Vitrumanthus flosculus Kise & Reimer 2024, sp. nov.

Vitrumanthus flosculus Kise & Reimer sp. nov. Figs 2, 3, 4, 5 Material examined. Holotype • NSMT -Co 1898, Shoho Seamount, Nishi-Shichito Ridge, Japan (32°19.73'N, 138°44.28'E), 400 m depth, November 29, 2020. Type locality. Shoho Seamount, Nishi-Shichito Ridge, Japan. Etymology. “ flosculus ” meaning “ small flower ” or “ floweret ” in Latin. Description. External morphology. Colonial macrocnemic zoantharians associated with host hexasterophoran sponge Farrea Bowerbank, 1862 (Fig. 2 A). Solitary or colonial polyps rise irregularly from all over the three-dimensional structure of host hexasterophoran sponge with base of polyps embedded in tissue of sponge (Fig. 3 A). Preserved specimens consist of cylindrical polyps (Fig. 3 B, C), dark brown in coloration. The living polyps and tentacles transparent yellowish in coloration. Surface of column smooth and ectoderm continuous (Fig. 3 C). No encrustations of sand and silica particles in ectoderm of capitulum but ectoderm of scapus encrusted with small-sized sand and silica particles. Contracted preserved polyps 1.5–2.5 mm in height, 1.0– 2.5 mm in diameter. Capitulary ridges indiscernible when contracted. Tentacles 22–26 in number. Internal morphology. Zooxanthellae absent. Cyclically transitional marginal musculature (Fig. 4 A – C). Encircling sinus or mesogleal canal present and basal canals of mesenteries absent (Fig. 4 E). Mesenteries 22–26 in number, in brachycnemic arrangement (Fig. 4 D). Mesoglea thickness 20–30 µm. Siphonoglyph distinct and V-shaped. Mesenterial filaments present. Complete mesenteries fertile (Fig. 4 E). Cnidae. Basitrichs and microbasic b-mastigophores, microbasic p-mastigophores, holotrichs, special b-mastigophores, and spirocysts (see Fig. 5, Table 1 for sizes and distributions). Habitat and distribution. Northwestern Pacific Ocean: known from the Shoho Seamount, Nishi-Shichito Ridge, Japan at a depth of 400 m. The new species was found on a glass sponge, Farrea sp., attached to rocks on the summit of the Shoho Seamount. Associated host. Farrea sp. Remarks. Regarding host sponges, Vitrumanthus flosculus sp. nov. is associated with Farrea, while other Vitrumanthus species are associated with other, different host sponges (Vitrumanthus schrieri: Verrucocoeloidea, Parahigginsia and Cyrtaulon, Vitrumanthus vanderlandi: Aphrocallistes, and Vitrumanthus oligomyarius: Tretochone). Vitrumanthus flosculus sp. nov. has holotrich nematocysts in all tissues we examined, while V. vanderlandi does not have holotrichs in any tissues. The surface of the column is smooth in Vitrumanthus flosculus sp. nov. with no encrustation of sand and silica particles in the ectoderm of capitulum, while the surface of the column is rough in V. schrieri with encrustation in the ectoderm of capitulum. The mesenteric arrangement of both Vitrumanthus flosculus sp. nov. and V. oligomyarius is brachycnemic, an exceptional characteristic for species within the suborder Macrocnemina. However, these two species can be distinguished by their numbers of tentacles and the sizes of the polyps; Vitrumanthus flosculus sp. nov. has 22–26 tentacles, while V. oligomyarius has 32–36 tentacles. Vitrumanthus flosculus sp. nov. has relatively smaller polyps than those of V. oligomyarius (1.5–2.5 mm in height and 1.0– 2.5 mm in diameter vs. 0.5–3.1 mm in height and 1.2–3.4 mm in diameter). Furthermore, the host sponges of Vitrumanthus flosculus sp. nov. and V. oligomyarius are different (Farrea vs. Tretochone). Parachurabana is a monotypic genus, and the diagnostic feature of this genus is described as having an association with Farreidae sponges. Although Vitrumanthus flosculus sp. nov. is associated with Farrea sp., Vitrumanthus flosculus sp. nov. can be easily distinguished from Parachurbana by the different shape of its sphincter muscle (cyclically transitional vs. cteniform endodermal marginal musculature) and different mesenterial arrangement (brachycnemic vs macrocnemic arrangement). The diagnosis of Parachurabana may need to be updated based on examinations of additional specimens.