Physical Description
The Diadema genus consists of 6 species, which can be difficult to identify as they all feature the long black spines that are characteristic of the genus (except for D. palmeri which is distinct in its red pigmentation) (Muthiga & McClanahan, 2007). Species of Diadema can be separated by external characteristics such as spine morphology, test colour and pedicellarie shape. The two species of Diadema that are common on Heron Island, D. setosum and D. savignyi, look identical at first glance, however with a closer look can easily be distinguished in the field.
Both D. savignyi and D. setosum grow to approximately 80mm in diameter, and have a round test that is slightly flattened in profile. Spines of both species are brittle and long, and may be up to 30cm in length. Sharp and tapered, the spines are round cross-sectionally and hollow. Tiny barbs circumnavigate each spine and point towards the body of the urchin, making the spines rough to touch from tip to base. Primary spines are longer, especially over the ambitus, and thicker than the secondary spines. Most individuals have dark purplish black spines, however some may have banded or completely white spines (Gopalakrishnakone, 1990).
The body of both urchins is primarily black; however the two species can be distinguished by coloration patterns: D. setosum has 5 iridescent blue spots on the interambulacral areas on the test and an orange ring around the tip of the anal cone, while D. savignyi has continuous blue lines down the interambulacra and a black or blue ringed anal cone. Pedicellaria of both species are tridentate, however are wide and spoon-like in D. setosum, and slim and needle-shaped in D. savigny (Lessio & Pearse, 1996).
The iridescent blue patterns on the test and surrounding the anal pore
of D. savignyi.
A microscopic view of a D. savignyi primary spine. Note the tiny barbs
that whorl the spine, making the spine rough to touch from tip to base.
 
|