The Green Beard
The green beard, Tolypothrix barba, is a species of cyanobacteria found exclusively in the Mediterranean sea. As such, the green beard is a photosynthetic prokaryote, and not an animal per se, but we shall make an exception this once.
The green beard got its name from its choice of habitat, which is usually right below the mouth of bigger fish. The green beard attaches itself to the fish, and profits from the nitrogen-rich environment the fish creates around its mouth (by eating). Thus, the fish looks like it has a beard.
In some of the more extreme cases observed to date, green beard algae have attained a total mass of almost a ton, and several meters in length, when growing around the mouth of wales. The size a green beard colony can grow to is directly proportional to the size of the fish/mammal it is attached to. Once it has reached its maximum size, it keeps on producing spores, which detach themselves from the main colony and float around the sea, waiting to find a host of their own.
Studies have shown that the green beard, although reducing the fitness of individuals it attaches itself to, due to increased water drag, greatly helps to maintain a diversified and healthy ecosystem by creating diverse habitats and food sources.
In recent years, due to pollution and a massive decrease in fish stock, the green beard population has known a rapid decline. Recent conservation efforts have been focusing on stabilizing the population, notably by using bio-coated submarines that the green beard can use as hosts. Whether this will stop, or even hamper, the decline of the algae remains to be seen.
Recent research by Dr. Brutus Katakaras, of the Institute for Marine Beards of Athens (IMBA), suggest that the green beard was already known in the time of the ancient Greeks, and animals that had it growing from their mouth were said to be messengers of Poseidon. Katakaras believes that the priests of the temples dedicated to the sea god were all sporting a green beard, regularly immersing their face in sea-water to keep the growth alive. In recent years, this trend seems to be picking up again, but more as a fashion statement than a religious practice.