This hard sponge is very variable in shape. The juvenile specimens are usually spherical, the older once often wide with a flattened top. You can often observe large specimens, that they have formed an additional support. The exterior colour is caused by the symbiotic bacteria (cyanobacteria), in the upper layer. The leopard slug (Discodoris atromaculata) feeds on this sponge.

Height about 7 to 10 cm and 20 cm wide.

The exterior can range from reddish brown to dark brown. Specimens in places with little light will be mottled with white patches or sometimes become nearly white in color. The inside is light yellow to white.

On hard substrate from the shalows up to a depth of 50.

Mediterranean sea.