The Asteroidea, or Sea Stars, are an extremely popular class of organisms on The GBR, with around 1,600 species found worldwide. Commonly found in shallow water, but their range extends down to 6,000m. They are quite distinctive, shaped like a star, with arms (typically 5) radiating away from a central disc. Like all echinoderms, they are a vital part of the benthic community and can be quite aggressive predators.
Asteroidea have a mouth on the underside and an anus directly above on the dorsal side. Feeding by wrapping their legs around their food and expelling their stomachs outside of their body. A number of digestive enzymes then break down the food into a milky soup. Common foods include Clams, Molluscs, Sponges, Anemones, Worms, Algae, dead animals and even Coral. Many starfish can even absorb dissolved organic material directly through their skin.
The tips on their arms end in terminal tentacles that are sensitive to chemicals in their surrounding environment. Small clusters of light sensitive receptors also fill this area. They cannot put together images of their environment, but they can smell and detect changes in light and dark.
Thin projections called papulae cover the dorsal surface, which are responsible for gas exchange. This also takes place in the tube feet. These little feet, called podia are connected to the water-vascular system and are responsible for movement.
Starfish have an amazing ability to regenerate. So long as the central disc is intact, they can replace a lost limb. Some Asteroidea can even grow an entirely new body from a single arm. Furthermore, a select few can reproduce asexually by deliberately shedding arms.
The majority have separate sexes and reproduce by spawning. Congregating together, they pick up on chemical cues released by other starfish spawning nearby, triggering the release of their own spawn. The sperm and eggs fuse together to form embryos. Most Starfish begin as planktonic larvae developing through a number of stages before settling as an adult. Some starfish hold their eggs in a brooding pouch, releasing them when they have developed further.1,2
Order – Fancy science word used to group organisms with similar characteristics