Histological Structure of Retina and Visual Characteristics of Turbot Scophthalmus maximus Fry

Abstract:

Knowledge on fish retina structure and associated characteristics helps better understanding their feeding behaviors. In this study we studied the structure of retina and visual characteristics of fries of Turbot Scophthalmus maximus using paraffin sectioning method. The experimental water temperature was (20.5±2.0)℃ and the water salinity was 29±1. Healthy fries were fixed after certain periods of light or dark adaptation that included newly hatched, 2 d, 5 d, 10 d, 16 d, 20 d 23 d, 26 d, 30 d, 34 d, 39 d, 43 d and 50 d. We also measured the pigment index (PI) after the adaptation of retina, the thickness of outer nuclear layer, internal nuclear layer and ganglion cell layer of retina, and the amounts of cone cells (CC), ganglion cells (GC) and nuclei of the outer nuclear layer (ONN) on the retina cross section. It was observed that the pigment layer and cone cell layer appeared in the retina of 2 d larvae, which was the visual support for the first feeding. The retina of 5 d larvae showed no motion response. From 16 d larvae to 30 d juveniles, the difference in the retinal PIs after light and dark adaptation (the former subtracted by the latter) significantly increased (P<0.01), which indicated the motion response in retina. During the development there was reduction in the amounts of CC and GC in the retina (P<0.01), while there was an increase in the amounts of ONN, the ratio of ONN to CC, and the ratio of ONN to GC (P<0.01). The retinas of turbot fries possessed high visual acuity and low photosensitivity before metamorphosis, however the former dropped and the latter increased after metamorphosis. This adapted to the weak light stimulation when their lifestyle changed from planktonic to benthic. The photo sensing system of post-metamorphosis benthic turbot was deficiently developed.