Petrosia is a genus of sponges belonging to the family Petrosiidae. Petrosia are well distributed from temperate zone waters to tropical and from intertidal zone to deep waters.

The genus has almost cosmopolitan distribution.

Sponges are effective filter feeders, and associated, undigested microorganisms can count up to 60% of the fresh weight of marine sponges. These microorganisms include bacteria, fungi, cyanobacteria and unicellular algae.

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