of cusps in the rachidian tooth and presence of two lateral teeth ( Fig. 3E). The gizzard spines in N. abyssalis sp. nov. are comparatively more developed and sharp ( Fig. 3F). Concerning the reproductive system, N. abyssalis sp. nov. has a conspicuously narrower and longer prostate and a simple unilobed penial papilla ( Fig. 3G, H), which is bilobed in N. antarctica ( Fig. 2H).

Moreover, these two species, although perhaps sympatric in some geographical areas ( Fig. 1), seem to inhabit different bathymetric zones; N. antarctica apparently does not occur bellow c. 700 m, while N. abyssalis sp. nov. was only found at the transition from the bathyal to the abyssal zone (2325–2893 m depth) sensu Vinogradova (1997).