On the Rôle of an Integumentary Pigment in Photoreception in Holothuria

The experiment recorded in this note was undertaken in 1917, with a view to testing the notion (Crozier, 1914-15) that the fluorescent, greenish yellow, integumentary pigment characteristic of some members of the genus ttolothuria is concerned with their photic sensitivity. A functional connection of this sort was suspected because the presence and relative amount of the pigment seemed to be correlated with the degrees of photic stimulation in three species of Holothuria studied at Bermuda; a pigment of similar nature seems to occur (Crozier, 1917) in the West Indian Actinopyga agassizii which, like these species of Holothuria, is negatively heliotropic. Dubois (1914), in his book, had earlier suggested a photosensitizing r61e for the fluorescent (dichromatic) substances occurring in various worms and echinoderms. I t was expected that the stimulating power of sunlight transmitted by a screen of this pigment might be different from that of sunlight directly falling upon one of these holothurians. I t is known that in photochemical action the effects of light are due to the absorbed wave-lengths. With the skin pigment of Holothuria there is pronounced absorption in the visible spectrum. Hence it was assumed that light deprived of these absorbable rays, by preliminary passage through a layer of the pigment, might perhaps be less effective for excitation. This expectation was not realized. The species employed was 11olothuria captiva, a form very sensitive to light, which was usually found under stones; in dark caves, however~ it was found creeping freely over the walls, showing that its occurrence under stones along the shore is in all probability due to its photic irritability. 57