Diadema ascensionis Mortensen, 1909

Coppard, Simon Edward & Campbell, Andrew C., 2006, Taxonomic significance of test morphology in the echinoid genera Diadema Gray, 1825 and Echinothrix Peters, 1853 (Echinodermata), Zoosystema 28 (1), pp. 93-112 : 97-98

Diadema ascensionis Mortensen, 1909

Specimens of D. ascensionis examined in this investigation were collected from the South Atlantic islands of Ascension and St. Helena. Adult specimens had a mean horizontal test diameter of 70 mm (SD ± 5.2 mm) and a vertical diameter of 40 mm (SD ± 3.4 mm). Maximum test diameters of 84 mm (h.d.) and 46 mm (v.d.) were found from 28 specimens examined.

Test colour was black, commonly with a red tinge. This was more noticeable in juvenile specimens. The same pattern of iridophores occurred in this species as in D. antillarum and D. savignyi , with lines of iridophores down the mid-lines of the interambulacra and a ring around the apical system. This feature was not as obvious as in the afore-mentioned species. White dots were present on the naked median areas of the interambulacra, being more obvious at night, not always visible during the day.

The shape of the test was very similar to that of D. antillarum . The ambulacra measured 25-30% of the width of the interambulacra measured at the ambitus and as reported by Mortensen (1940) showed no variation to that of D. antillarum .

The apical system ( Fig. 1B ) was distinctly depressed, hemicyclic and measured 20-26% (h.d.) in adults, a slightly larger percentage of the test’s horizontal diameter in juveniles. The periproct measured 12-18% (h.d.), with a small black anal cone with no distinguishing features. Both the apical system and the corresponding periproct tended to be larger in this species than in D. antillarum . The genital plates ( Fig. 1B ) had fewer tubercles (between zero and two) than D. antillarum , with distinct arched depressions seen on the denuded tests of juveniles. These markings tended to fade with age.

The mid-lines of the interambulacra on the naked test had no obvious markings. On living specimens, thin lines of iridophores were found in these areas. These lines bisected the naked median “spot” areas of the interambulacra.

The peristome was black with a purple tinge and measured 40-50% (h.d.).

Figures

FIG. 1. — Drawings of the apical systems, showing species specific characters of the periproct and genital plates: A, Diadema antillarum; B, D. ascensionis; C, D. mexicanum; D, D. palmeri; E, D. paucispinum; F, D. savignyi; G, D. setosum; H, Echinothrix calamaris (brown colour morph); I, E. calamaris (white colour morph); J, E. diadema. Scale bars: 5 mm.