Some of the most colorful and delicate reef invertebrates are the anemones, the so-called flowers of the sea. Like corals, anemones are cnidarians that only exist in the polyp stage. They have typical saclike, cnidarian bodies as well as nematocystladen tentacles around their mouths. Anemones do not build skeletons as their coral relatives do.
Anemones may occur in colonies or singly. The body of an anemone polyp is a thick column with two distinctive ends, both clearly visible in the upper color insert on page C-3. The upper end, or the oral disk, consists of a ring of tentacles around a narrow slit at the center, the mouth. Grooves beside the mouth bring in water continuously to provide oxygen to interior tissues. Depending on the species, oral disks vary in diameter from a fraction of an inch to 12 inches (30 cm).
The other end of the animal, the pedal disk, attaches to hard substrates. Some secrete a sticky adhesive to help hold them in place. Although the animals are classified as sessile, or sedentary, they can move around very slowly. They may shuffle across the reef floor on their pedal disks, similar to the way a snail moves on its foot, or somersault on their tentacles from one place to another. Some release their hold on the seafloor, inflate their bodies with air, and float to the surface, where they will rest for a while with their heads down.
Several species of anemones live on reefs. The maroon anemone (Actinia bermudensis) has short red tentacles bordered by bright blue warts. The rock anemone (Anthopleura krebsi) has greenish-yellow tentacles and rust-colored warts. The stinging anemone (Lebrunia dana) is duller in color, usually brown or white, but its tentacles deliver powerful stings that quickly kill prey and can injure humans.