Chelotropella alexrogersi Payne, Samaai & Gibbons 2025, sp. nov.

Chelotropella alexrogersi Payne, Samaai & Gibbons sp. nov. urn:lsid:zoobank.org:act: D058612D-BC8B-4BD0-B3B5-265BBFDA0953 Figure 3A–K Material examined. Holotype. SAMC-A096892 (cross-reference TS 2310 & WSL-INV50(1)): Walters Shoal Seamount, Grid WSL022, Station ALG10954, coll. RV Algoa (33°10.9’ S; 43°48.6’ E) - (33°11.2’ S; 43°50.2’ E), 72–170 m depth, 29 May 2014. Type locality. Walters Shoal Seamount, south of Madagascar on the Madagascar Ridge, Western Indian Ocean (Fig. 1). Description (Fig. 3A). Spherical form. Length 1.9 cm, diameter 1.5 cm and thickness 1.7 cm. Surface microhispid and prickly to the touch. One oscule (~ 3 mm) present at the top of the specimen. Thin cortex (~ 1 mm) present, separable from the choanosome. Texture firm and dense, not compressible. Colour in life dull dark brown externally, paler brown internally. In preservative, colour light brown. Slightly sticky exudate. Skeleton (Fig. 3B, C, D). Choanosomal skeleton comprises thick, radial tracts of oxeas and triaenes (~200– 400 µm across), forming two subdermal layers in the peripheral region.Calthrops arranged in a somewhat disorganized fashion, occasionally congregating in horizontal formations. Subectosomal skeleton comprises large subdermal cavities, triaenes orientated radially, with cladomes forming two layers parallel to the surface. Strongyloacanthasters concentrated in the ectosome (~300–500 µm), and scattered throughout the peripheral region, including around subdermal spaces. Spiculation. Megascleres. Oxeas, smooth, straight to slightly curved, distally fusiform: 2134 (1092–3016) × 20 (11–31) µm, n = 20 (Fig. 3E). Dichotriaenes (Fig. 3F, G), in two size classes: I) rhabdome 2812 (2615–3078) × 51 (49–52) µm, cladome 501 (361–573) µm, stout protoclads 135 (111–154) µm, long deuteroclads terminating in somewhat blunt points 169 (89–222) µm, n = 3; II) rhabdome 1169 (786–1520) × 20 (24–34) µm, cladome 291 (254–334) µm, stout protoclads 118 (103–139) µm, short deuteroclads terminating in somewhat sharp points 40 (21–50) µm, n = 20. Anatriaenes (Fig. 3H): 1153 (807–1458) × 9 (8–11) µm, with cladome 74 (53–89) µm, n = 20. Plagiotriaenes (Fig. 3I), short-shafted, rare: 302 (111–694) × 16 (10–22) µm, with cladome 145 (63–260) µm, and cladi 77 (35–136) µm, n = 12. Calthrops (Fig. 3J, K), regular in shape, found in two size classes (ray): I) 480 (404– 595) × 52 (43–61) µm, n = 20; II) 197 (134–259) × 24 (15–34) µm, n = 20. Microscleres. Strongyloacanthasters (Fig. 3L) with ~10 terminally hook-spined rays: 19 (14–23) µm, n = 20. Substratum, depth range and ecology. One specimen found in a single sled on rocky substrate, which was host to many bivalves and sponges. Depth range: 72– 170 m. Etymology. The name ‘alexrogersi’ was given in honour of Prof.Alex D. Rogers, for his pioneering expeditions all over the world as part of the Ocean Census programme to discover new species. Remarks. The present material conforms to Chelotropella Lendenfeld, 1907 as diagnosed by the presence of calthrops, oxeas and peripheral dichotriaenes that form a radial skeleton, and two subdermal layers in the peripheral region (Van Soest & Hooper 2002). Erected by Lendenfeld in 1907 for a single species, this genus comprises two described species to date: Chelotropella sphaerica Lendenfeld, 1907 and Chelotropella neocaledonica Lévi & Lévi, 1983, of which only the former occurs in the Indian Ocean, with the latter found in New Caledonia. Although similar to C. sphaerica with regards to morphology (spherical sponge of ~ 1.8 cm with granular surface as described by Lendenfeld in 1907), the spicule component of the present material differs. The material in this study has megascleres that are smaller and narrower than those described by Lendenfeld (1907) (oxeas: 3600–5600 × 50–80 µm; dichotriaenes: rhabdome 2800–4400 × 100–440 µm, cladome 650–1300 µm, clades 130–170 µm; calthrops: I) 700–1050 × 85–120 µm, II) 170–700 × 20–85 µm), with his species also lacking anatriaenes and plagiotriaenes (although intermediate forms between calthrops and triaenes are noted), but including the presence of various euaster morphologies. Pulitzer-Finali (1993) recorded C. sphaerica from Kenya, also with various euaster morphologies, but with megasclere size ranges more in accordance with the present material (oxeas: 3500–4500 × 27–45 µm; dichotriaenes: rhabdome 2400 × 80 µm, cladome 1600 µm, protoclads 270 µm, deuteroclads 500 µm; calthrops: 300–760 µm). Pulitzer-Finali (1993) also found long anatriaenes (rhabdome 4000 × 20–36 µm, cladome 150–170 µm), which led Van Soest & Hooper (2002) to suggest that Pulitzer-Finali’s specimen may be a new species distinct from C. sphaerica. Thus, due to the presence of anatriaenes (in a much smaller size range than recorded by Pulitzer-Finali in 1993) and plagiotriaenes, as well as the lack of diverse euaster morphologies (only one type found), the present material constitutes a new species.