Diadema mexicanum A. Agassiz, 1863

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 : 98

Diadema mexicanum A. Agassiz, 1863

Specimens of D. mexicanum studied in this investigation were collected from the Galapagos Islands, and from Acapulco, Mazatlan and Pochutla in Mexico. Adult specimens had a mean horizontal test diameter of 75 mm (SD ± 6.1 mm) and a mean vertical diameter of 40 mm (SD ± 4.2 mm). Maximum test diameters of 92 mm (h.d.) and 46 mm (v.d.) were recorded from 48 specimens examined.

As reported by H. L. Clark (1948), the test was black with a brown tinge. The patterns of iridophores were similar to those seen in D. antillarum and D. ascensionis . However, like D. ascensionis , the blue colour was not bold or distinct. Mortensen (1940) correctly reported the presence of faint blue lines that proceeded from each side of the genital pore down to the ambitus where they bisected the naked median areas. These faint parallel lines were not always continuous, with the presence of small side branches between secondary spines. Mortensen (1940) also reported that there were no white spots in the adapical naked interambulacral area. Obvious white spots were not typically seen on specimens during the day. However, at night distinct white spots with faint white lines were seen leading from the naked median areas to the genital pores. These features were visible on the denuded test of some specimens ( Fig. 2B , faint white marking surrounded by area of darker pigment), but were not visible on all specimens.

The shape of the test was distinctly rounded pentagonal rather than horizontally circular and more flattened in appearance than D. antillarum . The ambulacra measured 25-30% of the interambulacra measured at the ambitus.

The apical system ( Fig. 1C ) was slightly depressed, hemicyclic and measured 20-25% of the test’s horizontal diameter. The periproct measured 12-16% (h.d.) and was black with a small anal cone. The genital plates ( Fig. 1C ) were similar in shape to those of D. antillarum . However, they had fewer tubercles (between zero and two) while the genital pores were typically larger. Similar to D. savignyi , a distinct arch-shaped depression was present on the genital plates in both juveniles and adults, forming the corners of the pentamerous apical ring.

The peristome was deep brown in colour and measured 45-50% (h.d.).

Figures

FIG. 2. — A-I, photographs of the median areas of the interambulacra, showing the variation in characters that occur: A, Diadema antillarum; B, D. mexicanum; C, D, D. savignyi; E, D. setosum; F, Echinothrix calamaris (brown colour morph); G, E. calamaris (white colour morph); H, E. diadema; I, D. palmeri; J, D. palmeri (whole specimen). Scale bars: A-I, 5 mm; J, 50 mm.

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.