A Review of Adaptive Mechanisms in Fish Retinal Structure and Opsins Under Light Environment Regulation

Abstract

Light, as one of the most crucial environmental factors, plays an essential role in the growth, physiology, and evolutionary survival of fish. To cope with diverse light conditions in aquatic environments, fish adapt through photosensory systems composed of both visual and non-visual pathways. The retina is a key component of the visual system of fish, capable of converting external optical signals into neural electrical signals, making it crucial for visual formation. During the process of visual signal transduction, opsins serve as the molecular foundation for vision formation. They can be divided into two major categories: visual opsins and non-visual opsins. Among these, melanopsin, as a member of the non-visual opsin family, acts as a key upstream factor in the circadian phototransduction pathway of fish. In this review, we review the adaptability of fish retinal structures to light reception and introduce in detail the gene diversity and relative expression levels of fish opsins. At the same time, we comprehensively describe the molecular mechanism by which fish adapt to changes in the underwater light environment. We also concluded that melanopsin, as a non-imaging photoreceptor, possesses not only core light-sensing functions but also non-imaging visual functions such as circadian rhythm regulation, body coloration changes, and hormone secretion. This review suggests that future research should not only elucidate the physiological functions of melanopsin in fish but also comprehensively reveal the mechanisms underlying the multi-adaptive nature of fish vision across varying light environments. Through these studies, researchers can have a deeper understanding of the physiological regulation mechanism of fish in complex light environments, and then formulate fish light environment management strategies, optimize aquaculture practices, improve economic returns, and promote the development of related fields.

1. Introduction

As an ecological factor, a light environment mainly includes three elements: spectrum, light intensity, and photoperiod. Light plays a crucial role in various biological processes of fish, including growth and development, feeding and digestion, and biological rhythms [1,2]. Light is essentially a form of visible electromagnetic radiation composed of photons, characterized by both particle-like and wave-like properties [3]. Its parameters, including wavelength, phase, polarization direction, and propagation direction, vary considerably. Light can travel through transparent substances such as air and water. In natural water bodies, natural light is distributed differently in different depths of water bodies through refraction or reflection [2,4]. Owing to species-specific differences, varying developmental stages, and alterations in the ambient light environment of their habitats, fish are perpetually exposed to complex and dynamic photic conditions. This requires the visual system of fish to undergo adaptive adjustments to accommodate external lighting conditions.

In the study of fish vision, the retina is recognized as a crucial tissue for photodetection and signal transduction. Its laminated structure and cell populations provide the material basis for receiving and transmitting light signals [5]. However, relying solely on the retinal-level structure is insufficient to explain the photoadaptation exhibited by fish under different light environments. With the continuous advancement of research and the development of molecular biology and genomics, the study of the opsin family has increasingly become central to investigations of fish photoadaptation. Opsins enable fish to adapt to varying light environments through gene duplication, divergence, and differential expression. These processes provide crucial molecular information, thereby expanding their visual adaptive mechanisms and enhancing their ability to perceive and respond to diverse photic conditions [6,7]. Furthermore, melanopsin, as a class of non-image-forming photoreceptors, not only reveals the significant functions of the non-visual system in circadian regulation and physiological modulation but also broadens the understanding of the fish photosensory system [8].

While current research has deeply explored the phenomenon of photoadaptation in fish from perspectives such as retinal structure and opsin genes diversity, these perspectives have largely remained focused on isolated levels, resulting in a fragmented knowledge base within this field. The central aim of this review is to propose an integrative research framework, integratively analyzing from structure to molecule and from the visual to the non-visual systems, thereby systematically linking adaptive mechanisms across different levels.

Building upon a systematic review of existing research, this review is organized into three levels: I. It introduces the impact of the light environment on the retinal architecture of fish, highlighting its fundamental role in visual adaptation. II. Focusing on opsins, it delves into how gene diversity, differential expression, and regional co-expression aid fish in coping with diverse light environments. III. Extending the discussion to melanopsin, it explores its roles in circadian rhythms, hormone secretion, and body color regulation. This review aims to provide a comprehensive reference for understanding the physiological regulatory mechanisms underlying fish adaptation to complex light environments and to offer insights into potential future research directions and application prospects.

