A countershading reflex in cephalopods

Abstract

Most cephalopods have more chromatophores on the dorsal body surface than on the ventral and these tend to be kept tonically expanded. As a result the dorsal surface is usually darker than the ventral, an effect shown by many animals and known as countershading. We report here that when Sepia officinalis, Loligo vulgaris and Octopus vulgaris are rotated 180 degrees around the longitudinal body axis the ventral chromatophores expand, causing darkening, while the dorsal chromatophores retract, causing paling. When animals are rotated through only 90 degrees the chromatophores on the uppermost half of the ventral and dorsal surfaces expand, while those on the lower half retract. This response, which we term the countershading reflex, can be abolished by ablating the statocysts; and experiments in which the direction of incident light is reversed show that the reflex is not driven by sensory input from the eyes. The function of the reflex is presumably to maintain countershading while the animal is momentarily disoriented; this idea is supported by the fact that it lasts only a few seconds.