Onchidium and the Question of Adaptive Coloration

Abstract

VI. Onchidium floridanum Dall exhibits at Bermuda two chief types of pigmentation, a pale type tending to dull olive yellow, which tends to be concealingly colored, and a much more abundant type of dark blue-black appearance. No correlation can be established between the pigmentation of an Onchidium--which there is some reason to consider the result of genetic factors primarily--and the hue of the substratum over which the snail creeps in the open at low tide. O. floridanum possesses repugnatorial mantle-glands of an effective type, secreting a granular emulsion of substances having a strongly acid reaction and producing on moist surfaces of the human mouth a pronounced stinging sensation. Touch and pressure stimuli on the dorsal snrface of the mantle are characteristically involved in releasing the discharge of these glands, which shoot their contents, in the form of a fine spray, to a distance about ten times the length of the Onchidium and with conspicuous accuracy of direction toward the source of excitation. These facts are incompatible with the view that the coloration of Onchidium is determined or controlled by selection in the direction of homochromicity or concealment. Nor can they be understood in terms of "warning''' coloration.