Symmetromphalus mcleani Beck, 2023

Chen, Chong & Sigwart, Julia D., 2023, The lost vent gastropod species of Lothar A. Beck, Zootaxa 5270 (3), pp. 401-436 : 422-426

Symmetromphalus mcleani Beck , sp. nov.

Figs 15–17

[ZooBank LSID: urn:lsid:zoobank.org:act:D1AFECE2-136D-4CFD-A32C-47B5B300DC17 ]

Type material. Holotype (female) [MNHN-IM-2000-38690] from STARMER II PL20 and two paratypes (male and juvenile) from STARMER II PL20 [Paratype 1, male: MNHN-IM-2000-38691; Paratype 2, juvenile: MNHN-IM- 2000-38692] as well as two paratypes from [STARMER II, PL16 ] in NSMT [Paratype 3, female, NSMT-Mo 79425; Paratype 4, male, NSMT-Mo 79426], and two paratypes [from STARMER II, PL16 ] in SMF [Paratype 5, empty shell, SMF 370385 ; Paratype 6, empty shell, SMF 370386 ].

Type locality. North Fiji Basin, (White Lady) active hot vents, 16° 59.50′ S – 173° 55. 47′ W .

Material examined. The type material and from Lau Basin : BIOLAU, BL03, 1 male and 6 juvenile specimens [ SMF 370387 ]; BIOLAU, BL12, 4 males and 2 juvenile specimens [NSMT-Mo 79427]. From North Fiji Basin: STARMER II, PL16, 17 females, 2 males, and 19 juvenile specimens [ SMF 370388 ]; STARMER II, PL18, 1 female and 12 juvenile specimens [NSMT-Mo 79428]; STARMER II, PL19, 8 empty shells and 1 juvenile specimen [MNHN-IM-2020-12945]; STARMER II, PL20, 13 females, 5 males, and 2 empty shells [ SMF 370389 ] .

Distribution. North Fiji Basin and Lau Basin at bathyal hydrothermal vents.

Etymology. Species is named after James H. McLean, Los Angeles County Museum of Natural History.

Description. Shell ( Figs 15 , 16A–B ). Small for genus, almost limpet-shaped with remnants of shell-coiling at apex, even in adults, aperture irregularly oval in outline reflecting up to 12 radial broad ribs which appear not before 4-5 mm shell length; protoconch diameter 180 μm; juvenile teleoconch smooth, at shell length exceeding 1.0 mm with fine radial riblets (in adults more than 200 riblets present), which can only be seen by SEM, riblets finely knobbed; periostracum white to yellowish-brown, slightly overhanging and flexible at shell margin, in adults additionally slightly scaly at shell margin; apex approximately at mid-line, its position changes during shell growth from posterior shell margin to center of the shell; shell margin not in level but describing an irregular line corresponding to the surface of the substratum; interior of shell white, of silky brightness; horseshoe-shaped scars of shell muscles weakly marked; inner surface (except muscles scars) with numerous irregular micropores into which the filamentous papillae of the pallial surface fit, pores penetrate the ostracum but not the periostracum.

Dimensions. Largest specimen (empty shell) length 8.9 mm, width 5.1 mm, height 2.5 mm; holotype female 7.1 mm x 4.2 mm x 2.0 mm, paratype 1 (male) 5.9 mm x 3.8 mm x 2.0 mm.

Radula ( Figs 16D–F ). Rhipidoglossate, as is typical for genus, formula 10–12 x 4 x 1 x 4 x 10–12; rachidian and four lateral teeth of similar shape, rachidian cusp slightly larger than cusp of first lateral, cusps of laterals increasing in size distally, no serrations except at distal margin of cusp of fourth lateral; there, the lowermost serration is most prominent; only 10–12 marginal pairs of teeth present, bearing finely denticulated cusps with distal, tongue-like processes.

Soft parts ( Figs 15A, D, H , 16C, E , 17A–E ). Head with short tentacles directed posteriorly; in males, the left cephalic tentacle is transformed and enlarged to function as a penis, snout transformed, having no oral disc with marginal papillae but a large transverse furrow; neck elongated, in males with deeply marked seminal groove leading to the left cephalic tentacle; neck dorso-ventrally compressed with laterally thickened edges; eyes lacking; penis with longitudinal furrows and with a tubule at the tip. Footsole round in outline, anterior third broader than the rest, with minute slit-shaped opening of anterior foot gland; foot sides laterally and posteriorly with epipodial ridge bearing up to ten pairs of tentaculiform appendages, posterior appendages of approximately the same size as the cephalic tentacles; pallial margin smooth but pallial epithelium with numerous minute, filamentous papillae fitting into the shell micropores; pallial cavity deep containing the very large bipectinate gill which apparently is used for filter feeding, afferent membrane completely absent, massive efferent axis arises at posterior of pallial cavity on left side and reaches up to or beyond anterior margin of head, gill lamellae thin and elongate (curled in contracted condition) increasing in length from posterior to anterior; visceral mass comparatively small, gonad appears to be its essential part, rectum leads to the anus at anterior right comer of pallial cavity; shell muscle horseshoe-shaped with longish left arm and slightly shorter and broader right arm, both arms posteriorly connected by an extremely thin muscular ligament. Operculum present in all specimens examined, very thin and translucent, last [re]volutions enlarged rapidly.

Remarks. S. mcleani sp. nov. is the third species of the genus Symmetromphalus which lives at active hot vents in the western Pacific. The type species, S. regularis from Mariana vents, is reported to live in dense aggregations on basalt boulders in the path of hydrothermal effluents ( McLean 1990). The same is true for S. hageni reported from the surface of active smokers at Manus vents ( Beck 1992a). All morphological evidence [from gill and head-foot] of the new species described here point out that S. mcleani sp. nov. is also sessile and lives as a filter-feeder. The new species’ morphology is clearly distinguished from the known species by the following characters: shell with up to 12 broad radial ribs and a slightly scaly periostracum at shell margin; shell dimensions smaller; number of marginal teeth reduced to 10–12 pairs. All specimens have intact shells with no mineral encrustations or damaged periostraca as is usual for other hot-vent taxa (e.g. species described earlier in this paper). However, as a consequence of a sessile mode of life, soft parts are in some places colonized by bacteria. At the epipodium threads of organisms similar to Beggiatoa are common. At seminal groove, coccal organisms were found to exist in dense colonies.

Figures

FIGURE 15. Symmetromphalus mcleani sp. nov. A–C. Holotype (shell length 7.1 mm), female. A. Ventral view. B. Dorsal view. C. Viewed from left. D. Paratype 4 (shell length 3.5 mm), male, ventral view. E. Paratype 1 (shell length 5.9 mm), male, ventral view. E. Paratype 4, male, viewed from left. G–H. Paratype 2 (shell length 1.3 mm), juvenile specimen. G. Dorsal view. H. Ventral view.

FIGURE 16. Symmetromphalus mcleani sp. nov. A. Periostracum at shell margin. B. Interior of shell with micropores. C. External surface of pallial skirt with filamentous papillae fitting to the shell micropores. D. Radula, overview. E. Dorsal jaws and protruded buccal mass with radula. F. Radula, close-up. [Scale bars: A, 500 μm; B–C, 10 μm; D, 50 μm; E, 100 μm; F, 50 μm.]

FIGURE 17. Symmetromphalus mcleani sp. nov. A–D. Male specimen. A. Front view of the head. B. Dorsal view of the head. C. Coccal bacteria in seminal groove. D. Coccal bacteria, close-up. E. Female specimen, ventral view. [Scale bars: A–B, 1 mm; C, 50 μm; D, 5 μm; E, 2 mm.]