Red Urchin, Astropyga radiata

The Red Urchin, False Fire Urchin, Fire Urchin or Blue Spotted Urchin (Astropyga radiata) is a species of sea urchin belonging to the family Diadematidae. It’s a large species with long spines and is found within the tropical Indo-Pacific region. It was initially described in 1778 by the German naturalist Nathaniel Gottfried Leske.

This species is a large urchin with a test diameter of up to 8 inches, flattened or slightly concave on the aboral, meaning upper, side. The spines are up to 1.6 inches long and are assembled in five vertical clusters in between which are V-shaped areas with no spines corresponding to the interambulacral plates. These bare areas are red with lines of iridescent blue colored dots while the color of the rest of the test and spines varies from a reddish brown to purple, dark brown or almost black. The anal sac is brown with a dark colored tip. The juveniles have spines with transverse banding and this trait sometimes continues into adulthood.

It’s located in the Indo-Pacific Ocean at a maximum depth of about 230 feet but typically at 33 to 98 feet. Its range extends from the African coast to Hawaii and the waters of Australia. It’s frequently found in lagoons and bays where the substrate is sand, coral rubble, and shingle. Occasionally, many urchins gather together in one locality in dense aggregations.

This species is mainly nocturnal and feeds by grazing on algae. The mouth is at the center of the oral surface where there are five powerful teeth set into an arrangement known as an Aristotle’s lantern. This urchin is sensitive to light and can angle its spines towards an approaching threat.

The sexes are separate within this species. The eggs and sperm are release into the water column. After the fertilization process, the larvae are plank tonic and develop through a handful of stages before settling on the seabed and undergoing metamorphosis into a juvenile urchin.

Numerous crustaceans and fish reside in association with this urchin. This includes shrimp and crabs. Dorippe frascone is a symbiont and carries the urchin on its back. Certain juvenile fish also list amongst the spines including young emperor red snappers, cardinal fish, and zebra fish.

Image Caption: A group of Astropyga radiata (red sea urchin) seen in Nyali, Kenya. Credit: FredD/Wikipedia (CC BY-SA 3.0)