Halichondria (Halichondria) madagascarensis Payne, Samaai & Gibbons, 2025

Payne, Robyn P., Samaai, Toufiek, Janson, Liesl, Kerwath, Sven E. & Gibbons, Mark J., 2025, Eleven new heteroscleromorph Demospongiae (Porifera), and a new record of the tetractinellid Ancorina corticata, from Walters Shoal, a shallow seamount on the Madagascar Ridge in the South West Indian Ocean (SWIO), Zootaxa 5575 (1), pp. 1-56 : 23-26

Halichondria (Halichondria) madagascarensis Payne, Samaai & Gibbons sp. nov.

urn:lsid:zoobank.org:act:131665ED-C41B-49A2-993E-7CCC06378A01

Figure 8A–F , Table 6

Material examined. Holotype. SAMC-A096901 (cross-reference TS 2339 & WSL-INV94(11)): 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 . Paratypes. SAMC-A096898 (cross-reference TS 2336 & WSL-INV94(7)), SAMC- A096900 (cross-reference TS 2338 & WSL-INV94(10)) , SAMC-A096902 (cross-reference TS 2340 & WSL-INV94(12)): 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 2350 ( WSL-INV94 (22)): 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. TS 2373 ( WSL-INV75 (14)), TS 2374 ( WSL-INV75 (15)), TS 2375 ( WSLINV75 (16)), TS 2377 ( WSL-INV75 (18)), TS 2378 ( WSL-INV75 (19)), TS 2379 ( WSLINV75 (20)), TS 2380 ( WSL-INV75 (21)): Walters Shoal Seamount, coll. RV Algoa , (33°11.2' S; 43°50.7' E), 29 m depth, 30 May 2014. TS 2381 ( WSL-INV83 (1)), TS 2383 ( WSL-INV83 (3)), TS 2384 ( WSL-INV83 (4)), TS 2385 ( WSL-INV83 (5)), TS 2387 ( WSL-INV83 (7)): Walters Shoal Seamount, coll. RV Algoa , (33°10.6' S; 43°51.0' E), 29 m depth, 30 May 2014. TS 2390 ( WSL-INV92 (1)), TS 2391 ( WSL-INV92 (2)), TS 2392 ( WSLINV92 (3)), TS 2393 ( WSL-INV92 (4)): Walters Shoal Seamount , Grid WSL043, Station ALG10975, coll. RV Algoa , (33°13.8’ S; 43°55.5’ E) - (33°13.1’ S; 43°55.8’ E), 28–30 m depth, 02 June 2014. TS 2481 ( WSL-INV84 (10)), TS 2482 ( WSL-INV84 (11)), TS 2483 ( WSL-INV84 (12)), TS 2484 ( WSL-INV84 (13)), TS 2485 ( WSL-INV84 (14)), TS 2486 ( WSL-INV84 (15)), TS 2490 ( WSL-INV84 (19)), TS 2492 ( WSL-INV84 (21)), TS 2499 ( WSL-INV84 (28)): 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. 8A ). Thickly encrusting, semi-spherical form. Length 5.0 cm, diameter 4.0 cm and thickness 2.5 cm. Surface smooth, uneven, with various ridge-like structures. Oscules (1–2 mm) scattered randomly on the upper surface. Membrane present that covers the exterior. Texture spongy and dense, of medium compressibility and easily torn. Colour in life dark brown, with very dark brown (almost black) regions, light brown with dark brown regions in preservative. Specimen smells like soil and leaves a brown exudate on tissue paper.

Skeleton ( Fig. 8B, C ). Confused choanosomal skeleton, typically halichondrid, with oxeas of variable length arranged in a disorderly fashion (spicules distributed randomly), showing little tendency to form ascending tracts, and separated by well-developed subdermal spaces. The ectosomal skeleton typically comprises a tangential spicule layer of varying thickness (~100–300 µm), often becoming confused via intercrossing spicules. Spicules do not penetrate the surface. Ectosome not readily detachable from choanosome.

Spiculation ( Table 6 ). Megascleres. Oxeas, smooth, straight to slightly curved, fusiform, in three size classes: I) 416 (350–468) × 11 (6–14) µm, n = 20 ( Fig. 8D ); II) 244 (208–288) × 8 (6–10) µm, n = 20 ( Fig. 8E ); III) 144 (113–199) × 6 (5–7) µm, n = 20 ( Fig. 8F ). Microscleres. Absent.

Substratum, depth range and ecology. Thirty specimens found on rocky substrata in three sleds and both dives, with ascidians, tube worms, coralline algae and/or hydroids. Depth range: 25– 34 m.

Etymology. Named for the Madagascar Ridge, where Walters Shoal is located.

Remarks. The present material has a typically halichondrid skeleton ( Erpenbeck & Van Soest 2002), with megascleres that are exclusively oxeas and oscules scattered randomly on the upper surface. Therefore, it is classified in the genus Halichondria Fleming, 1828 . Due to the smooth, ridge-like surface, the species is further placed in the subgenus Halichondria Fleming, 1828 .

