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A Circeaster pullus starfish everting its stomach to feed on coral. Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals. [24] [58] The crown-of-thorns starfish consumes coral polyps, [59] while other species are detritivores, feeding on decomposing organic material and faecal matter.
To feed on one of these, the starfish moves over it, attaches its tube feet and exerts pressure on the valves by arching its back. When a small gap between the valves is formed, the starfish inserts part of its stomach into the prey, excretes digestive enzymes and slowly liquefies the soft body parts.
The common starfish, common sea star or sugar starfish (Asterias rubens) is the most common and familiar starfish in the north-east Atlantic. Belonging to the family Asteriidae , it has five arms and usually grows to between 10–30 cm across, although larger specimens (up to 52 cm across) are known.
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, such as the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on brittle stars, and have only a feeding function in feather stars. They are part of the water vascular system.
The starfish must broaden their diet from their preferred species, colony size, and shape. The starfish often aggregate during feeding, even at low densities, but during high densities, the cleared coral patches become almost or completely continuous. Second-order effects exist for these large areas of preyed coral:
Crown-of-thorns starfish (Acanthaster planci), which feed on large swaths of western Pacific coral reefs, are notable unidirectional regenerators. [9] Starfish of this invasive species are extremely difficult to eradicate because of their ability to regrow when half or more of the original starfish is intact. [9]
When this happens, aggregations of starfish do feed on it. This is despite laboratory feeding tests demonstrating that it prefers abalone. It seems that the abalone has some behavioural responses to attack that make it more likely to escape, one of which seems to be the production of a chemical deterrent by the mollusc. [3]
The species is also considered in reef aquariums. It feeds on the surface of the stones in thin layers of algae, so it can live only in an old well-ripened aquarium. If the algae are not growing fast enough, supplemental feeding is usually unsuccessful, and this starfish dies of starvation.