Introduction
Environmental conditions are a crucial part of shaping the development and organization of the visual system in fish. In aquatic environments light variability, physicochemical properties of water and habitat complexity are only a few factors that directly influence visual performance, while they may also include structural adaptations within the visual system . The retina, as the primary site of photoreception and initial signal processing, exhibits considerable plasticity in response to environmental conditions. In natural habitats fish are exposed to multifaceted and dynamic visual stimuli, which stimulate complex development of retinal structure . In contrast, artificial systems, such as recirculating aquaculture systems (RAS), provide relatively simplified and controlled environments with reduced sensory input . These differences in the fundamental charactersitics of the environment may influence the development and organization of the visual system in farmed fish species. European perch (Perca fluviatilis) is a visually oriented predator which in latest years has display increasing importance in aquaculture. While the majority of its biology is well studied, the effects of domestication on perch vision are poorly understood. Considering short history of domestication and recent technological breakthroughs in production systems of this species , European perch represents a suitable model for studying environmentally-driven visual plasticity. Therefore, this study aimed to compare the retinal structure and optic tectum organization of wild and domesticated perch to assess the influence of rearing conditions on the visual system.
Materials and Methods
European perch individuals were collected from two environments: a domesticated population reared in RAS (n = 25) and a wild population from pond conditions (n = 25). Morphometric measurements included standard length, total length, body mass, eyeball diameter and pupil diameters. Whole eyeballs were dissected and fixed (Davidson's solution) for histological analysis using standard paraffin techniques and staining protocols (HE and RGB). Retinal morphometry was conducted on microphotographs (n = 36), with retinal layer thickness expressed as a percentage of total retinal thickness. Statistical analysis included one-way ANOVA with significance set at p < 0.05.
Results
Morphometric measurements proved that domesticated fish exhibited significantly lower standardized eye diameter along with higher standardized shortest pupil diameter compared to wild individuals. Measurements of retinal layer thickness revealed distinct differences in retinal organization. Wild fish showed greater proportional thickness of inner retinal layers (ganglion cell layer, inner plexiform layer and inner nuclear layer), while domesticated fish exhibited higher proportions of outer layers (outer plexiform layer and outer nuclear layer). Additionally, domesticated fish showed greater proportional thickness of photoreceptor layer (both inner and outer segments). No significant differences were noted in the thickness of the nerve fibre layer between analyzed populations.
Discussion
The observed differences indicate that environmental conditions strongly influence the structural organization of the visual system in European perch. Reduced eye size in domesticated fish may reflect decreased reliance on high-resolution vision under simplified and predictable conditions, while increased pupil size may compensate for reduced light capture efficiency . Enhanced development of inner retinal layers in wild fish suggests greater capacity for signal integration and processing, consistent with the demands of complex natural environments. In contrast, the increased contribution of photoreceptor layers in domesticated fish indicates a shift toward primary signal reception and modulation under reduced sensory input .
These findings demonstrate that domestication drives functional reorganization of the eyes of European perch.
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