2. Light Environment and Retinal Structure

Based on wavelength, light is classified into ultraviolet, visible, and infrared light. Visible light is further subdivided into three spectral ranges: long-wavelength (590–740 nm), medium-wavelength (500–590 nm), and short-wavelength (380–485 nm) light. The aquatic light environment is highly dynamic. In seawater, for instance, spectral composition varies significantly across different water depths. This is due, in part, to the differential absorption efficiency of seawater for various wavelengths: long-wavelength light is absorbed rapidly, while short-wavelength light is more prone to scattering. On the other hand, the physicochemical properties of the water body also influence light transmission through processes such as refraction. These include turbidity, suspended particles, colored dissolved organic matter (CDOM), and phytoplankton [4]. The combined effects of these factors shape the survival environment inhabited by fish and other aquatic organisms. Generally, the surface layer of natural seawater exhibits a full spectrum (400–800 nm). The benthic zone in coastal waters is dominated by a green light spectrum (495–570 nm), whereas the benthic zone of oceanic waters is characterized by a blue light spectrum (450–495 nm) (Figure 1) [4,9]. Current research on the effects of the light environment has progressed significantly, focusing primarily on five key areas: growth [10,11], physiology [12], vision [13], body coloration [14], and behavior [15]. Among these, vision holds a dominant position within the sensory system; for fish, it is a vital perceptual tool for survival, reproduction, and environmental adaptation.

The retina, serving as the physiological foundation for vision formation in fish, is a crucial structure. It is the primary site where light signals are converted into electrical signals. Similarly to other vertebrates, the fish retina is composed of ten distinct layers (exceptions may exist). These include the pigment epithelium layer (PEL), photoreceptor layer (PCL), outer limiting membrane (OLM), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL), ganglion cells layer (GCL), nerve fiber layer (NFL), and inner limiting membrane (ILM) (Figure 2) [16]. Each layer has a specialized function, working in concert to ensure visual perception and information transmission. The integrity of the retinal structure and its morphological changes reflect adaptive adjustments of the fish retina in response to variations in the light environment.

2.1. Adaptations of Fish Retinal Structure to Different Light Environments

Variations in the light environment can induce adaptive modifications in retinal structure, thereby enhancing visual photosensitivity under different photic conditions—a phenomenon that has been clearly documented across multiple fish species. For example, the flashlight fish (Anomalops katoptron), which inhabits nocturnal and dim light environments, exhibits a relatively high thickness ratio of its outer nuclear layer to inner nuclear layer [17]. The Korean rockfish (Sebastes schlegelii), dwelling in blue light-dominated water layers, possesses a thicker inner nuclear layer to enhance visual resolution and color discrimination [18,19,20]. Studies have found that under green light illumination at 1000 mW/m² and 1500 mW/m², the thicknesses of the retinal pigment epithelium layer, inner nuclear layer, and ganglion cell layer in the tiger puffer (Takifugu rubripes) increase in response to light stimulation [21]. In addition, prolonged light exposure can result in the thinning of the photoreceptor layer in the European sea bass (Dicentrarchus labrax), Atlantic cod (Gadus morhua), and Atlantic salmon (Salmo salar) [15,22]. Therefore, an appropriate light environment is a crucial condition for fish growth and development. Prolonged exposure to unsuitable light conditions can disrupt the function of their visual system, impair their ability to perceive the light environment, and consequently adversely affect their growth and survival. In summary, fish effectively adapt to varying light conditions through adaptive changes in the relative thickness of the retinal layers. However, interspecies variations exist. To elucidate the response patterns of their retinae under varying light conditions, it is necessary to conduct species-specific studies to better understand the adaptive mechanisms of the fish visual system.

