Asteroidea, commonly known as sea stars or starfish, are a diverse group, estimated to include somewhere between 1,500 and 1,800 species. They are found in ocean basins around the world, including deep depths and other extreme environments.
Asteroidea are anywhere from 2 to 24 centimetres in diameter, although most fall in the upper half of the range. They are pentagonal or star shaped in form and can have as many as 40 or even 50 arms, sometimes called rays. Although similar in appearance, all members of this group are not true starfish, leading to some controversy over their taxonomy.
Asteroidea require brackish water, and as such, are ocean dwelling creatures. They have evolved to survive in a wide range of harsh conditions, perhaps accounting for their variety of forms. They can adjust to saltwater aquariums if properly treated.
Larvae and Early Development
Asteroidea are primarily gonochoristic, that is, they have two separate sexes, although some are hermaphroditic. Most are free spawners, releasing sperm and eggs into the water and leaving them to their fate. Hermaphroditic Asteroidea sometimes brood their young, especially in harsher environments. Some species can reproduce asexually.
Larvae are bilaterally symmetrical and must undergo a sessile, or anchored, stage before adulthood. They are attacked by adult pheromones, which lead them to an area in proximity to an adult, where they remain until hormones induce maturity. In some species, metamorphosis is triggered by adult pheromones.
Adulthood and Biology
Adults are radially symmetrical and can live up to 35 years, depending on the species. They are primarily scavengers and carnivores and ingest food via a suspension system where plankton and organic material stick to the mucus on their body and are moved by cilia to the mouth. Some species can turn out their stomach from their mouth and begin to digest food outside their bodies.
Most Asteroidea have red pigmented eye spots at the end of each arm that function as photoreceptors. Due to their non-centralised nervous systems, they can sense their environment from all sides.
Movement and Regeneration
They move by means of a water vascular system in which internal muscles contract and move water to the tube feet, which elongate. Some have suckers, while other have pointed tube feet. Almost all Asteroidea can regenerate or regrow their arms.