The occurrence of melanin in the sea-urchin, Diadema antillarum Philippi.

The characteristic dark color of Diadema antillarum is chiefly attributable to a black pigment, most of which is contained in large epidermal chromatophores (see Fig. 1). A red pigment is also present, but is easily separable from the black, being readily soluble in ethanol and acetone, whereas the black is not. The nature of the red pigment is unknown and we have not studied its properties. Correlated with the amount of black pigment is the remarkable sensitivity of the animal to changes in light intensity (1). The superficial location of the concentrated pigment (Fig. 1), makes it readily accessible, and cytolytic agents in disrupting the extremely delicate epidermis, liberate large quantities of pigment. Indeed, avoiding contamination of the sea water in which the animals are kept, by escaping pigment, is a major problem in the laboratory. Loss of vitality, temperature changes, dilution of the sea water etc., all bring about copious discharge of the black pigment. Large quantities are therefore easily obtained. Diadema antillarum and closely allied species, occur throughout the Antilles, from Florida and Bermuda to Surinam, in the warmer waters of the eastern Atlantic, the Indo-Pacific and from the Gulf of California to the Gulf of Panama (2). Only Diadema antillarum has claimed our attention, but it would be surprising indeed if the pigment of allied species did not prove equally suitable and accessible for study. In view of this, and the abundance of these animals, it was considered appropriate to draw the attention of investigators to these facts.