Paraphelliactis tangi, Li, Yang & Xu, Kuidong, 2016

Li, Yang & Xu, Kuidong, 2016, Paraphelliactis tangi n. sp. and Phelliactis yapensis n. sp., two new deep-sea species of Hormathiidae (Cnidaria: Anthozoa: Actiniaria) from a seamount in the tropical Western Pacific, Zootaxa 4072 (3), pp. 358-372 : 360-365

Paraphelliactis tangi n. sp.

( Figs. 1–4 ; Tables 1, 2)

Material examined. Holotype: Y 30046 , attached on stones, collected on 16 December 2014 from FX-Dive 17 (137°49.90′E, 9°0.82′N), 1980 m, foraminiferal ooze bottom. Paratypes: Y 30051 , two specimens, embraced sponge spicules, collected on 16 December 2014 from FX-Dive 17 (137°49.81′E, 8°59.80′N), 1928 m, foraminiferal ooze bottom.

Body and Size. Pedal disc attached to stone or grasping sponge spicules. Column covered with brown cuticle, darker proximally than distally ( Fig. 2 ). In preservation, column sub-cylindrical, height 13–70 mm (70 mm in holotype), diameter of pedal disc 27–89 mm (70 mm in holotype), larger than that of proximal column (18–55 mm; 45 mm in holotype), greatest diameter of distal column 17–74 mm (74 mm in holotype) ( Fig. 2 B, C). Column divisible into scapus and scapulus: former provided with layer of cuticle and larger tubercles; latter short, without cuticle, longitudinally furrowed, with smaller tubercles ( Fig. 2 ). Tubercles mainly in distal column, typically conical in holotype and rounded in smaller specimens, of different sizes, diameters of base to 7 mm and height to 9 mm, irregularly arranged. Mesogloea of column thick, to 3 mm between tubercles and about same thickness along column.

Oral Disc and Tentacles. Oral disc pink, elliptical, long axis diameter 6–54 mm (54 mm in holotype). Actinopharynx well developed, length to 25 mm, occupying about 1/3 of column length in holotype. Tentacles marginal, retractile, smooth, tapered, and without mesogloeal thickenings on aboral side ( Figs. 2 A). Length to 30 mm in life, to 10 mm long and 3 mm wide in preservation. Tentacles hexamerously arranged in six cycles, inner ones wider and longer than outer ones, 181 in holotype (12 cut off with margin for histological sections), 191 in larger paratype (15 cut off with margin for histological sections), and 92 in smaller paratype; full complement of large individuals likely 192 (6 + 6 + 12 + 24 + 48 + 96).

Internal Anatomy. Two symmetrical siphonoglyphs well developed, each attached to pair of directive mesenteries. Mesenteries not divisible into macrocnemes and microcnemes. Equal number of mesenteries at margin and limbus. In large specimens (holotype and larger paratype), 96 pairs hexamerously arranged in five cycles. In holotype, six pairs of mesenteries of first cycle and one mesentery of second cycle perfect and sterile (mesentery pairing with perfect one of second cycle fertile). In larger paratype, mesenteries of first cycle except for one perfect and sterile (imperfect mesentery also sterile). Mesenteries of second-fourth cycle fertile (except for perfect mesentery in holotype), with oocytes larger than 250 Μm in diameter. Mesenteries of fifth cycle sterile. In smaller paratype, 48 pairs of mesenteries regularly arranged in four cycles: first cycle perfect and sterile, second and third cycles fertile, fourth cycle sterile. No cinclides. Larger mesenteries bear acontia proximally.

Sphincter mesogloeal, alveolar, not very strong ( Fig. 3 A). Longitudinal muscles of tentacles ectodermal. Radial muscles of oral disc meso-ectodermal, and mesogloea much thicker in parts corresponding to stronger endocoels than in exocoels ( Fig. 3 B). Parietobasilar muscles weak ( Fig. 3 C). Longitudinal retractor muscles diffuse, weak ( Fig. 3 C).

Cnidom. Spirocysts, basitrichs, microbasic p -mastigophores ( Fig. 4 ). See Table 1 for size and distribution.

Distribution and Habitat. Paraphelliactis tangi n. sp. has been found only from a seamount near the Yap Trench in the tropical Western Pacific, where the water depth ranged from 1,928 m to 1,980 m and the sediment was foraminiferal ooze. Some other individuals were also observed by ROV nearby, and the holotype attached on a stone and the paratypes embraced sponge spicules with pedal disc.

