Echinothrix diadema (Linnaeus, 1758)

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 : 102-104

Echinothrix diadema (Linnaeus, 1758)

Specimens were collected from Malmahera ( Indonesia), Haunomu Bay, Kahe Point (Hawaii), Bougainville Is. ( Papua New Guinea), Mariana Is. ( Guam), Ignoitijala ( Maldives), Luzon ( Philippines), Dravuni Is., Taveuni Is., Yanuca Is., Suva ( Fiji), and Nouméa ( New Caledonia).

Th e tests were black with a blue-green sheen in young specimens. Th is feature was lost with age. Juveniles had very diff erent colouration to the adults. However, this was mainly associated with the spines as the test remained black.

Adult specimens had mean test diameters of 92 mm (h.d.) (SD ± 6.3 mm) and 55 mm (v.d.) (SD ± 4.4 mm). Th e maximum test diameters recorded for this species in this study were 110 mm (h.d.) by 61 mm (v.d.) from 200 specimens examined. Th e tests were not as flattened aborally as seen in E. calamaris , while the ambulacra were not conspicuously raised or the interambulacra conspicuously sunken. Th e tests had a greater vertical diameter to horizontal diameter ratio than E. calamaris and thus had a more rounded appearance. Th e ambulacra measured 22-26% of the interambulacra measured at the ambitus, increasing in width towards the periproct but not increasing in width towards the peristome.

Th e apical system was monocyclic ( Fig. 1J ) and measured 20-25% (h.d.). Th erefore, it was smaller than in E. calamaris . Th e periproct measured 12- 15% (h.d.) and had a small black anal cone (not swollen as in E. calamaris ) lacking white platelets or other distinguishing features.

Th e genital plates ( Fig. 1J ) had four to eight tubercles present, not just along their inner edge but distributed all over the plate beneath the gonopores. Th e gonopores were particularly large in this species, made more noticeable by the reduced size of the apical system.

Th e peristome was black and measured 38-46% (h.d.), being somewhat smaller than in E. calamaris .

COMPARISON BETWEEN SPECIES WITHIN GENERA

Comparisons using ordination by multi-dimensional scaling (MDS), and cluster analysis using Euclidean distances of taxonomic similarity, based on 28 test features, are illustrated in Figures 3 and 4 .

From the dendrogram ( Fig. 4 ) a distinct association can be seen between the white and brown colour morphs of E. calamaris . Although this shows a reasonable level of similarity, it is not as high as would be expected for two colour morphs of the same species (zero Euclidean distance). Th is level of similarity is considerably less than for different species within the genus Diadema . Th e MDS plot ( Fig. 3 ) substantiates this, with a large distance between the white and the brown colour morphs of E. calamaris . From this information it can be surmised that based on test structures and proportions, the brown and white colour morphs of E. calamaris are possibly separate species.

The next group incorporates all the species of the genus Diadema , as well as E. diadema . Echinothrix diadema is the first divergent species, indicating only slight association with species of Diadema . Each of the two species in a group composed of D. palmeri and D. setosum , possessed species-specific test features. This is supported by the MDS plot which indicates their disassociation from other members of the genus. The next group comprises all other species of Diadema . Diadema paucispinum is the first divergent species from this grouping, which then subdivides into two groups composed of D. mexicanum and D. savignyi , and D. antillarum and D. ascensionis . These clusterings are illustrated on the MDS plot, with D. antillarum and D. ascensionis showing the closest association.

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.

FIG. 3. — MDS plots of taxonomic similarity based on 28 test characters.

FIG. 4. — Cluster analysis based on 28 test characters (zero Euclidean distance = greatest similarity). Abbreviations: b, brown colour morph; w, white colour morph.