Cnidarians use nematocysts and their tentacles to capture their prey, and these carnivorous creatures usually snack on members of class Crustacea. While crabs, shrimp, and barnacles can make a good meal, some species of cnidarians get their food from a symbiotic relationship with other organisms.
There are two main types of symbionts: yellow-green algae, known as zooxanthellae; and unicellular green algae, known as zoochlorellae.
Anemones profit from these organisms that inhabit their gastrodermal cells by feeding on photosynthetic products such as glycerol, glucose, and alanine, that the algae produce as they turn sunlight into energy during photosynthesis.
Since polyps are attached to the ocean floor, these sedentary animals are not exactly the jet setters of the aquatic world. But despite their sluggish appearance, some polyps can get a move on. Using their pedal discs located at the end of the body opposite the mouth, sea anemones can glide along the substrate at a nearly imperceptible pace. Movement may be necessary to find more lucrative hunting grounds, or secure a new mate.
The sea anemone is considered sedentary because it has limited locomotive capabilities. Mature coral, however, is considered sessile because it is permanently fixed to the substrate.
When the going gets tough, anemones get shape shifting. In low tide conditions when battering waves threaten polyps’ delicate bodies, sea anemones will flatten themselves to avoid injury, or retract their tentacles to avoid drying out.