Ontogeny of the eye of meagre (Argyrosomus regius) from hatching to juvenile and implications to commercial larval rearing

Abstract

The histological development of the visual system of meagre (Argyrosomus regius) was studied from hatching to juvenile stage (44days post hatching, dph), under aquaculture rearing conditions. The cones, horizontal, bipolar, amacrine and ganglia cells appeared already at 3dph at a total body length (TL) of 3.11±0.21mm, at the stage of mouth opening. Rods appeared at 6dph (TL 4.16±0.73mm) and increased their density continually during the monitoring period. On the contrary at the same period, the evolution of the number of cones, horizontal, bipolar, amacrine and ganglia cells in the retina were reduced. While photopic sensitivity increased over time, meagre may be considered as a species with well-developed scotopic sensitivity as well, already by 17dph (TL 8.14±1.64mm). This observation suggests that instead of the commonly used 24-h continuous high light intensities used in larval rearing protocols of meagre, lower light intensities should be employed, especially during the night hours. Histological visual acuity increased with larval development and appeared to stabilize by the early juvenile stage. Based on the estimated histological visual acuity and the theoretical distance at which rotifers and Artemia may be identified by meagre larvae, minimum prey densities should be 0.5 rotifers and 0.009Artemiaml−1 during their first use as feed items. The study demonstrated that the majority of ontogenetic events that are related with retinal differentiation and the function of the eye occurred very early during larval development in meagre. The results provide important information for the biology of the species, as well as for the optimization of larval rearing conditions regarding light management and prey item density under aquaculture conditions.