Tissue Cnida type N n Range, in µm

Tentacle Robust spirocysts (A) 3/3 65 (21.5) 25.5–75.0 × 3.2–9.0

Gracile spirocysts (B) 3/3 45 25.0–49.0 × 3.0–4.0 Basitrichs (C) 3/3 52 26.0–52.0 (56.0) × 2.5–4.0 Basitrichs (D) 2/3 7 16.0–21.0 × 1.5–3.0

Middle column Basitrichs (E) 3/3 40 14.0–24.0 × 2.0–4.0 Basitrichs (F) 3/3 33 9.5–14.0 × 1.5–2.0

Microbasic p -mastigophores (G) 1/3 18 18.2–21.0 ×3.0–4.0

Actinopharynx Basitrichs (H) 3/3 61 25.5–55.0 × 2.9–4.0 Basitrichs (I) 3/3 25 17.5–24.5 × 2.0

Microbasic p -mastigophores (J) 3/3 46 25.0–41.0 (50.0) × 4.0–4.9

Mesenterial filament Basitrichs (K) 3/3 35 35.0–60.0 × 3.0–4.0 (5.0)

Basitrichs (L) 3/3 43 16.0–26.0 × 1.5–3.0 Microbasic p -mastigophores (M) 3/3 24 24.0–37.0 × 3.0–5.5

Acontia Basitrichs (N) 2/2* 65 49.0–62.0 × 3.0–4.1 Basitrichs ( O) 2/2 16 17.0–27.0 × 2.0 Etymology. Named in honor of the late Chinese marine biologist, Prof. Zhican TANG, for his great contribution to the taxonomy of Cnidaria in China.

Remarks. The genus Paraphelliactis was established by Carlgren (1928a) for Pa. spinosa based on its resemblance of appearance and structure with Phelliactis Simon, 1892 . Subsequently a second species, Pa. michaelsarsi Carlgren, 1934 , was described. Carlgren (1942, 1949) distinguished Paraphelliactis from Phelliactis mainly by the different arrangement of the radial muscles of the oral disc and the (probably) greater number of mesenteries at the margin than at the limbus. Riemann-Zürneck (1973) suggested the radial muscles of the oral disc used to separate Paraphelliactis and Phelliactis were variable and thus synonymized Paraphelliactis with Phelliactis . Later, Dunn (1982) described the third species of Paraphelliactis , Pa. pabista . Sanamyan & Sanamyan (2007) regarded Paraphelliactis as a valid genus after discussing the relation of the number of mesenteries at the margin to that at the limbus in Paraphelliactis and Phelliactis , where Ph. hertwigi Simon, 1892 (the type species) and many other species of Phelliactis have fewer mesenteries at the margin than at the limbus, while species of Paraphelliactis have more mesenteries at the margin than at the limbus. Molodtsova et al. (2008) reported a specimen, which has almost the equal number of mesenteries at the margin and at the limbus, and identified it as Phelliactis michaelsarsi ( Carlgren, 1934) .

The new species Paraphelliactis tangi n. sp. has two unusual features: a complete fifth cycle of mesenteries and an equal number of mesenteries throughout the column. The former feature is absent in all three known species of Paraphelliactis , but exists in three of 20 known species of Phelliactis ( Ph. americana Widersten, 1976 , Ph. callicyclus Riemann-Zürneck, 1973 and Ph. lophohelia Riemann-Zürneck, 1973 ). The latter feature probably exists also in the three known species of Phelliactis and in the specimen identified by Molodtsova et al. (2008) as Ph. michaelsarsi . In addition, Pa. tangi n. sp. has another feature not observed in other Paraphelliactis or Phelliactis , viz., tentacles without mesogloeal thickenings on the aboral side. Nonetheless, the new species is covered with a thick cuticle, a typical feature of the genus Paraphelliactis , while in the species of Phelliactis the cuticle is usually thin and is easily stripped off. Thus, we assigned the new species to the genus Paraphelliactis rather than Phelliactis . We extend the generic diagnosis of Paraphelliactis as follows to include the new species by stating that the number of tentacles can be equal to that of mesenteries at the limbus and the fifth cycle of mesenteries may be complete.

Improved diagnosis of Paraphelliactis Carlgren, 1928 (adapted from Carlgren 1949). Hormathiidae with well developed pedal disc. Column divisible into scapus and scapulus, the former strongly tuberculated and provided with a thick cuticle. Sphincter mesogloeal, alveolar. Tentacles arranged in more than five cycles, more than or almost equal to mesenteries at the limbus, with or without mesogloeal thickenings on the aboral side. Longitudinal muscles of tentacles ectodermal, radial muscles of oral disc ectodermal or more or less mesogloeal. Two well developed siphonoglyphs. Mesenteries hexamerously arranged in five cycles, usually six pairs perfect and sterile, and the last cycle incomplete or complete. Retractors of mesenteries diffuse and weak. Parietobasilar muscles weak. Acontia well developed. No cinclides. Cnidom: Robust and gracile spirocysts, basitrichs and microbasic p -mastigophores.

