Crown-of-thorns starfish

The crown-of-thorns seastar, Acanthaster planci, is a large starfish that preys upon hard, or stony, coral polyps (Scleractinia). The crown-of-thorns starfish receives its name from venomous thorn-like spines that cover its upper surface, resembling the biblical crown of thorns. It is one of the largest starfish in the world. A. planci has a very wide Indo-Pacific distribution. It is perhaps most common in Australia, but can occur at tropical and subtropical latitudes from the Red Sea and the east African coast across the Indian Ocean, and across the Pacific Ocean to the west coast of Central America. It occurs where coral reefs or hard coral communities occur in this region. The body form of the crown-of-thorns starfish is fundamentally the same as that of a typical starfish, with a central disk and radiating arms. Its special traits, however, include being disc-shaped, multiple-armed, flexible, prehensile, and heavily spined, and having a large ratio of stomach surface to body mass. Its prehensile ability arises from the two rows of numerous tube feet that extend to the tip of each arm. In being multiple-armed, it has lost the five-fold symmetry (pentamerism) typical of starfish, although it begins its life cycle with this symmetry. Adult crown-of-thorns starfish normally range in size from 25 to 35 cm (9.8 to 13.8 in). They have up to 21 arms. Although the body of the crown of thorns has a stiff appearance, it is able to bend and twist to fit around the contours of the corals on which it feeds. The underside of each arm has a series of closely fitting plates which form a groove and extend in rows to the mouth. They are usually of subdued colours, pale brown to grey-green, but they may be garish with bright warning colours in some parts of their wide range. The long, sharp spines on the sides of the starfish's arms and upper (aboral) surface resemble thorns and create a crown-like shape, giving the creature its name. The spines are stiff and very sharp, and readily pierce through soft surfaces. Despite the battery of sharp spines on the aboral surface and blunt spines on the oral surface, the crown-of-thorns starfish's general body surface is membranous and soft. When the starfish is removed from the water, the body surface ruptures and the body fluid leaks out, so the body collapses and flattens. The spines bend over and flatten, as well. They recover their shape when reimmersed, if they are still alive. The family Acanthasteridae is monogeneric; its position within the Asteroides is unsettled. It is generally recognised as a distinctly isolated taxon. Recently, Blake concluded from comparative morphology studies of Acanthaster planci that it has strong similarities with various members of the Oreasteridae. He transferred Acanthasteridae from the Spinulosida to the Valvatida and assigned it a position near to the Oreasteridae, from which it appears to be derived. He attributed Acanthaster morphology as possibly evolving in association with its locomotion over irregular coral surfaces in higher energy environments. There is a complication, however, in that Acanthaster is not a monospecific genus and any consideration of the genus must also take into account another species, Acanthaster brevispinus, which lives in a completely different environment. A. brevispinus lives on soft substrates, perhaps buried in the substrate at times like other soft substrate-inhabiting starfish, at moderate depths where presumably the surface is regular and there is little wave action. Acanthaster planci has a long history in the scientific literature with great confusion in the generic and species names from the outset, with a long list of complex synonyms. Georg Eberhard Rhumphius first described it in 1705, naming it Stella marina quindecium radiotorum. Later, Carl Linnaeus described it as Asterias planci based on an illustration by Plancus and Gualtieri (1743), when he introduced his system of binomial nomenclature. No type specimens are known; the specimen described by Plancus and Gualtieri (1743) is no longer extant. Subsequent generic names used for the crown-of-thorns starfish included Stellonia, Echinaster and Echinites, before settling on Acanthaster (Gervais 1841). Species names included echintes, solaris, mauritensis, ellisii, and ellisii pseudoplanci (with subspecies). Most of these names arose from confusion in the historical literature, but Acanthaster ellisii came to be used for the distinctive starfish in the eastern Pacific Gulf of California.