2.2. Effects of the Light Environment on Retinal Cells

The adaptation of the retina to changing light environments involves not only structural alterations and variations in cell number but is also influenced by cellular properties (such as different cone cell types) as well as their specific arrangement and density within the retina [23,24]. Photoreceptor cells, as the primary structural components of the visual system, are chiefly categorized into two broad types: rod cells and cone cells. The quantity and spatial distribution of these two cell types determine the level of photosensitivity and visual acuity in the fish retina [13]. Furthermore, the total number of ganglion cells, the types of opsins, and their amino acid sequences are also key factors regulating the photosensitivity and visual acuity of the fish retina [23,24,25]. Rod cells, characterized by higher photosensitivity, are primarily responsible for light detection under dim light conditions. Cone cells, with lower photosensitivity, are mainly involved in photodetection and color vision under bright light. For example, due to their long-term habitation in dim light environments, the retinas of deep-sea fish have mostly evolved to contain only rod cells. In addition, research has found that some deep-sea fish (e.g., in pearlsides) have developed a unique type of photoreceptor: the rod-like cone cell [25,26]. Studies indicate that the tongue sole (Cynoglossus semilaevis) exhibits a lower proportion of cone cells in dim light environments to enhance its photosensitivity during nocturnal or deep-water activities [27]. The mandarin fish (Siniperca chuatsi) undergoes significant visual adaptations during its development. In the visual transition period, the outer segments of its rod cells elongate markedly to enhance low-light predation. Furthermore, when shifting from surface-feeding to ambush-feeding near the substrate, the distribution density of cone cells in the ventral retina increases significantly, thereby improving its hunting efficiency [28]. For surface-dwelling fish, the rod cells of the zebrafish (Danio rerio) are generally shorter [29,30], In contrast, the diurnally active crucian carp (Carassius auratus) possesses abundant populations of both rod and cone cells, thereby maintaining flexible visual adaptation in changing light environments [17]. In a blue light environment, the medium-to-long-wavelength-sensitive cone cells of the blue acara (Aequidens pulcher) significantly increase their outer segment length to maintain color vision balance, while the number of short-wavelength-sensitive cone cells decreases by 65% [31]. Collectively, these studies demonstrate that the number of retinal cells in fish can adaptively adjust in response to changes in the ambient light environment of their habitat. This phenomenon not only reveals the long-term adaptation of fish to distinct ecological niches but also confirms the important role of the light environment as a driving force in shaping the diversity of their visual systems.

There are many shared characteristics between the visual systems of fish and humans [32,33]. The main differences lie in the accessory ocular structures and photoprotective mechanisms. For instance, teleost fishes typically lack eyelids (though some possess an adipose membrane) and cannot protect the retina from intense light damage by adjusting the iris; therefore, fish often adapt to their ambient light environment by adjusting the distribution of melanin granules in the pigment epithelium layer or the expression of photoreceptors, thereby reducing damage to the retina [34,35,36,37]. Furthermore, MCH (Melanin-Concentrating Hormone), as a byproduct of melanin and a key regulator of physiological body color change, modulates the aggregation of pigment granules within chromatophores via the G protein-coupled receptor (Melanin-Concentrating Hormone Receptor, MCHR). This leads to body color lightening while simultaneously protecting the retina through the melanin granule layer [32,33]. The goldfish (Carassius auratus) retina exhibits extensive cell apoptosis, and the expression level of MCHR continuously increases [38,39]. Therefore, this unique adaptive mechanism not only protects the retina from light-induced damage but also enables fish to better adapt to different light environments, thereby enhancing their survival capacity.

Overall, the response of retinal structure to the light environment can be regarded as the biological foundation of visual adaptation. To meet the demands of specific light environments, the thickness and cellular composition of the various layers in the fish retina undergo adaptive changes. This optimizes the efficiency of light signal transmission and ensures the accurate perception of environmental information. However, research on fish retinal structure exhibits relative rigidity and may struggle to meet the demands of rapidly changing environments. Therefore, deeper adaptive mechanisms rely on molecular-level regulation, specifically manifested in the diversity of opsin genes and the plasticity of their expression control.

3. Molecular Adaptation of Opsins to the Light Environment

Opsins belong to the G protein-coupled receptor (GPCR) family [40]. Following light absorption by a visual pigment, a chromophore undergoes isomerization and dissociates from an opsin. The activated opsin then engages transducin (a G protein), initiating a phosphorylation cascade. This cascade leads to the closure of ion channels in the cell membrane, resulting in membrane hyperpolarization. The resulting electrical signal is transmitted to the brain, where visual perception is generated [41]. Its primary function is to combine with a chromophore to form visual pigments, participating in fish adaptation to the external light environment, and it serves as the molecular basis of vision [42]. Similarly to most vertebrates, fish typically possess five classes of visual opsins: rhodopsin (RH1) in rod cells, along with four cone opsin classes—short-wavelength-sensitive type 1 (SWS1, ultraviolet-sensitive); short-wavelength-sensitive type 2 (SWS2, blue-sensitive); medium-wavelength-sensitive (RH2, green-sensitive); and long-wavelength-sensitive (M/LWS, red-sensitive) (Figure 3) [43,44]. As research has progressed, opsin expression has been found not only in rod and cone cells but also in various other tissues, including the pineal gland [45], retinal ganglion cells [46], pigment epithelial cells [47], brain [48], skin [49], kidneys, and heart. Interestingly, opsins are also expressed in in vitro cultured cells [50,51].

