Acanthasteridae Family of Crown of Thorns Sea Stars

Crown-of-Thorns Sea Star, Ancanthaster planci.  A representative of the Acanthasteridae Family of Crown of Thorns Sea Stars.

Phylogeny: Crown of Thorns Sea Stars of the Acanthasteridae Family, like sea urchins and sea cucumbers, are in the phylum Echinodermata. Echinoderms (meaning spiny skin) are characterized by a larval stage with bilateral symmetry and an adult stage with 5-rayed radial symmetry. They are unsegmented and can be cucumber, disk, sphere or star in shape. They have a water vascular system, tube feet, and a complete digestive system, but they lack a head, eyes, nervous system, or excretory system. They are found only in marine environments. Acanthasteridae Sea Stars are in the class Asteroidea, which is characterized by having a flattened, star shape. There is a central disk, surrounded by multiple (5-40) radiating arms (or rays). The disk contains most of the organs, with the mouth on the ventral side and the anus and madreporite (entry plate for the water vascular system) on the dorsal surface. Asteroidea Sea Stars are one of two major classes of Asterozoans. The other class is Ophiuroidea.  Asteroideans are easily distinguished from Ophiuroideans by the arms of Asteroideans having a broad attachment point where the arms join the disk. Their arms taper as they extend from the disk. Ophiuroideans have arms that are relatively the same thickness their entire length, giving them a narrow attachment point at the disk. All Asteroideans have the ability to regenerate amputated limbs. The Acanthasteridae Family is one of fifteen families in the order Valvatida. The Acanthasteridae Family is small, containing only one genus and four species. The name Acanthasteridae comes from the Greek words meaning “spine star” or “thorn star”. Species in this family are also commonly called Acanthasterids.

Morphology:   Acanthasterids have a central disk that is surrounded by 8 – 21 radiating arms.  The arms are relatively short compared to most sea stars. The arms are no longer than the diameter of the disk. Light sensitive photoreceptors are located at the tips of the arms. The aboral (dorsal) surface is covered with long, stiff, sharp spines. The spines can reach up to 5 cm (1.9 inches) in length. The spines are venomous and can cause a painful sting. 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 mouth is located on the ventral surface. They feed via inserting the stomach out through the mouth surrounding the prey, secreting enzymes to digest the food and then retracting the stomach when finished eating. Specimens in this family can be brightly colored and coloration within a species can vary based on diet. Acanthasterids reach a maximum diameter of 80 cm (2 feet 7 inches).

Habitat and Distribution: Crown of Thorns Sea Stars may be found on coral, rocks, or sand between reefs. They live within the lower intertidal zone and to depths up to 65 m (213) feet. They are found in the subtropical to tropical regions of the Indian and Pacific Oceans, including the Red Sea. Only one member of the Acanthasteridae Family is found in Mexican waters and that in the Pacific Ocean.

Reproduction: The Crown-of-Thorns 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.

Ecosystem Roles: Crown of Thorns Sea Stars are widely known for eating polyps from hard corals, but they also consume soft corals, mollusks, sponges, and other Crown of Thorns Sea Stars. Juveniles eat calcareous algae. Adult Crown of Thorns Sea Stars are preyed upon by fish and large gastropods. Juveniles are preyed upon by crabs, shrimp, and polychaete worms.