Aquatic and Land Animal Diversity

Aquatic Animals

What is a hydra?

The Portuguese man-of-war (Physalia physalis) is a floating hydrozoan and is actually a colony of four types of polyps—each with a “duty,” such as allowing the organism to float or gathering food. Certain fish, such as the yellowjack and the clownfish, all live within the tentacles. Some of these fish, in particular the clownfish, produce a slimy mucus that causes the man-of-war not to fire its stingers, while other types of fish rely on a specialized swimming pattern—swimming near the surface in various directions to avoid the man-of-war’s stings.

A hydra, a well-known member of phylum Cnidaria, is a tiny (0.4 inch or 1 centimeter in length) organism found in freshwater ponds. It exists as a single polyp that sits on a basal disk that it uses to glide around—it can also move by somersaulting. It usually has six to ten tentacles, which it uses to capture food, and they reproduce both sexually and asexually (budding). Hydras are named after the multiheaded monster of Greek mythology that was able to grow two new heads for each head cut off. When a hydra is cut into several pieces, each piece is able to regrow all the missing parts and become a whole animal.


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