In studies of light environment adaptation, the mechanisms of opsins involve multiple levels, for instance, the genetic repertoire provided by gene diversity; differential expression allows fish to achieve context-dependent tuning of visual spectral sensitivity in response to developmental and environmental changes; and regional co-expression leading to the specialization of spectral sensitivity zones within the retina. These mechanisms work in concert to significantly optimize visual perception.

3.1. The Diverse Repertoire of Fish Opsin Genes

Opsin genes play a crucial role in the evolution and functional diversity of fish. During long-term evolutionary processes, fish have continuously adjusted their visual systems to adapt to dynamically changing external light environments. Although the fish visual system retains a repertoire of five major opsin genes families, distinct species-specific differences exist in the preservation and copy number of these genes due to variations in evolutionary history and habitat environment. Studies have found that cartilaginous fishes such as the shark (Prionace glauca) and ray (Okamejei kenojei) lack the short-wavelength-sensitive opsin genes (SWS1 and SWS2). This evolutionary adaptation likely helps them avoid ocular damage from short-wavelength light in aquatic environments [52]. Additionally, fish can also continuously increase the diversity of their opsins through the duplication and divergence of opsin genes. Generally, freshwater fish tend to possess a greater number of LWS genes compared with marine fish. In contrast to shallow-water species, deep-water fish not only have more RH2 and SWS2 genes for perceiving medium- and short-wavelength light, but also involve the dim-vision RH1 gene, showing a marked distinction from their shallow-water counterparts. For example, the guppy (Poecilia reticulata) genome contains four LWS genes [51]. The Pacific bluefin tuna (Thunnus orientalis) possesses five RH2 genes [53]. The benthic tongue sole (Cynoglossus semilaevis) lacks the SWS1 gene [54]. The deep-sea silver spinyfin (Diretmus argenteus) possesses as many as 38 RH1 genes [55].

Gene duplication serves as the core driving force for the diversity of fish opsin genes. As the degree of divergence among different gene subtypes evolves throughout the evolutionary process, their color vision function may also undergo adaptive changes. Duplication of the RH2 and LWS genes is among the most common occurrences in opsins. For instance, the zebrafish (Danio rerio) possesses four RH2 genes, and the Japanese medaka (Oryzias latipes) has three RH2 genes [56,57]. Consequently, the fish visual system enhances its adaptability to the light environment through such gene duplication events. Studies have shown that the divergence of fish opsin genes is also closely correlated with water layer depth. A study on the evolution of RH1 in 11 species of squirrelfishes and soldierfishes (Beryciformes, Holocentridae) revealed that the λmax (lambda max) of RH1 in species inhabiting water layers of 0–10 m underwent a red shift, while that of species dwelling below 60 m exhibited a blue shift, thereby enabling survival adaptation across varying water depths [58].

It is noteworthy that some opsin genes may also degenerate into pseudogenes during the evolutionary process. For example, as the spotted halibut (Verasper moseri) develops and transitions its habitat from shallow to deeper waters, this change in light environment leads to the inactivation of its RH2A gene due to a premature stop codon [59]. Similarly, the RH2-2 gene in the tiger puffer (Takifugu rubripes) and the spotted green pufferfish (Tetraodon nigroviridis) has also evolved into a pseudogene over the long term, likely related to alterations in their photic environments [60]. These results indicate that the opsins gene family not only provides functional diversity through duplication but may also achieve functional optimization through degeneration.

Collectively, the diversity of opsin genes provides fish with a broad range of spectral sensitivity. The flexibility of these genes enables diverse species to undergo adaptive changes across various light environments, thereby playing a key role in ecological niches and population differentiation.

