Phymorhynchus oculatus Zhang & Zhang 2017, sp. nov.

Phymorhynchus oculatus sp. nov.

Type specimens: Holotype. MBM285087, height: 16.0 mm, width: 8.4 mm, 3 °43′S 151°40′E, 1740 m deep, June 12, 2015; Paratype. MBM 285088, height: 16.6 mm, width: 9.4 mm, collected with the holotype at the type locality. All type specimens have been deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China.

Type locality. A hydrothermal vent site in Manus Back-Arc Basin (3°43′S 151°40′E), at depth of 1740 m. For detailed information of this vent site, please see Hashimoto et al. (1999) and Fourre et al. (2006).

Etymology. Latin adjective oculatus, having eyes, referring to the presence of eyes on cephalic tentacles.

Description Shell (Figs. 1 A–D) fusiform, thin, white under a dull smooth olive-green periostracum, decollate but indicating more than three teleoconch whorls. Suture moderately constricted; teleoconch whorls convex. Spiral sculpture on the last whorl of about 25 wide, flattened spiral cords, with fine, equal interstices; axial sculpture lacking. Surface of the penultimate whorl heavily eroded, but traces of spiral cords still visible. Aperture ovate, inner surface whitish, outer lip thin, fragile; columellar lip smooth, anterior part slightly curved. Siphonal canal short and wide, not recurved.

Soft parts (Fig. 1 E). Foot large, fleshy. Operculum lacking. Head cylindrical, fully overlaid by the mantle. Cephalic tentacles large; stout, blackish eyes located at the middle part. Penis large, bent backward, distal end truncated without seminal papilla. Ctenidium moderately large. Osphradium relatively small, situated at left anterior of ctenidium.

Radula (Fig. 1 F) toxoglossate, with a formula of 1+0+0+0+1. Marginal teeth needle-like in shape, slightly bent near base. The distal end sharply pointed with a well-developed barb. An individual marginal tooth approximately 365 µm in length. Each single tooth with wide basal and distal openings that are connected by a distinct suture.

Remarks. Phymorhynchus oculatus sp. nov. is characterized by its fusiform, slender shell with flattened, regularly spaced spiral sculpture; its cephalic tentacles with blackish eyes; and by its long and slender marginal teeth with a pointed, barbed distal end. These characters distinguish Phymorhynchus oculatus sp. nov. from other congeners.

The new species is most similar to Phymorhynchus buccinoides Okutani, Fujikura & Sasaki, 1993 from a seep site off Hatsushima, Japan, which has a slender shell with wide, flattened spiral cords like Phymorhynchus oculatus sp. nov. However, the spiral cords in Phymorhynchus buccinoides Okutani, Fujikura & Sasaki, 1993 are exclusively present on base of shell. Furthermore, Phymorhynchus buccinoides is very different from the new species by having much shorter marginal tooth (ca.100 µm vs. ca. 365 µm) without a barb near the distal end (see Fujikura et al. 2009; Sasaki et al. 2010).

Species Distribution

P. buccinoides Okutani, Fujikura & Sasaki, 1993 Hatsushima, Japan, 35°00′N 139°14′E, 1160 m, seep.

P. carinatus Warén & Bouchet, 2001 Mid-Atlantic Ridge, 14°45′N 44°59′W, 3040 m, vent.

P. cingulatus Warén & Bouchet, 2009 Regab site off West Africa, 05°48′S 09°43′E, 3150 m, seep. P. coseli Warén & Bouchet, 2009 Regab site off West Africa, 05°48′S 09°43′E, 3150 m, seep. P. hyfifluxi Beck, 1996 North Fiji Basin, 16°60′S 173°55′E, 2003m, vent.

P. major Warén & Bouchet, 2001 East Pacific Rise, 09°50′N and 104°17′N, ca. 2500-2600 m, vent. P. moskalevi Sysoev & Kantor, 1995 Mid-Atlantic Ridge, between 26°N and 23°N, 3400-3700 m, vent. P. ovatus Warén & Bouchet, 2001 Mid-Atlantic Ridge, 14°45′N 44°49′W, 1600-3500 m, vent. P. starmeri Okutani & Ohta, 1993 North Fiji Basin, 2750 m; Manus Back-Arc Basin. vent. P. turris Okutani & Iwasaki, 2003 Nankai Trough, Japan, 32°21′N 134°32′E, 3540-3581m, seep. P. wareni Sysoev & Kantor, 1995 Edison Seamount, south of Lihir Island, 3°19′N 152°35′E, 1483 m; off

Manus Island, 3°10′N 150°17′W, 2450 m. vent.

Phymorhynchus wareni Sysoev & Kantor, 1995, which occurs at a hydrothermal vent site off Manus Island, is a congener with the closest geographic proximity to Phymorhynchus oculatus sp. nov. However, Phymorhynchus wareni can be clearly separated from the new species by its much broader shell, different sculpture pattern, and absence of eyes on cephalic tentacles.

The conoidean gastropods are an important group of predators in deep-sea environments (Bouchet & Warén 1980; Sysoev 1996; Kantor et al. 2016). Among them, the most well-known component in vent/seep environments is the genus Phymorhynchus. To date, about 11 species of Phymorhynchus from vent/seep areas have been described (see Table 1).

Eyes are often reduced or absent in deep-sea gastropod species. Within the genus Phymorhynchus, eyes were previously known only in Phymorhynchus sulcifera (Bush, 1893) (see Bouchet & Warén 1980), a species that is not associated with a vent/seep environment. Thus, Phymorhynchus oculatus sp. nov. represents the first Phymorhynchus species that occurs in a chemosynthetic environment that possesses eyes. The presence of prominent eyes is extraordinary for a resident of deep-water hydrothermal vents, which indicates that the invasion of Phymorhynchus oculatus sp. nov. to the vent habitat may happened recently. In another case, however, the larvae may undergo migration to the epipelagic zone, as do some deep-sea conoideans (Bouchet & Warén 1994). But so far, development of members of Phymorhynchus is still poorly known and so there is no evidence to indicate that larvae of these species migrate to the epipelagic zone. Further study will be needed to solve this issue.

This research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA1103040102, XDA1102030505) and Key Research Program of Frontier Sciences, CAS (QYZDB-SSW- DQC036). We would like to express our sincere thanks to the crews of R/V KEXUE for their cooperation during the survey. Special gratitude to Dr. Yu. I. Kantor (Severtsov Institute of Ecology and Evolution, Russia) for providing us with helpful reference.