Diadema setosum

Diadema setosum is a species of long-spined sea urchin in the family Diadematidae. It’s a typical sea urchin, which exceptionally long and hollow spines that are mildly venomous. D. setosum is different from other Diadema with five distinctive white colored dots that can be found on its body. The species is located throughout the Indo-Pacific region, from Australia and Africa to Japan and the Red Sea. Although it is capable of painful stings when stepped upon, the urchin is only somewhat venomous and doesn’t pose a serious threat to humans.

As a member of the class Echinoidea, the anatomy of Diadema setosum is that of a usual sea urchin. All of the internal organs are enclosed within the spherical black test that is fundamentally the body of the organism. However, the body isn’t perfectly spherical; Diadema tests are somewhat dorso-ventrally compressed. Protruding outwards from the central body are the long spines iconic of a sea urchin’s appearance. Like the other members of the Diadematidae family, the spines of D. setosum are exceptionally long and narrow in proportion to the body. The spines, most of the time black but occasionally brown-banded, are hollow and contain a mild venom. D setosum can be distinguished from the other species within the genus Diadema by its five white colored spots on the animal’s test, intentionally located between the urchin’s ambulacra grooves.

Additionally, a clear characteristic trait of this species is the bright orange ring around the urchin’s periproctal cone, a structure that is commonly referred to as the urchin’s “anus”. A few other insignificant characteristics regarding this species include bluish colored spots on the genital plates and similar blue spots positioned in a linear fashion along its test. It lacks an apical ring, along with calcareous platelets on its apical cone. A sexually mature individual averages from 35 to 80 grams in weight. The adults average a size of no more than 70 millimeters in diameter of the test and around 40 millimeters in height.

As a widely distributed species, its range stretches throughout the Indo-Pacific basin, longitudinally from the Red Sea and then towards the east to the Australian coast. Latitudinally, it can be found as far north as Japan and as far south as the southern tip of the African east coast.

It’s been introduced into other localities not within its natural range. In the year 2006, two live specimens were found in waters off of the Kas peninsula in Turkey. The discovery and subsequent collection of these individuals makes this species the first invasive Erythrean sea urchin in the Mediterranean. Several hypotheses have been suggested for the finding of these individuals. Larvae of this species may have traveled through the Suez Canal into the Mediterranean from the Gulf of Suez, where the species has a prosperous population. An additional proposed vector is that of foreign ships bringing in individuals by means of their ballasts. A final possibility was that the individuals were intentionally released by aquarists.

This species is commonly associated with coral reefs, but also is found on sand flats and in sea grass beds. Along with the other members of the family, D. setosum is a productive grazer. They’re known to feed on various algal species that are common on tropical coral reefs. The ecological significance of the taxon as a whole has been stressed due to its herbivorous habits.

It’s been known to spawn both seasonally and year-round depending on the location of the spawning population. It’s been suggested that Diadema setosum populations are dependent on the temperature in their spawning seasonalities. Some temperatures higher than 25 degrees Celsius have been cited as a possible spawning cue. Equatorial populations are those documented to spawn at no specific times throughout the year. Other cues, such as the phases of the moon, have been observed to affect the spawning of this species. It has been found to trigger spawning events in concordance with the appearance of a full moon.

Like other venomous sea urchins, the venom of this species is only mild and not at all fatal to humans. The toxin mainly causes swelling and pain, and gradually diffuses over several hours. More danger is presented by the delivery system. The urchin’s spines, which are extremely brittle and needle-like, easily break off within flesh and are quite a challenge to extract.

Image Caption: Diadema setosum in Prague sea aquarium, Czech Republic. Credit: Karelj/Wikipedia