Light control of vertical movements of larvae and juvenile sole (Solea<i>solea</i>L.)

Abstract

To detail the respective role of geotaxis, phototaxis and photokinesis variations in the control of distribution and vertical migrations of sole larvae and juveniles during development, sets of experiments were carried out in 3 types of vertical tanks: cylindrical tank illuminated from the surface, cylindrical tank illuminated from below, U shape tank illuminated from one surface. Automatic detection with infrared barriers was used to record vertical movements. Under irradiances of diurnal levels, geopositivity maintained newly hatched larvae and metamorphosed animals near the bottom, despite their permanent photopositivity; old larvae exhibited a geonegative behaviour and swam in the whole water column, regardless the light orientation. During light ‐ darkness changes or under day ‐ night alternances, upwards movements occured at irradiances lower than a threshold value, characteristic of each developmental stage (5.10‐2 μW. cm‐2 in larvae, from 10‐3 to 10‐4 μW. cm‐2 in young juvenile). These upward movements were controlled respectively in young larvae by the reverse of geotaxis and a negative photoorthokinesis, in old larvae by an increase of photopositivity at irradiances of crepuscular and nocturnal levels, and in juvenile by both geotaxis reversal and photopositivity below the threshold of this geotaxis reversal. This control was clearly demonstrated by a nocturnal unilateral accumulation at the illuminated surface in U shape tank experiment. These experimental results suggest hypotheses concerning the specific role of light in the control of vertical migrations. Key words: Solea Developmental stagesVertical movementsGeotaxisKinesisPhototaxis