Callyspongia (Callyspongia) cacumen Payne, Samaai & Gibbons 2025, sp. nov.

Callyspongia (Callyspongia) cacumen Payne, Samaai & Gibbons sp. nov. urn:lsid:zoobank.org:act: CE128D4B-8436-4C06-A2BC-BC29283866A2 Figure 13A–E, Table 12 Material examined. Holotype. SAMC-A096914 (cross-reference TS 2369 & WSL-INV75(10)): Walters Shoal Seamount, coll. RV Algoa, 29 m depth, 30 May 2014. Paratypes. SAMC-A096897 (cross-reference TS 2330 & WSL-INV94(1)), SAMC-A096903 (cross-reference TS 2341 & WSL-INV94(13)), SAMC-A096906 (cross-reference TS 2353 & WSL-INV94(25)): Walters Shoal Seamount, Grid WSL044, Station ALG10976, coll. RV Algoa, (33°14.0’ S; 43°55.5’ E) - (33°13.7’ S; 43°55.6’ E), 25–28 m depth, 02 June 2014. Additional material. TS 2370 (WSL-INV75 (11)), TS 2371 (WSL-INV75 (12)): Walters Shoal Seamount, coll. RV Algoa, 29 m depth, 30 May 2014. TS 2382 (WSL-INV83 (2)): Walters Shoal Seamount, coll. RV Algoa, 29 m depth, 30 May 2014. TS 2479 (WSL-INV84 (8)): Walters Shoal Seamount, Grid WSL042, Station ALG10974, coll. RV Algoa, (33°11.2’ S; 43°51.0’ E) - (33°11.2’ S; 43°50.7’ E), 28–34 m depth, 02 June 2014. TS 2537 (WSL-INV102 (3)): Walters Shoal Seamount, Grid WSL045, Station ALG10977, coll. RV Algoa, (33°13.8’ S; 43°56.1’ E) - (33°14.2’ S; 43°55.9’ E), 80 m depth, 02 June 2014. Type locality. Walters Shoal Seamount, south of Madagascar on the Madagascar Ridge, Western Indian Ocean (Fig. 1). Description (Fig. 13A, B). Massive, predominantly ramose but tubular, growing from a common base, from which upright clusters of finger-like projections arise, interconnected laterally. Tubes usually coalesced for a greater or lesser distance, occasionally united along entire length. Length 6.0 cm, width 9.1 cm and thickness 1.4 cm. Tube diameter 3 cm. Surface smooth and velvety to the touch. Oscules (3–9 mm diameter) present at the apex of the tubes, which become fibrous at the tips. Transparent membrane covering exterior. Texture soft and spongy, compressible and easily torn. Colour in life bright blue, turning beige with purple tips above water. In preservative, pale yellow. Skeleton (Fig. 13C, D). Choanosome with a regularly rectangular-meshed skeleton formed by multispicular primary spongin fibres ~30 µm wide, and ~71–430 µm apart, interconnected often perpendicularly by secondary unispicular fibres ~20 µm thick, forming meshes ~110–250 µm wide. Unispicular tertiary fibres sometimes present, ~25 µm thick, forming meshes ~80 µm thick, which interconnect secondary fibres perpendicularly. Specialised ectosomal skeleton absent. Spiculation (Fig. 13E; Table 12). Megascleres. Oxeas, short, smooth, straight to slightly curved medially, hastate: 64 (57–69) × 3 (2–4) µm, = 20. Microscleres. Absent. Substratum, depth range and ecology. Nine specimens found on rocky substrata in three sleds and both dives, often with crinoids as epifauna. Depth range: 25– 80 m. Etymology. The species name cacumen is derived from the Latin noun "cacumen," which is gender-neutral and means "summit." This name was chosen to reflect the species' occurrence on the summit or crown area of the shallow regions of Walters Shoal. Remarks. The present material conforms to the genus Callyspongia (Callyspongia) Duchassaing & Michelotti, 1864 as diagnosed by a single ectosomal non-hispid layer, multispicular well-defined choanosomal fibres with a distinct spongin sheath, forming a rectangular mesh without free spicules, and a smooth surface (Desqueyroux-Faúndez & Valentine 2002). The present material is not conspecific with the three species of this subgenus that have been recorded from the region of interest (de Voogd et al. 2024): Callyspongia (Callyspongia) differentiata (Dendy, 1922), Callyspongia (Callyspongia) reticutis (Dendy, 1905) and Callyspongia (Callyspongia) tubulosa (Linnaeus, 1759) sensu (Esper, 1797), the latter of which was re-described by Samaai & Gibbons in 2005. Both C. (C.) differentiata and C. (C.) reticutis have slightly larger spicule sizes (80 × 3 µm and 72 × 2.6 µm, respectively) but also differ from the present material with regards to skeletal structure, with the former having secondary fibres devoid of spicules and the latter having multispicular secondary fibres. Callyspongia (C.) tubulosa is most similar morphologically to the present material, but has larger oxeas (110–140 × 13 µm) with multispicular secondary fibres (Table 12).