Crown-of-Thorns Sea Star, Acanthaster planci

Crown-of-Thorns Sea Star, Acanthaster planci. Underwater photograph taken in coastal waters off Isle Coronado, Baja California Sur, May 2023. Photograph courtesy of Dr. Tom Bartol, Carlsbad, California.

Phylogeny: The Crown-of-Thorns Sea Star, Acanthaster planci (Linnaeus,1758), is member of the Acanthasteridae Family of Crown of Thorns Sea Stars. The Acanthasteridae Family is small, containing only this genus and four species. This species is known in Mexico as Corona de Espinas. Some sources recognize the Eastern Pacific population of this species as a separate species – the Pacific Crown-of-Thorns Sea Star, Acanthaster solaris. The World Registry of Marine Species (WoRMS) does not accept this species. The Genus name Acanthaster comes from the Greek words meaning “thorn star”. The species name planci comes from the Latin pseudonym, Janus Plancus, of the 18th century naturalist Giovanni Bianchi. Bianchi was a physician, botanist, zoologist, and ardent collector of natural specimens.

Morphology:  The Crown-of-Thorns Sea Star has eight to twenty-one arms that are relatively short compared to the large central disk. The aboral (dorsal) surface of the disk and arms is covered with long venomous spines. The spines can reach up to 5 cm (1.9 inches) in length. The venomous quality of these spines is not fully understood; saponin has been discovered in the spines’ underlying tissue, though the quantity is not sufficient to trigger the painful reactions seen in humans who have come into contact with the spines. There is no evidence that the spines inject toxins. The oral surface (underside) is covered with blunt spines. The aboral spines are cylindrical, while the ventral spines are flattened. Despite these stiff spines, their general body surface is membranous and soft. They are soft and pliable compared to most sea stars. The arms are equipped with suckers on the oral side.  It’s adult form does not possess the five-fold symmetry that is common of most starfish, although juvenile forms have five-fold symmetry. They vary in color and may be gray, red, orange, or purple. The color is thought to be caused by diet.  The diameter of Crown-of-Thorns Sea Stars ranges from 10 cm (3.9 inches) to 70 cm (27.6 inches).

Habitat and Distribution:  The Crown-of-Thorns Sea Star is found within the lower intertidal zone, and at depths up to (46 m) 150 feet. They prefer rocky reefs, particularly on offshore islands, where rich coral and gorgonian growth occurs. Crown-of-Thorns Sea Stars are found in subtropical to tropical waters of the Indian and Pacific Oceans.  In Mexican waters they are distributed from the Central Gulf of California to Isla Clarion. Despite their affinity for a diet of coral, they are curiously absent from the Gulf of California’s only coral reef at Cabo Pulmo, Baja California Sur.

Diet:  Coral polyps primarily make up the diet of this sea star, though they also eat sessile invertebrates and carrion. They feed by everting their stomach over their prey, digesting it externally, and absorbing the nutrients through the stomach wall. They can live up to nine months without eating. One individual can eat through up to ten square meters of coral a year.

Predators:  Adult Crown of Thorns Sea Stars are preyed upon by  large gastropods, harlequin shrimp, fireworms, and fish such as triggerfish and pufferfish. Only the gastropods consume the entire animal, the other take small bites. Juveniles are preyed upon by all these, and crabs.

Reproduction: The Crown-of-Thorns Sea Stars are gonochoric (male or female for life) and reproduce sexually through broadcast spawning. The eggs hatch into planktonic larvae. After several weeks the larvae settle to the bottom and metamorphose into tiny five-armed juvenile sea stars. Crown-of-Thorns Sea Stars have exceptional reproductive capacity. Females produce as many as 60 million eggs throughout a season and the become sexually mature at two years of age. Unlike most sea stars, Crown-of-Thorns Sea Stars are not known to reproduce asexually.

Ecosystem Interactions: The presence and density of the Crown-of-Thorns has implications on the health of coral reefs. Shell collectors taking the large gastropod predators of this species have allowed Crown of Thorns Sea Stars to reach artificially high population levels. Prohibitions against shell collecting have helped with this imbalance. Several copepod species and the Commensal Shrimp, Periclimenes soror, live among the sharp and venomous spines of these sea stars as epibionts.

Human Interactions: Crown of Thorns Sea Stars have a very limited impact on human activities. Their venomous spines can sting if the animal is stepped on or handled.  I personally have handled many of these animal and been stung only once. The sting was moderately painful, bled profusely, and left the surrounding skin numb for several years. Allergic reactions to the sting have been reported.  From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms:  Acanthaster echinites, Acanthaster echinus, Acanthaster ellisi, Acanthaster ellisi pseudoplanci, Acanthaster pseudoplanci, Acanthaster solaris, Asterias echinites, Asterias echinus, Asterias planci, Asterias solaris, Echinaster ellisi, Echinaster solaris, Echinities solaris, and Stellonia echinites.