Phelliactis yapensis Li & Xu, 2016, n. sp.

Phelliactis yapensis n. sp.

(Figs. 1, 5–7; Tables 3, 4) Material examined. Holotype: Y 30039, grasped sponge spicules, collected on 15 December 2014 from FX-Dive 16 (137°44.84′E, 8°51.90′N), 855 m, foraminiferal ooze bottom. Paratypes: Y 30040, one specimen, collected together with the holotype and attached to sponges; Y 30064, one specimen, attached to sponges, collected on 18 December 2014 from FX-Dive 18 (137°48.00′E, 8°53.98′N), 879 m, foraminiferal ooze associated with manganese nodules.

Body and Size. Individuals usually grasping or attached to sponges (Fig. 5 A, C, D). In preservation, column sub-cylindrical, height 27–104 mm (104 mm in holotype), greatest diameter of pedal disc 18–98 mm (98 mm in holotype), greatest diameter of column 22–78 mm (78 mm in holotype), greatest diameter of oral disc 11–50 mm (50 mm in holotype). Column divisible into scapus and scapulus. Scapus with thin layer of cuticle, and irregularly arranged tubercles in distal column; tubercles hemispherical, diameter of base to 6 mm. Scapulus without cuticle, but with tubercles smaller than those of scapus; tubercles of varying shapes and sizes, irregularly arranged (Fig. 5 B). Ectoderm of column very thin, mostly stripped off. Mesogloea thick, about 1.5 mm in distal column between tubercles, and about 2.5 mm in margin and proximal column of holotype.

Oral disc and Tentacles. Oral disc red, bilobed and asymmetric in holotype, larger part possesses 90 tentacles and 46 pairs of mesenteries, smaller part possesses 75 tentacles and 37 pairs of mesenteries (Fig. 5 B–D). Mouth ovoid, elevated in center of oral disc, same color as oral disc. Actinopharynx well developed, length to 33 mm, occupying near 1/3 of column length. Tentacles marginal, smooth, tapered, with mesogloeal thickenings on aboral side (Fig. 6 A); length to 10 mm, diameter of base to 6 mm in preservation. Tentacles alternately arranged in two cycles, with inner and outer ones subequal. Number 165 in holotype (six of them cut off with margin for histological sections), 82 in paratype Y 30040, and 91 in Y 30064.

Internal Anatomy. Two elongate, symmetrical siphonoglyphs; each attached to pair of directive mesenteries. Mesenteries not divisible into macrocnemes and microcnemes, arranged in five cycles (Fig. 6 D). Mesenteries more numerous at limbus than margin: holotype has 176 mesenteries at limbus, 168 at middle column, and 166 at margin; paratype Y 30040 has 128 mesenteries at limbus and 114 at margin; paratype Y 30064 has 106 mesenteries at limbus, and 96 at margin. In holotype, mesenteries of first cycle perfect and sterile; those of second cycle rarely with gametogenic tissue, and two pairs of them perfect and sterile; those of third and fourth cycles fertile, with oocytes larger than 180 Μm in diameter; those of fifth cycle incomplete, some fertile, and one excrescent pair (Fig. 6 D). Mesentery arrangement undeterminable in paratypes as their damage in course of dissection and counting number of mesenteries. Longitudinal retractor muscles diffuse, weak (Fig. 6 C). No cinclides. Acontia well developed, not coiled, usually one acontium arises from each larger mesentery proximally.

Sphincter mesogloeal, alveolar, and moderately strong (Fig. 6 B). Longitudinal muscles of tentacles and radial muscles of oral disc ectodermal (Fig. 6 A, E). Radial muscles of oral disc weaker and mesoglea thicker over endocoels than over exocoels (Fig. 6 E). Parietobasilar muscles weak (Fig. 6 C).

Cnidom. Spirocysts, large basitrichs, small basitrichs, microbasic p -mastigophores (Fig. 7). See Table 3 for distribution and size.

Distribution and Habitat. Phelliactis yapensis n. sp. has been found only from a seamount near the Yap Trench in the tropical Western Pacific, where the water depth ranged from 855 m to 879 m and the sediment was foraminiferal ooze sometimes associated with manganese nodules. The holotype grasped sponge spicules with pedal disc and the paratypes attached to sponges.

Etymology. Named after the seamount/location (near the Yap Trench) where the species was discovered.

Remarks. Phelliactis yapensis n. sp. matches well with the definition of Phelliactis Simon, 1892 in Carlgren (1949). It differs from all congeners by the combination of body size, column structure, the arrangement of mesenteries and tentacles, and the size of cnidae (Table 4). Phelliactis yapensis n. sp. possesses characteristic, very large basitrichs of mesenterial filaments (Fig. 7 J; Table 3), a basitrich type not observed in the known species of Phelliactis. Among the 20 Phelliactis species recognized by Fautin (2013), only three were described by Wassilieff (1908) from the eastern sea area of Japan in the Western Pacific: Ph. crassa, Ph. japonica, and Ph. magna (Table 4). Phelliactis yapensis n. sp. is the first species found from the tropical Western Pacific Ocean (Fig. 1). It differs from the three known species by the distinct scapulus above the scapus (vs. no clear differentiation between the scapus and capitulum) (Stephenson 1918, 1920). In addition, Ph. yapensis n. sp. differs from Ph. crassa by its larger body size (up to 104 mm high and 98 mm wide vs. 50 mm high and 25 mm wide); from Ph. japonica by the stronger sphincter (vs. weak); and from Ph. magna by the strongly asymmetric oral disc (vs. slightly asymmetric), stronger sphincter (vs. weak), fewer tentacles (maximum 165 in 2 cycles vs. 192 in 6 cycles), and the higher number of perfect mesenteries (8 pairs vs. 6 pairs and 2 unpaired).

According to Riemann-Zürneck (1973), the species of Phelliactis can be classified into three groups. Phelliactis yapensis n. sp. has six pairs of perfect mesenteries in the first cycle and two additional pairs in the second cycle, and thus belongs to the Phelliactis hertwigi group - together with its ten congeners Ph. algoaensis Carlgren, 1928; Ph. capensis Carlgren, 1938; Ph. capricornis Riemann-Zürneck, 1973; Ph. coccinea (Stephenson, 1918); Ph. hertwigii Simon, 1892; Ph. incerta Carlgren, 1934; Ph. magna (Wassilieff, 1908); Ph. pelophila Riemann-Zürneck, 1973; Ph. pulchra (Stephenson, 1918); and Ph. siberutiensis Carlgren, 1928 (Table 4). By contrast, the seven species of Phelliactis that have only six pairs of perfect mesenteries belong to the Phelliactis robusta group: Ph. callicyclus Riemann-Zürneck, 1973; Ph. carlgreni Doumenc, 1975; Ph. crassa (Wassilieff, 1908); Ph. hydrothermala Sanamyan & Sanamyan, 2007; Ph. japonica (Wassilieff, 1908); Ph. robusta Carlgren, 1928; and Ph. somaliensis Carlgren, 1928. The remaining three species have 12 pairs of perfect mesenteries: Ph. americana Widersten, 1976; Ph. gigantea (Carlgren, 1941); and Ph. lophohelia Riemann-Zürneck, 1973 (Table 4; and references therein). Our holotype has also an excrescent pair of mesenteries in the fifth mesentery cycle. Such a pattern is likely a minor irregularity in the arrangement of mesenteries. Similar case was also observed in Ph. americana Widersten, 1976.

TABLE 4. Cοmparisοn οf Phelliactis yapensis n. sp. with knοwn species οf Phelliactis Simοn, 1892. −, Data nοt available.