Bathyacmaea nadinae Beck, 2023

Chen, Chong & Sigwart, Julia D., 2023, The lost vent gastropod species of Lothar A. Beck, Zootaxa 5270 (3), pp. 401-436 : 408-412

Bathyacmaea nadinae Beck , sp. nov.

Figs 2–5

[ZooBank LSID: urn:lsid:zoobank.org:act:1F6BE1B6-5801-4BB6-B280-90CF4B1C44B9 ]

Type material. Holotype [MNHN-IM-2000-38676] and one paratype in MNHN [Paratype 1: MNHN-IM-2000- 38677], two paratypes in NSMT [Paratype 2: NSMT-Mo 79402; Paratype 3: NSMT-Mo 79403], two paratypes in SMF [Paratype 4: SMF 370350 ; Paratype 5: SMF 370351 ].

Type locality. Lau Basin (Hine Hina) 22°32′S –l 76°43′W; BIOLAU BL02.

Material examined. The type material and BIOLAU, BL02, 2 specimens [ SMF 370352 ]; BIOLAU, BL08, 4 shells and 1 shell fragment, dry [NSMT-Mo 79404]; STARMER II, PL18, 3 specimens [NSMT-Mo 79405]; STARMER II, PL20, 10 empty shells, dry [ SMF 370353 ]; STARMER II, PL21, 2 empty shells, dry [MNHN-IM-2022-12940] .

Distribution. Lau Basin and [North] Fiji Basin in bathyal areas of diffuse hydrothermal venting.

Etymology. Named after [Beck’s] daughter Nadine who had to suffer from [his] time-consuming job. Species name refers to the latinized form of the name Nadine.

Description. Shell ( Figs 2A–F , 3A, B ). In adults: very solid, with oval outline, profile low to medium, anterior slope somewhat steeper than posterior slope, slopes with fine irregular growth-lines and up to three conspicuous steps of growth, apex on mid-line, strongly eroded, protoconch lost, external shell surface smooth except the effects of growth and hydrothermal black incrustations. Internal surface with transparent outer margin (parallel-lamellar prismatic layer) and barely marked muscle scar (myostracum). In juveniles: shell thin and fragile without an inner, radial, crossed-lamellar layer, external surface with irregular, weak, radial riblets; sporadically, the crescent shaped scar of the protoconch is preserved. Shell structure ( Figs 3A–B ) shows three layers: an outer, parallel—lamellar (foliated), a middle, concentric, crossed—lamellar and an inner, compact, prismatic layer with amorphous fillings. In the apical region, the foliated layer usually is eroded. The myostracum is not demonstrated here.

Dimensions. Holotype: length: 23.0 mm, width 18.9 mm, height 8.3 mm; paratype 1: 17.7 x 14.2 x 6.7 mm; paratype 2: 16.0 x 12.5 x 5.9 mm; paratype 3: 11.0 x 8.3 x 3.2 mm; paratype 4: 11.1 x 8.2 x 3.4 mm; paratype 5: 8.1 x 6.0 x 2.3 mm.

Radula ( Fig. 4 ). Docoglossate, central teeth and marginal teeth completely lacking, formula 0.1.0.1.0, lateral teeth asymmetrically arranged within a transverse row, the distal third of each tooth is impregnated with ferrous oxides causing the dark brown colour that can be seen by using a microscope. Lateral tooth with long basal plate and a trifid distal part which shows a short and slender inner ramus, a broad, middle, non-determined ramus and a welldeveloped, outer ramus, the middle ramus towers somewhat above the inner and outer rami. In juveniles (specimen of 3.0 mm shell length), the basal plate appears to be weaker and the inner and outer rami appear to be stronger.

Soft parts ( Fig. 3C–E ). Operculum lacking.Animal of white colour. Head with small tapering cephalic tentacles, no eyes, snout tapering with a small mouth-opening, oral fringe folded in retracted condition and weakly papillated. Surface of odontophore ( Figs 3D, E ) equipped with rows of minute pins. Foot with a large, ciliated foot sole without a foot gland opening, no epipodial appendages; pallial edge with numerous, minute papillae, pallial skirt thin and transparent in the posterior two thirds but thicker and non-transparent in the anterior third; the pallial margin is attached to the shell. Shell muscle horseshoe-shaped, anterior ends bent to midline, while the end of the right arm is somewhat larger than the end of the left arm; shell muscle arranged in bundles of unequal size; pallial cavity shallow extending to about one third of animal length from anterior end; gill of moderate size, bipectinated, dorsal and ventral gill lamellae of equal size, gill on left side delimited by efferent branchial vein, on right side by the afferent branchial vein, finger-shaped anus papilla and urogenital-papilla on right dorsal side of the cavity; at nuchal region and pallial floor loops of oesophagus, intestine and oesophageal pouches can hardly be observed.

Visceral mass. The loops of the gut system continue into the visceral mass whereas the large intestine is recognizable by black mineral crystals and greyish flaky material, the stomach is covered by the digestive gland which is situated apically, the rectum crosses the visceral mass diagonally, running parallel to one loop of the intestine. The right kidney and its opening (the urogenital papilla) can be recognized in the excretory system, but the left kidney, which is usually situated anterior to the rectum in Bathyacmaea , is not clearly detectable without cross sectioning. The gonad lies under the digestive gland and reaches to the posterior end of the visceral mass. Of the circulatory system, the region of ventricle and auricle can be seen on left side at the base of the gill; a circumpallial vein as well as efferent and afferent branchial veins are clearly visible; capillary vessels could not be detected. Apparently, osphradia are not present.

Figures

FIGURE 2. Bathyacmaea nadinae sp. nov., shells and soft parts. A–C. Holotype (shell length 23.0 mm). A. Apical view. B. Ventral view showing soft parts in retracted condition. C. Lateral view of left side. D–F. Shell of paratype 3 (shell length 11.0 mm). D. Apical view, apex eroded. E. Ventral view. F. Lateral view of right side showing conspicuous steps of growth.

FIGURE 3. Bathyacmaea nadinae sp. nov., SEM micrographs of shell and soft parts. A, B. Tangential shell fracture from the mid-point at the middle between apex and shell margin showing three layers: A. Shell layers (from top to bottom) concentric, crossed-lamellar and radial-lamellar prismatic. B. Concentric crossed-lamellar and radial crossed-lamellar layer. C–E. Soft parts. C. Anterior part, oblique view from right. D. Snout with mouth-opening, oral fringe and slightly protruded odontophore.E. Surface of odontophore equipped with rows of minute pins. [Scale bars:A, 200 μm; B, 100 μm; C, 2 mm; D, 500 μm; E, 50 μm.]

FIGURE 4. Bathyacmaea nadinae sp. nov., SEM micrographs of radulae. A. Specimen from BIOLAU BL02. B. Specimen from STARMER II PL18. C. Juvenile specimen from BIOLAU BL07 (shell length 3 mm). [Scale bars: A, 100 μm; B, 20 μm; C, 50 μm.]

FIGURE 5. Bathyacmaea nadinae sp. nov. A. Dorsal view of soft parts, neck and gill seen by transparency. B. Single, right, lateral, trifid tooth of radula.