Stony Sponge – Petrosia ficiformis
The Stony Sponge is a species in the class of the Demospongiae and thus belongs to the phylum of the Porifera. Petrosia ficiformis belongs to the genus and sub-genus Petrosia and therefore should be called Petrosia (Petrosia) ficiformis. It was first described in 1789 by Poiret.
Due to the symbiosis with cyanobacteria the Stony Sponge usually has a violet to brown colour. However, Petrosia ficiformis can also be white in case it is in a completely dark environment. Its consistency is hard, which is why it is called the Stony Sponge.
Its surface is smooth and has several oval Oscula, openings through which water and metabolic products are released. The shape is variable.
Petrosia ficiformis reaches a height of up to 20cm and can occupy an area of more than one square meter.
Habitat and Distribution
The Stony Sponge lives on a solid ground, between 5 and 50m of depth, preferring shady areas. But it can appear also in bright locations.
When diving in the Canaries, Petrosia ficiformis is very common. If you look between rocks, in crevices and caves, you will be guaranteed to find it.
Its distribution area spreads over the Mediterranean Sea and the North Atlantic. Petrosia ficiformis is also found around the Azores, Cape Verde and Madeira.
The Stony Sponge is a filterer and feeds on micro-organisms such as plankton.
Its biggest predator is Peltodoris atromaculata, the Dotted Sea-Slug. By eating the Stony Sponge, the Dotted Sea-Slug is also supplied with chemicals for defence .
The reproduction of Petrosia ficiformis takes place from May to December. Eggs and sperm are excreted through the Oscula into the open water, where they are then fertilized. As a larva, they live planktonic before they settle down and grow into a new sponge.
The sponge can also reproduce asexually. If a part of Petrosia ficiformis breaks off and regrows in a suitable place, a new sponge can be formed.