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A star polyp is a thumb-shaped individual example of a star coral colony, a species of stony corals which are round or boulder-shaped. Sea anemones and jellyfish also have a polyp stage, and all produce a medusa stage. Medusas are free swimming or floating versions of a coral’s star polyp that break off from the main body at sea, and can be tendril or disc-shaped. Corals universally undergo a polyp stage, but the medusa stage may be dominant, such as in the class Scyphozoa, without the presence of the polyp stage at all.
Growing star polyps in an aquarium is pretty easy as they are non-invasive and spread out rapidly from a central core. Common names used to describe the expanding shapes of star corals include Eight Tentacle, Brain, Daisy, and Starburst, and they are usually white or brown in color, but can also be green and blue. The great star coral can grow to over 5 feet in diameter (1.52 meters) and is composed of green star polyps, which can change to a bright red or orange color during the daytime.
The structure of an individual polyp is an upright sac-shape with eight tentacles at the top, and a cylindrical wall composed of two layers of cells, with a basal plate that attaches to a hard surface. The tentacles surround a mouth and serve both as sensory organs and as an array of fingers used to capture food. The star polyp typically reproduces by asexual budding, but, in the marine environment, they are also known to utilize sexual reproduction by releasing sperm and eggs into the water that merge far off from the initial coral colony.
Caring for star polyps requires that they have significant water flow over the surface and they need to be kept separate from other corals that can dominate the local environment. The algae Zooxanthellae must be introduced into the water as they host it within their bodies and they can then derive the majority of their nutrition from it. Algae generates energy through photosynthesis and for this reason a star polyp needs to be elevated in the tank to a point where it has good exposure to a light source. Introducing micro-plankton and fish larvae into the water will also serve as an extra food source for a star polyp.
Propagating a star polyp colony is easily done if they are started on pieces of broken rock, that can then be separated and moved to several different sections of a tank as they grow. In order to keep new colonies healthy and thriving, it’s important not to overstock the tank with fish or invasive algae. A small amount of phosphate in the water will help them grow, but it will also encourage nuisance algae growth for a star polyp. Snails in the clade of sea slugs known as Nudibranchia can act as predators for a star polyp and should not be included in the tank.