The sponge Sarcotragus spinosulus belongs to the class Demospongiae, order Dictyoceratida and family Irciniidae. It is a sponge found in the waters of the western Mediterranean Sea, the Adriatic Sea and the coasts of southern Europe. It inhabits rocky bottoms, vertical walls and overhangs in shallow and somewhat shaded waters, although it is also possible to find it at depths of up to 300 meters.
Sarcotragus spinosulus is brownish black or dark gray in color, and its surface has numerous small conical bumps. It has a firm and rough consistency, compressible and extremely tough. Its body rarely exceeds 10 cm in diameter and is usually somewhat spherical. Its base is slightly narrower than the rest of the body. It has numerous more or less conical-shaped oscular openings, which are distributed irregularly along its body.
The feeding of Sarcotragus spinosulus is by filtration of the surrounding water column. Through a multitude of small pores throughout its body, it inhales the surrounding water, and by means of specialized flagellated cells, creates the necessary current to generate the water current through its body, and thus trap small organic particles or bacteria from which it obtains the energy and compounds necessary for growth.
It is common to find specimens of Sarcotragus spinosulus on the shells of Pinna nobilis, in a relationship called epibiont, that is, it is a harmless relationship for the host and therefore could be considered as a commensalist relationship.
The reproduction of Sarcotragus spinosulus is viviparous, with individuals presenting hermaphroditism. Both sperm and eggs are generated during the months of June to October in the same individual, although cross-fertilization does not usually occur. Instead, sperm are released into the stream so that when they enter the tissue of another specimen, they fertilize the eggs present there.
Sarcotragus spinosulus can be confused with specimens of Scalarispongia scalaris, as they are externally very similar and to differentiate them it will be necessary to examine them under a microscope. They can also be confused with Sarcotragus foetidus, although the latter tends to have a much more irregular surface and cracks. Also the conical bumps are larger and more widely spaced than in the case of Sarcotragus spinosulus.Photos: