Amphimedon cf. sulcata Fromont, 1993

Calcinai, Barbara, Bastari, Azzurra, Bavestrello, Giorgio, Bertolino, Marco, Horcajadas, Santiago Bueno, Pansini, Maurizio, Makapedua, Daisy M. & Cerrano, Carlo, 2017, Demosponge diversity from North Sulawesi, with the description of six new species, ZooKeys 680, pp. 105-150 : 110

Amphimedon cf. sulcata Fromont, 1993 Figure 6

Material examined.

BU-560, 26/06/2004, Bualo (Bunaken Island), unknown depth. BU-575, 27/06/2004, Alung Bauna (Bunaken Island), 27 m depth.

Description.

The sponge is flat, with a roundish contour, about 1 cm thick, without visible oscules. It is completely free of epibiotic organisms. Colour in situ may be greyish-white (Figs 5A, 6A) or pale cerulean (Figs 5B, 6B), off-white to greyish in the preserved state. The sponge shows ridges and grooves, covered by a very thin membrane, that give a typical convoluted or brain-like aspect to its surface (Fig. 6B).

Skeleton. The ectosomal skeleton is a reticulation of pauci-spicular tracts (3-4 spicules) (Fig. 6C) organised in quite regular triangular meshes with scarce spongin at the nodes. The choanosomal skeleton (Fig. 6D) is formed by a reticulation of multi-spicular tracts and round meshes of approximately 60 µm in diameter, with abundant scattered spicules. The spicule tract extremities barely protrude from the sponge surface, causing micro-hispidation.

Spicules. Megascleres are straight or slightly curved oxeas with sharp tips; they measure 125 - (188.9 ± 33.5) - 247.5 × 2 - (5.2 ± 3.4) - 12.5 µm (Fig. 6E); numerous thin oxeas are present (Fig. 6F); microscleres are very thin, C-shaped, sigmas 10 - (12.9 ± 1.5) - 15 × ≤ 1µm (Fig. 6G).

Remarks.

The sponge here described has a skeleton organisation fitting with the diagnosis of the genus Amphimedon that is characterised by an ectosomal skeleton of tangential fibres forming meshes, covered by a thin membrane and by a choanosomal skeleton formed by a plumose, irregular reticulation of multispicular tracts ( Desqueyroux-Fáundez and Valentine 2002).

Our specimens are similar to A. sulcata , especially for the very characteristic surface: "meandering parallel ridges, interspersed with spaces, give a convolute or brain-like appearance to the surface" ( Fromont 1993), for the thin membrane covering the ridges and the absence of abundant spongin.

Among the Indo-Pacific species of Amphimedon , only A. sulcata has sigmas similar in size (13 - (15.9) - 16.9 µm) to our specimens, but its oxeas (122 - (139) - 153 × 3 - (4.5) - 5.3 µm) are smaller than those we observed. Another difference is in the colour: "mauve alive, cream or fawn in alcohol" in A. sulcata (Fromont, 1993).

Remarks on the association.

Amphimedon cf. sulcata is not tightly attached to Rhabdastrella distincta , and the two sponges can be separated rather easily. Frequently, wide areas of R. distincta are not covered by the outer sponge (Figs 5A, B, 6A, B), and exhalant and probably also inhalant parts of R. distincta are in these portions, free from the epibiont.

In the boundary between the two sponges, a thin collagenous layer is present. Both in the histological preparations and in SEM images, the oxeas of R. distincta are clearly visible, protruding out of the surface and penetrating inside the tissues of the external sponge (Fig. 5C, D), as it is usual in similar associations ( Ávila et al. 2007). This association was frequently observed in North Sulawesi, usually below a depth of 30 m.