The response of squid and fish to changes in the angular distribution of light

Abstract

This paper describes the responses of a squid (Alloteuthis subulata) and a fish (Trachurus trachurus) to changes in the angular distribution of light.  An apparatus was made that simulated the angular distribution of daylight in the sea. The apparatus enabled the direction of the brightest light to be changed and the positions of the animals in response to these changes were observed. Squid viewed head-on were observed to roll by a maximum of 20° when the incident light source was at angles between 20° and 90° (where 0° is vertically downwards). When viewed laterally, i.e. in the pitch plane, the squid were observed to position themselves more closely with respect to the angle of the light source, they swam in a near vertical plane when the incident light source was at an angle of 90°. Swimming movements in the roll and pitch plane became more horizontal with positions of the light source between 90 and 180°. Horse mackerel, in contrast, inclined their dorsal surfaces to almost perfectly match the angle of the incident light source, even swimming upside-down when light came from below. These experiments also revealed that squid display a counter-shading chromatophore pattern (‘Flexible Countershading’) in response to light coming from the sides, which involves darkening the side of the body facing the brightest light. The use of chromatophores in this way may explain why the dorsal light reflex in squid is so weak compared to that of fish.