Synhalcurias kahakui sp. nov.

(New Japanese name: kobito-seitaka-kawari-ginchaku)

Figs. 4, 5; Table 1

Material examined. Holotype. NSMT-Co 1695: dissected specimen, July 11, 2016, off northwestern Otouto-jima Island, Ogasawara Islands (27°13.13ʹN, 142°5.43ʹE), at 157 m in depth, biological dredge, coll. T. Izumi . Paratypes. NSMT-Co 1696: dissected specimen (damaged on its surface), collected May 23, 2019, west off Amami Island (28°22.42ʹN, 129°15.14ʹE), at 315 m in depth, using beam trawl, during research cruise of RV Toyoshio-Maru, coll. I. Kobayashi. CMNH-ZG 09759: a dissected specimen and histological slides; same date, locality, collector, and method as NSMT-Co 1696. CMNH-ZG 09760: a whole specimen preserved in 99 % ethanol; same date, locality, collector, and method as NSMT-Co 1696 .

......continued on the next page

Description. External anatomy. Body cylindrical (Fig. 5A, C), up to ca. 50 mm in height and ca. 30 mm in width in preserved specimens; 40 mm in height and 27 mm in height and 15 mm in width in preserved holotype. Column surface smooth, without tenaculi or nematocyst batteries, pale whitish orange (or translucent) ectoderm layer and transversal discontinuous yellow line. Column has spirocysts and very sparsely distributed nematocysts. Surface of column easily peeled off. Distal column slightly expanded, simple and not developed into lobes (Fig. 5B). Tentacles simple, all marginal, 8–16 mm length, tapering at tip, no thickenings including at aboral base, pale white to pale orange in color when alive (Fig. 5A–B); 68 in number on oral disc; inner and outer tentacles alternatively bared. Tentacles less contractile and less adhesive. Aboral end developed as pedal disc, opaque, with mesenterial insertions invisible. Oral disc pale white, diameter almost same as column. Mouth at center of oral disc, highly swollen, lip-like, with radial grooves on surface, pale orange, with fluorescent yellow patch on dorsal and aboral sides when alive (Fig. 5B).

Internal anatomy. All mesenteries perfect. Sixty-eight mesenteries (Fig. 5H) at actinopharynx level: 12, including four directives, in first cycle; eight in second cycle; 16 in third cycle; 32 in fourth cycle. Mesenteries of second and younger cycles in endcoels of mesenteries of first cycle. All mesenteries perfect distally, and each tentacle between either exo- or endocoelic. Tentacular longitudinal and circular muscles too weak to observe in historical section (Fig. 5D–G); retractor muscles comparatively weak, restricted near body wall or diffused and integrated to parietal muscle (Fig. 5I). Muscle processes very short, simple, 40–100 in each muscle pennon. Parietal muscles of mesenteries weak, indistinct (Fig. 5I). Mesoglea thickest in body wall and actinopharynx (Fig. 5H), up to 3 mm, far thicker than ectoderm and endoderm; mesoglea thinner in tentacles than in other body parts, but far thicker than endo- and ectoderm (Fig. 5D–G); same in mesenteries (Fig. 5I). Two siphonoglyphs connected to actinopharynx on dorsal and ventral sides (Fig. 5H), with 12 additional longitudinal grooves shallower than syphonoglyphs. Marginal sphincter muscle absent. Basilar muscle absent. Possibly gonochoric, with immature oocytes in holotype. Mesenteries of first to third cycles fertile.

Cnidom. basitrichs, spirocysts, microbasic b -mastigophores, and microbasic p -mastigophores. See Table 1 and Fig. 4M–Y for size and distribution.

Etymology. This species name is derived from the Japanese abbreviated name “Kahaku” of the National Museum of Nature and Science, Tokyo. The survey during which this species was collected was part of a project organized by this museum. Derivation of Japanese name: “kobito” means small, as S. kahakui sp. nov. is far smaller than S. elegans .

Distribution. Off Ogasawara Islands, Tokyo Met., and Amami Island, Kagoshima Pref. Both localities are around remote islands south of the mainland of Japan (Fig. 1).

Remarks. This species is remarkably similar to S. elegans and clearly belongs to Synhalcurias . However, there are several differences between S. kahakui sp. nov. and S. elegans: the sizes of bodies ( S. kahakui shorter than 50 mm when alive), even when at sexual maturity; the number of perfect mesenteries ( S. kahakui has 68 (first to fourth cycle of mesenteries), whereas S. elegans has at least 76 (but up to 108); the column of S. kahakui has no nematocyst batteries.