Paraphelliactis tangi n. sp. differs from Pa. spinosa Carlgren, 1928 , type of the genus, in the presence of the large basitrichs of its mesenterial filaments (vs. absent) and larger basitrichs of acontia (length 49–62 Μm vs. 33.8– 42 Μm) ( Carlgren 1942). It differs from Pa. michaelsarsi Carlgren, 1934 in its smaller body size (maximum TABLE 2. Comparison of Paraphelliactis tangi n. sp. with known species of Paraphelliactis Carlgren, 1928 . −, Data not available.

Characteristics Pa. tangi n. sp. Pa. spinosa Carlgren, 1928 Pa. michaelsarsi Carlgren, 1934 Pa. pabista Dunn, 1982

Size (mm) Up to 70 high and 74 wide Up to 50 high and wide Up to 140 high, over 100 wide Up to 80 high and 73 wide

Body shape Sub-cylindrical, pedal disc − Cup shaped, pedal disc narrower than Cup shaped, pedal disc narrower than

wider than limbus column column

Scapulus Distinct Distinct Indistinct Distinct

Column Solid Solid Spongy Solid

mesogloea

Tubercles Conical; irregularly Acuminated; irregularly arranged Columnar, some with pointed tips; Conical; regularly arranged in 24

arranged irregularly arranged rows

Sphincter Moderately strong Strong Weak Weak and short

Tentacles Near 192; unthickened Up to 168; thickened aborally Up to 152; thickened aborally Maximun>150; thickened aborally

aborally

Mesenteries Same number distally and More distally than proximally More distally than proximally, one More distally than proximally

proximally specimen about equal number

Fifth cycle Complete Incomplete Incomplete Incomplete

mesenteries

Distribution W Pacific N Atlantic N Atlantic NE Pacific

References Present study Carlgren 1928a, 1942, 1949; Dunn Carlgren 1949; Dunn 1982; Dunn 1982; Sanamyan & Sanamyan 1982; Sanamyan & Sanamyan 2007 Riemann-Zürneck 1986; Sanamyan & 2007; Eash-Loucks& & Fautin 2012 Sanamyan 2007; Molodtsova et al. 2008

height 70 mm and width 74 mm vs. height up to 140 mm and width over 100 mm), the sub-cylindrical body shape (vs. cup shaped with pedal disc narrower than column), the solid mesogloea of column (vs. spongy), distinct scapulus (vs. indistinct), and stronger sphincter. Paraphelliactis tangi n. sp. differs from Pa. pabista Dunn, 1982 by the sub-cylindrical body shape (vs. cup shaped with pedal disc narrower than column), the irregularly arranged tubercles (vs. regularly arranged), the stronger sphincter, and the larger basitrichs of mesenterial filaments (length 35–60 Μm vs. 23–34 Μm) (Table 2, and references therein). So far, Pa. tangi n. sp. is the fourth species of the genus and the first known in the Western Pacific.

Figures

FIGURE 1. Distribution of Paraphelliactis tangi n. sp. and Phelliactis yapensis n. sp. as well as their congeners in the Western Pacific Ocean and Indian Ocean.

FIGURE 2. External morphology of Paraphelliactis tangi n. sp. A, B. The holotype in situ and in preservation. C. The larger paratype in preservation. Scale bars = 40 mm.

FIGURE 3. Internal anatomy of Paraphelliactis tangi n. sp. A. Longitudinal section through the margin of the holotype, showing marginal sphincter muscle (S). B. Transverse section through the oral disc of the holotype, note much thicker mesogloea over an endocoel (arrow). C. Cross sections of mesenteries through the actinopharynx of the larger paratype, showing weak parietobasilar muscle (P), diffuse retractors (R) and oocytes (O). Numbers indicate the cycles of mesenteries. Scale bars = 0.5 mm (A, B) and 1 mm (C).

FIGURE 4. Cnidae from Paraphelliactis tangi n. sp. Tentacles: A. Robust spirocyst. B. Gracile spirocyst. C. Basitrich. D. Basitrich. Column: E. Basitrich. F. Basitrich. G. Microbasic p - mastigophore. Actinopharynx: H. Basitrich. I. Basitrich. J. Microbasic p - mastigophore. Filament: K. Basitrich. L. Basitrich. M. Microbasic p - mastigophore. Acontia: N. Basitrich. O. Basitrich. Scale bar = 20 µm.