Ontogenetic changes in the retinal topography of the European hake, Merluccius merluccius : implications for feeding and depth distribution

Abstract

Changes in the distribution pattern of cells in the ganglion cell layer were studied in the retina of the European hake, Merlucciusmerluccius (L.), to identify the possible adaptations of visual capabilities to different bathymetric distributions and feeding habits. From early juveniles to adults, the eye diameter increased eightfold; thus, retinal surface increased dramatically with size also. In early juveniles the retinal topography of the cells in the ganglion cell layer showed a concentric arrangement with respect to the centre of the retina. Two specialised areas were found, located at the ventral and dorso-rostral periphery, where the cell density reached 47,900 cells mm–2, which corresponds to a theoretical visual acuity of 21′ (minutes of arc). The visual axes were located upwards and downwards at around 80° from the geometric centre of the retina. In juveniles, the retina underwent important changes as the concentric topographic pattern transformed: the ventral specialised area progressively disappeared, the dorso-rostral area relocated to a rostral position and a new specialised area formed in the temporal retinal region. The visual axes were directed forward and backward. For fish with a total length of 12 cm or more, a horizontal visual streak formed along the rostro-temporal axis of the retina and a new specialised area was formed in the temporo-central region of the visual streak. In adults, acute vision could be identified with the two specialised areas at the temporal and rostral periphery, where the ganglion cell density peak decreased to 3,200–3,600 cells mm–2 and the resolving power increased to 10′. As visual acuity is partially dependent on the cell types in the ganglion cell layer, cell populations in this layer were distinguished into either ganglion cells or displaced amacrine cells, using morphometric and histological criteria. The proportion of displaced amacrine cells was fairly uniform throughout the retinal surface, always representing between 32% and 39% of all cells in early and advanced juveniles. Only in adults did their density increase to 50%, probably as an adaptation to low light levels, which fish encounter as their distribution increases in depth. A small population of giant ganglion cells was also present in the retina. In the young and adult retinae, they represented 1.2% and 2.7% of the total cell population, respectively. Therefore, it has been shown how in M. merluccius the retinal topography undergoes important changes in relation to varying environmental demands.