Ontogenic variations in the phototaxis of larval and juvenile sole Solea solea L.

Abstract

Ontogenic variations in phototaxis and photokinesis of larval and juvenile Solea solea L. were studied in horizontal tanks. Short-duration experiments were carried out under artificial white light on dark- and light-adapted animals. The results were compared with those obtained in 24-h experiments under continuous natural illumination. After 4–5-day period of neutrality, young larvae became photopositive in the range of 10 −3 to 10 μW·cm −2. Associated with the increased in ocular sensitivity, phototaxis was strongly negative at < 1 μW·cm −2, but only in short-duration experiments; in 24-h experiments, phototaxis always remained positive at diurnal, crepuscular and nocturnal irradiance. At metamorphosis, phototaxis became positive, whatever the intensity, in both short and 24-h experiments; this photopositivity persisted until juveniles reached a length of 30 mm. In 24-h experiments older juveniles were photonegative at both diurnal and nocturnal irradiance, although a slight photopositivity appeared at dusk and dawn. No phototactic circadian rhythm was demonstrated in larvae and juveniles. Kinesis was weak in very young larvae increased in 8–15-day-old larvae; it was maximum at diurnal irradiance. In metamorphosing larvae, a photoinhibition appeared at strong diurnal irradiance and kinesis became maximum at nocturnal levels of illumination. In light gradients, movements of the different stages under vertical illumination reflected the variations in phototaxis. The ecological consequences of phototactic behaviour of the young sole are discussed.