Plakortis pulvillus sp. nov.

(Fig. 2 A–D)

Material examined. Holotype. SAMC–A24721 (cross-reference TS 909 & Saf 3-Sod74), Deep-sponge reef, Sodwana Bay, South Africa (27.5216°S, 32.6835°E), 0 6 November 2003, collected by T. Samaai, depth 30 m.

Type locality. Sodwana Bay, east coast of South Africa .

Description. Small, thickly encrusting, cushion-shaped sponge, 40 × 20 × 20 mm diameter, body attached to substratum (Fig. 2A). Surface smooth with few oscules, 2 mm in diameter, randomly scattered on the surface of the sponge. Texture firm, rubbery to the touch, barely compressible, difficult to tear. Colour in life, liver brownish yellow; in preservative, pale brown-yellow.

Skeleton (Fig. 2B, C). Choanosomal skeleton is confused, consisting of diods in three size classes. The diplodal canal system is richly developed and extends into the ectosome. Larger diods appear in the choanosome whereas the smaller diods are found at the base of the choanosome or within the subdermal cavities. The ectosomal skeleton is poorly developed with a palisade of smaller diods. Basal cavities are absent.

Spicules (Fig. 2D). Diods, irregular and lumpy in places with centrally located double kinks, in three size classes: I) 106 (91–115) × 3 (3) µm, n = 10; II) 64 (42–78) × 2 (2) µm, n = 10; III) 22 (8–32) × 1(1), n = 10. Triods absent.

Substratum, depth range and ecology. This sponge was found on a rocky ledge at a depth of 30 m.

Etymology. Named for the small, cushion-shaped morphology of this species ( pulvillus, Latin).

Remarks. The genus Plakortis, which includes 29 valid species (Van Soest et al. 2017), is allegedly cosmopolitan in distribution, with 14 species in the Tropical Western Atlantic (TWA) (Domingos et al. 2013; Vicente et al. 2016; Van Soest et al. 2017). Species belonging to the genus Plakortis are a valuable source of novel cyclic peroxides, pyrroloacridine alkaloids and polyketides with biologically active properties (see Muricy & Diaz 2002; Muricy 2011; Vicente et al. 2016). This chemical diversity makes the genus Plakortis an interesting target group for pharmacological studies.

Five species ( Plakortis copiosa Pulitzer-Finali, 1993; P. kenyensis Pulitzer-Finali, 1993; P. erythraena Lévi, 1958; P. nigra Lévi, 1953; P. simplex Schulze, 1880 & P. lita de Laubenfels, 1954) are recorded from the Western Indian Ocean (WIO) region. Pulitzer-Finali (1993) described P. copiosa and P. kenyensis and recorded P. simplex from Kenya, while Thomas (1973, 1979b, 1981) recorded P. simplex from Mahe Island in the Seychelles and Paradise Island in Mozambique, respectively. Lévi (1956) described P. erythraeana and P. nigra from the Red Sea and Van Soest (1994) reported P. lita from the Seychelles. This is the first record of the genus in South African waters (Delagoa ecoregion), although P. simplex has been recorded further north by Thomas (1979), around Paradise Island in Mozambique.

Schulze (1880) originally described P. simplex sensu stricto as a thin encrustation (maximum 5 mm thick), light-coloured (light brown, tan, yellow or white) with diods (60–150 µm long) and triods (actines 20–50 µm long) as spicules. In contract, P. pulvillus sp. nov. is thickly encrusting (cushion-shaped), dark-coloured (liver-brown) with only diods (22–106 µm long) as spicules.

Plakortis simplex sensu Pulitzer-Finali (1993) recorded from Kenya, P. lita sensu Van Soest (1994, 1993) and P. copiosa (Pulitzer-Finali, 1993) are similar in external morphology to P. pulvillus sp. nov., all being cushionshaped and brown in colouration. However, P. simplex sensu Pulitzer-Finali (1993) possesses diods and rare triods, 95–115 µm long, and P. simplex sensu Thomas (1973, 1979b, 1981) possesses triods in variable abundance, in addition to diods. Plakortis copiosa from Kenya possesses triods that are more numerous than diods while no triods have been observed in P. pulvillus sp. nov. Plakortis lita possesses numerous microrhabds, a character only shared with P. microrhabdifera Moraes & Muricy, 2003 described from Brazil. The absence of triods (and microrhabds) in P. pulvillus sp. nov. does not place it among the P. simplex species group (Muricy 2011).

Most species of Plakortis have restricted distributions (Muricy 2011, Domingos et al. 2013) and low dispersal abilities, which raises the question of whether P. simplex from the western Mediterranean and P. lita from Micronesia are really present in the WIO. Both species were reported in the WIO with short or no descriptions (e.g. see Thomas 1973, 1979b, 1981; Pulitzer-Finali 1993; Van Soest 1994). On biogeographical grounds alone, the occurrence of any European species in the Western Indian Ocean is highly improbable. Plakortis pulvillus sp. nov. is grouped under Plakortis with only diods, which include three species, that is, P. nigra, P. halichondrioides (Wilson, 1902) and P. zyggompha (de Laubenfels, 1934) . Both P. halichondrioides and P. zyggompha are described from the Caribbean region, whereas P. nigra are decribed from the southern Red Sea. These records represent complexes of cryptic species and the WIO populations probably deserve new species names. The distribution of P. copiosa, P. kenyensis, P. erythraena and P. nigra is much more restricted than that of the P. simplex complex, which occurs in five ecoregions (see Van Soest et al. 2017).

This conspecific character within the genus is due to the simple and non-diagnostic nature of their skeleton and external morphology (Moraes & Muricy 2003), and hence, more stringent examinations of these specimens/species are needed. Considering the generally poor larval dispersal potential of sponges (Maldonado & Bergquist 2002, Muricy 2011), and the differences in spicule complement and dimensions between morphologically similar species, P. pulvillus sp. nov. is distinguished from other Plakortis species.

Key diagnostic characters.

• Sponge small, thinly cushion-shaped.

• Diods in three size categories.

• Diods often double kinked

• Triods absent.