Seven species of Halichondria (Halichondria) Fleming, 1828 are described from the WIO and South Africa. These are Halichondria (Halichondria) capensis Samaai & Gibbons, 2005 , Halichondria (Halichondria) gilvus Samaai & Gibbons, 2005 , Halichondria (Halichondria) cartilaginea (Esper, 1794) , Halichondria (Halichondria) lendenfeldi Lévi, 1961 , Halichondria (Halichondria) aldabrensis Lévi, 1961 , Halichondria (Halichondria) tenuiramosa Dendy, 1922 and Halichondria (Halichondria) prostrata Thiele, 1905 ( de Voogd et al. 2024).

Halichondria (Halichondria) madagascarensis Payne, Samaai & Gibbons sp. nov. is distinct from the seven species of Halichondria (Halichondria) Fleming, 1828 that have been identified in South Africa and the WIO region. This distinction is based on morphological differences and the presence of three size classes of oxeas.

Halichondria (Halichondria) capensis and Halichondria (Halichondria) gilvus , both described by Samaai & Gibbons (2005) from the west coast of South Africa, have one size class of oxeas (333 (319–355) × 12 (12) µm, and 391 (328–437) × 15 (9–18) µm, respectively) with a relatively narrow size range. In addition, both H. (H.) capensis and H. (H.) gilvus have conspicuous papillae, as opposed to the present material which has irregular, spongy, easily deformed turrets. Within the WIO, Halichondria (Halichondria) cartilaginea and Halichondria (Halichondria) lendenfeldi also both have one size class of oxeas (185–203 × 3–4 µm; 400–600 × 11–13 µm, respectively) ( Table 6 ). These two species also differ from the present material morphologically, with H. (H.) cartilaginea having a bushy form, with a slightly brittle consistency and small (0.02 mm) pores, while H. (H.) lendenfeldi has a hispid, velvety surface with many pores distributed over the entire surface. Halichondria (Halichondria) aldabrensis has two size classes of oxeas (I) 275–650 × 4–10 µm; II) 275–650 × 4–10 µm), which are larger than those found in the present material. Halichondria (Halichondria) tenuiramosa which occurs extensively in the Indian Ocean, has one size class of very small oxeas (210 × 6 µm), with a creeping, branching form. In contrast, Halichondria (Halichondria) prostrata , with its type locality in Tumbes, Quiriquina, Chile, has one size class of long, thin oxeas (300–320 × 9 µm) that are shorter than those found in the new species.

Figures

FIGURE 1. A) Map showing the location of Walters Shoal Seamount (red circle) within the bathymetric context of the Western Indian Ocean region (generated using QGIS v.2.6.1; available: qgis.osgeo.org/en/site/). B) Bathymetric map of Walters Shoal Seamount (generated using Surfer 9; Golden Software, available: www.goldensoftware.com). C) Bathymetric map of Walters Shoal Seamount (generated using Surfer 9; Golden Software, available: www.goldensoftware.com), with sled (S), dive (D) and lobster trap (LT) sampling locations.

FIGURE 8. External morphology, skeletal architecture and spicule complement of Halichondria (Halichondria) madagascarensis sp. nov. A—Lab photograph of whole-body image of preserved Holotype specimen, B, C—Skeletal architecture on transverse view. D—Oxea I. E—Oxea II. F—Oxea III. Scale bars on figure.

Tables

TABLE 6. Comparative spicule dimensions of Halichondria (Halichondria) madagascarensis Payne, Samaai & Gibbons sp. nov. specimens and the species of Halichondria occurring in the region of interest (South Africa, WIO, Indo-Pacific). Micrometric values are in µm, n = 20 where applicable.

	Specimen	Oxea I	Oxea II	Oxea III
Halichondria (Halichondria) madagascarensis sp. nov.	Holotype TS 2339	416 (350–468) × 11 (6–14)	244 (208–288) × 8 (6–10)	144 (113–199) × 6 (5–7)
	Paratype TS 2336	415 (315–469) × 8 (6–9)	239 (203–284) × 7 (4–10)	155 (113–173) × 6 (4–8)
	Paratype TS 2338	435 (377–513) × 11 (8–14)	255 (209–287) × 8 (6–10)	136 (114–161) × 5 (4–6)
	Paratypes TS 2340	444 (418–474) × 11 (8–13)	238 (211–274) × 7 (5–9)	127 (120–142) × 5 (4–6)
	Halichondria (Halichondria) capensis Samaai & Gibbons, 2005	333 (319–355) × 12 (12)
	Halichondria (Halichondria) gilvus Samaai & Gibbons, 2005	391 (328–437) × 15 (9–18)
	Halichondria (Halichondria) cartilaginea (Esper, 1794)	(185–203 × 3–4
	Halichondria (Halichondria) lendenfeldi Lévi, 1961	400–600 × 11–13
	Halichondria (Halichondria) aldabrensis Lévi, 1961	275–650 × 4–10	275–650 × 4–10
	Halichondria (Halichondria) tenuiramosa Dendy, 1922	210 × 6
	Halichondria (Halichondria) prostrata Thiele, 1905	300–320 × 9