Lichens have traditionally been referred to as a prime example of a symbiotic relationship. Each lichen consists of an intimate association between a fungus and an algae. The algae within the fungus photosynthesizes, providing food for both organisms. The fungus protects the alga from harmful light intensities, produces a substance that accelerates photosynthesis in the algae, and absorbs and retains water and minerals for both organisms. There is physiological evidence that suggests the fungus parasitizes the algae in a controlled fashion and, in some instances, actually destroys the algal cells. There are about 25,000 species of lichen known and they are capable of living in environmental conditions that kill most other forms of life. The number of aquatic lichen is limited as most live under the blazing sun or on bare rocks. Aquatic lichens typically live in the intertidal zone along sea shores or in shallow streams.