The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region, and the Northern Pacific seastar is on the list of the World's 100 Worst Invasive Alien Species.

They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements.

The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected.

[8] In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface.

This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.

These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove.

[20] Some burrowing species from the genera Astropecten and Luidia have points rather than suckers on their long tube feet and are capable of much more rapid motion, "gliding" across the ocean floor.

The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion.

These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water.

Although some species can tolerate relatively low salinity, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments.

[27] Starfish produce a large number of secondary metabolites in the form of lipids, including steroidal derivatives of cholesterol, and fatty acid amides of sphingosine.

In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryos and larvae live as part of the plankton.

[34] In these brooding species, the eggs are relatively large, and supplied with yolk, and they generally develop directly into miniature starfish without an intervening larval stage.

[8] Some species of starfish in the three families Asterinidae, Asteriidae and Solasteridae are able to reproduce asexually as adults either by fission of their central discs[46] or by autotomy of one or more of their arms.

[55] Echinoderms, including starfish, maintain a delicate internal electrolyte balance that is in equilibrium with sea water, making it impossible for them to live in a freshwater habitat.

Habitats range from tropical coral reefs, rocky shores, tidal pools, mud, and sand to kelp forests, seagrass meadows[56] and the deep-sea floor down to at least 6,000 m (20,000 ft).

[62] Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues.

[65] When studying the low intertidal coasts of Washington state, Paine found that predation by P. ochraceus was a major factor in the diversity of species.

Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of Mytilus mussels, which were able to outcompete other organisms for space and resources.

[67] The feeding activity of the omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of microorganisms.

[77] Some starfish such as Astropecten polyacanthus also include powerful toxins such as tetrodotoxin among their chemical armoury, and the slime star can ooze out large quantities of repellent mucus.

[79] Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.

[12] Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres.

The taxonomy of the group is relatively stable but there is ongoing debate about the status of the Paxillosida, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida, do not fit easily in any accepted lineage.

[2] The phylogeny proposed by Gale in 1987 is:[2][122] † Palaeozoic Asteroids Paxillosida Valvatida, including Velatida, Spinulosida (not a clade)[2] Forcipulatida, including Brisingida The phylogeny proposed by Blake in 1987 is:[2][123] † Palaeozoic Asteroids † Calliasterellidae † Compasteridae † Trichasteropsida Brisingida Forcipulatida Spinulosida Velatida Notomyotida Valvatida Paxillosida Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.

[125] Starfish oocytes are well suited for this research as they are large and easy to handle, transparent, simple to maintain in sea water at room temperature, and they develop rapidly.

[129] An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959)[130] tells how some animals needed a canoe to cross the ocean.

Among the "uncreated gods" described early in the song are the male Kumulipo ("Creation") and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.

[132] Georg Eberhard Rumpf's 1705 The Ambonese Curiosity Cabinet describes the tropical varieties of Stella Marina or Bintang Laut, "Sea Star", in Latin and Malay respectively, known in the waters around Ambon.

[153] In World War II, Starfish sites were large-scale night-time decoys created during The Blitz to simulate burning British cities.