Aaptos incrustans Payne, Samaai & Gibbons sp. nov.

urn:lsid:zoobank.org:act: A9182140-153C-4386-985E-59C2CF13AA18

Figure 9A–H, Table 7

Material examined. Holotype. SAMC-A096926 (cross-reference TS2502 & WSL-INV84(31)): 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 . Paratype. SAMC-A096927 (cross-reference TS2503 & WSL-INV84(32)): 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 .

Type locality. Walters Shoal Seamount, south of Madagascar on the Madagascar Ridge, Western Indian Ocean (Fig. 1).

Description (Fig. 9A). Thickly encrusting form. Length 1.9 cm, diameter 1.7 cm and thickness 0.7 cm. A dense array of spicules at the surface (~ 1 mm), arranged in a confused fashion rendering the surface prickly to the touch. No visible oscules. Texture dense and firm, barely compressible specimens tear so-so. Colour in life dull black externally, almost appearing grey due to visible spicules at the surface. Internal colour in life beige. Colour in preservative dull brown externally, internally grey-beige.

Skeleton (Fig. 9B, C, D). Choanosomal skeleton comprises dense tracts of megascleres (~230–290 µm wide) that arise from the base and radiate vertically through the choanosome, fanning out and forming brushes into the ectosome.These brushes form a dense palisade at the surface, with smaller spicules intermingled (often perpendicular to surface) between the larger spicules. Subectosomal region consists of a layer of densely packed, tangentially orientated megascleres. Ectosome consists of small styles and larger intermediate styles, which form palisades of vertically arranged brushes. The distal ends of these megascleres protrude through the sponge surface.

Spiculation (Table 7). Megascleres. Strongyloxeas, smooth, straight to slightly bent, thickest centrally with reduced apices, distally fusiform: 988 (678–1285) × 16 (8–20) µm, n = 20 (Fig. 9E). Styles, smooth, straight to slightly bent, often thickest centrally in largest size class, distally fusiform, in three size classes: I) 925 (674–1282) × 27 (24–32) µm, n = 20 (Fig. 9F); II) 444 (348–576) × 13 (8–20) µm, n = 20 (Fig. 9G); III) 197 (128–291) × 5 (3–7) µm, n = 20 (Fig. 9H). Microscleres. Absent.

Substratum, depth range and ecology. Two specimens found in one sled on rocky substrata. Depth range: 28– 34 m.

Etymology. This species is named after the encrusting habit.

Remarks. The present material has a radiate skeleton of strongyloxeas, with a dense ectosomal palisade and is thus placed in the genus Aaptos Gray, 1867 (Van Soest 2002). Aaptos incrustans Payne, Samaai & Gibbons sp. nov. is distinct from the two species of Aaptos that have been recorded from South Africa: Aaptos alphiensis Samaai & Gibbons, 2005 and Aaptos nuda (Kirkpatrick, 1903) . Aaptos alphiensis was described by Samaai & Gibbons (2005) from the west coast of South Africa, as having both primary and intermediate subtylostyles, intermediate styles and dermal tylostyles, while Kirkpatrick (1903) notes the presence of only oxeas in A. nuda . The comparative spicule measurements are in Table 7.

There have been several records of Aaptos aaptos (Schmidt, 1864), which has both strongyloxeas and styles, in the Indian Ocean. However, the World Porifera Database (de Voogd et al. 2024) suggests that these records are inaccurate due to the type location being in the Adriatic Sea.