Echinothrix diadema, commonly known as ‘Blue-black urchin‘ is a species of sea urchin in the family Diadematidae. The genus name ‘Echinothrix‘ is derived from the Greek ‘echinos‘ meaning ‘hedgehog‘ and ‘thrix‘ meaning ‘hair‘ referring to the long, thin spines characteristic of this group of sea urchins. The epithet ‘diadem‘ refers to a crown or diadem, probably alluding to the shape of the spines that surround its shell, giving it a majestic appearance. This species is native to the tropical regions of the Indian and Pacific Oceans, being found from the east coast of Africa and the Red Sea to the Pacific islands, including Hawaii, the Great Barrier Reef in Australia, and southwest Asia. Echinothrix diadema mainly inhabits coral reefs, lagoons and rocky areas, from intertidal areas to depths of about 70 meters. It is an active species during nighttime hours, hiding in crevices or under rocks during the day.
Echinothrix diadema is a medium-sized sea urchin that can reach a diameter of up to 10-12 centimeters in its carapace (testa), not counting its spines. This sea urchin is characterized by extremely long and sharp spines, which can reach up to three times the diameter of its body, reaching up to 30-40 centimeters in length. The spines are generally black or dark brown, although some may show bluish tones. Like other species of its genus, it has two types of spines: longer and more fragile ones used for defense, and other shorter and more robust ones that help in locomotion and manipulation of the substrate. Echinothrix diadema can be differentiated from Echinothrix calamaris by the absence of short white or translucent spines and by having a more uniform and darker coloration in its long spines.
As for its diet, Echinothrix diadema is mainly herbivorous, although it can be considered an opportunistic omnivore. Its diet consists mainly of encrusting coralline algae, macroalgae (Codium geppiorum) and organic detritus. It uses its specialized mouth structure, called ‘Aristotle’s Lantern’ to scrape and crush algae and organic material from the substrate. It can also consume small amounts of animal material, such as sponges and coral fragments. Its role in coral ecosystems is significant, as it contributes to the control of algal growth, allowing corals to recover and grow. However, in areas where the sea urchin population is very high due to the absence of natural predators, Echinothrix diadema can cause considerable erosion of coral reefs by feeding on encrusting algae.
Reproduction of Echinothrix diadema is sexual and occurs through external fertilization. During the reproductive season, which usually coincides with the warmer months of the year, sea urchins release their gametes (eggs and sperm) directly into the water column. This synchronous release increases the likelihood of fertilization. Fertilized eggs develop into planktonic larvae called ‘pluteus‘ which float in the ocean for several weeks before settling on the substrate and metamorphosing into juveniles. This planktonic phase allows the species to disperse and contributes to its wide geographic distribution on coral reefs.
An interesting curiosity about Echinothrix diadema is its defensive behavior: in the presence of predators or disturbances, it moves its longest spines defensively, raising them and creating an almost impenetrable barrier. Although it is not poisonous, contact with its spines can be painful and can cause wounds, due to the fragility of the spines, which tend to break and become embedded in the skin. These wounds can easily become infected if not properly treated. In addition, Echinothrix diadema is considered a bioindicator of coral reef health; its abundance may reflect changes in ecosystem dynamics, such as the decline of predators or reef degradation due to human activities. Specimens of Echinothrix diadema may host various commensal species, such as the shrimp Stegopontonia commensalis, or Saron marmoratus that stay nearby for protection. Many other fish species also seek refuge in the surroundings or among the long spines when they feel in danger, as is the case of many fishes of the families Apogonidae (cardinal fishes) and Centriscidae (razorfishes and relatives).
Photos:
from