Family Esperiopsidae Hentschel, 1923 Genus Amphilectus Vosmaer, 1880 Amphilectus sp.

(Fig. 20 A–F)

Examined material. Sample Bugor 327: Indonesia, North Sulawesi, Siladen Island, Siladen Barat, 5 m, January 2007.

Description. The sponge encrusts several branches of Carijoa riisei (up to 12 cm long) as a thin film of about 0.5 mm thick (Fig. 20 A); on some branches the sponge becomes thicker and massively encrusting (Fig. 20 A); in all the branches the anthocodiae are free to expand and retract (Fig. 20 B). Consistency is gelatinous, surface smooth. Light yellow in ethanol (Fig. 20 A, B). Numerous larvae and eggs are dispersed in the tissue (Fig. 20 B).

Skeleton. The skeleton organisation is difficult to determine, due to the consistency of the sponge. Scattered, tangential styles are present on the sponge surface; sinuous tracts of styles, vaguely interconnecting and running without a clear direction, are present in the inner part of the sponge. Scattered spicules among tracts (Fig. 20 C).

Spicules. Straight, smooth styles (Fig. 20 D), 167.5 – (180.8 ± 9.1) – 195 x 2 – (2.4 ± 0.2) – 2.5 μm. Sigmas “C” shaped (Fig. 20 E), 15 – (22.3 ± 8.5) – 40 μm. Anchorate isochelae with three narrow teeth and a groove along the dorsal margin of the spicule, forming a crest (Fig. 20 F), 12.5 – (13 ± 1.1) – 15 μm.

Remarks. The species here described has chelae that remind of the arcuate, often unguiferate chelae typical of the genera Strongylacidon and Chondropsis (family Chondropsidae). However, Strongylacidon is characterised by strongyles as megascleres and by an ectosomal skeleton consisting of brushes of strongyles deriving from the choanosomal tracts (van Soest, 2002b). Moreover, sand grains and foreign material are conspicuous in this genus and have a structural role in the fibres. These features do not seem to characterise the present specimen. On the other hand, the skeleton is not clearly interpretable. The presence of an ectosomal skeleton of tangential styles, absent in species of Chondropsis, is here detectable but structural sand or foreign material (typical of Chondropsis) are not present. On the basis of these considerations we prefer to attribute this specimen to Amphilectus, taking choanosomal architecure as a better performing character than microsclere shape (Hajdu & van Soest, 1996).

The presence of styles, isochelae and sigmas characterizes the genera Esperiopsis and Amphilectus. Amphilectus is very close to Esperiopsis and the main discriminating characters are the size of the styles (<400 μm) and the absence of sigmas in Amphilectus (van Soest & Hajdu 2002a). However, at least one species of Amphilectus was described with sigmas (A. glaber Brøndsted, 1924) and at least two species of Esperiopsis have styles <400 μm. In van Soest & Hajdu (2002a) these two genera are described as possessing a different skeleton: Amphilectus with regularly isodictyal, anisotropic skeleton, and Esperiopsis with irregularly anastomosing spicule tracts.

Amphilectus glaber (Brøndsted, 1924), A. munitus Whitelegge, 1907, and A. unciger (Topsent, 1928) are all reported from the Indo-Pacific (van Soest et al. 2011). Amphilectus munitus and A. unciger do not have sigmas. Amphilectus glaber has larger styles (200 – 370 x 10 μm) and the choanosomal skeleton consists of a regular network of rectangular meshes. Numerous species of Esperiopsis are known from the Indo-Pacific (van Soest et al. 2011), but only two of these species have styles that are <400 μm: E. diasolenia Lévi, 1993 and E. magnifolia Lévi, 1993 (250–300 μm and 350–450 μm, respectively); nevertheless these species are devoid of sigmas, and chelae are longer (28–40 μm in E. diasolenia and 30 μm, and 40–60 μm in E. magnifolia). These species are clearly different from our Indonesian specimen, which is characterised by short styles (<400 μm) and by a single category of chelae and sigmas. This spicule complement, in particular the short styles, prompts us to attribute this specimen to the genus Amphilectus. Unfortunately the small size of the sample prevents a clear interpretation of the skeleton, and therefore we refrain from formally describing a new species from this material.