3.2. Differential Expression of Fish Opsin Genes in Response to Different Light Environments

In addition to gene diversity, the differential expression of opsin genes is also a key mechanism for fish to adapt to complex and varied light environments. Research indicates that the five RH2 opsin paralogs generated through gene duplication in the Pacific bluefin tuna (Thunnus orientalis) exhibit differential expression in the deep-sea environment. Amino acid sequences in some paralogs have undergone a blue shift, contributing to their diverse visual system and adaptation to complex predatory light environments [55]; the expression levels of RH2B1, SWS1, and SWS2 in the turbot (Scophthalmus maximus) also show significant variation under different spectral environments [61]. Likewise, the expression of the LWS gene in the guppy (Poecilia reticulata) also varies with light spectrum, being significantly higher under orange light than under green light [62]. This indicates that changes in the light spectrum may influence opsin expression levels. Similarly, changes in light parameters, such as photoperiod and intensity, can also modulate the expression of opsin genes. For instance, after 30 days, opsin gene expression under constant light (24L:0D) was significantly higher than that in the 0L:12D (0 h light/12 h dark) control group [63], while the expression levels in the tiger puffer (Takifugu rubripes) also varied with changes in green light intensity, as previously reported [20].

Fish experience varying light environments across different developmental stages, and their opsin expression undergoes corresponding alterations. For instance, in black porgy (Acanthopagrus schlege), RH1 expression comprises 60% of total opsin in larvae; this proportion rises to 90% in adults. This process coincides with the juvenile fish’s transition from shallow to deeper water habitats, where elevated RH1 expression adapts to the changes in the ambient light environment [64]; when juvenile sockeye salmon (Oncorhynchus nerka) migrate from shallow, ultraviolet-rich (<390 nm) waters to deeper zones, their SWS1 opsin expression declines accordingly [65]. Similarly, concomitant with the crimson snapper’s (Lutjanus erythropterus) shift from a pelagic to a benthic existence, the predominant opsin in its retina is replaced from LWS to RH1 [66]. In rainbow trout (Oncorhynchus mykiss), during individual development, single cone cells progressively decrease the expression of SWS1 and increase the expression of SWS2 [67]. In addition to differences in opsin expression driven by developmental habits, alterations in the habitat environment of fish can also lead to the differential expression of opsin genes. Research indicates that in the bluefin killifish (Lucania goodei), the expression of SWS2B and SWS1 increases with higher water clarity, while LWS and RH2 expression shows the opposite trend. Similarly, in adult red shiners (Cyprinella lutrensis), LWS expression increases with elevated turbidity, thereby enhancing their sensitivity to red light [68]. These findings indicate that, due to differences in growth habits and the light environments of their habitats, various fish species can meet their survival needs by enhancing photosensitivity through opsin expression.

Taken together, through the dual strategies of opsin gene diversity and differential expression levels, fish effectively respond to complex and variable light environments. These two strategies constitute the core molecular foundation of their visual adaptation.

3.3. The Regional Co-Expression of Fish Opsin Genes

The expression of opsins within the retina exhibits specific spatial characteristics. This feature contributes to some extent to the differentiation in spectral sensitivity across different retinal regions, and may well be correlated with the ambient light environment. For example, a study on the African cichlid (Hemitaeniochromis) revealed that its retinal dorsal and ventral regions co-express RH2Aα, RH2Aβ, RH2B, and LWS [69]. This establishes a regional differentiation in spectral sensitivity, thereby optimizing photoreceptive capabilities and enhancing visual adaptability. This finding may also be related to behavioral preferences. For instance, in behavioral preference experiments, the bighead carp (Aristichthys nobilis), an inhabitant of mid-upper water columns [70], displays a pronounced preference for the red and white spectrum, whereas the benthic fish schizothoracin (Schizothorax prenanti) shows a distinct preference for the green and blue spectrum [71]. This indicates that fish may also possess a certain degree of spectral selectivity, which is likely related to factors such as their habitat and foraging environments [72]. Therefore, regional co-expression provides spatial flexibility for visual adaptation. This mechanism, working in synergy with gene diversity and expression plasticity, collectively constitutes the molecular strategy enabling fish to cope with complex light environments.

In summary, opsins play a central role in the photoadaptation of fish. Their gene diversity, achieved through duplication and evolution, provides a broad range of spectral sensitivities. Differential expression endows the visual system of fish with plasticity to adapt to changing growth habits and environmental shifts, thereby accommodating gradually varying light conditions. As for regional co-expression, it further optimizes the spatial distribution of photosensitivity. These three aspects work synergistically, collectively establishing the molecular adaptive mechanisms by which the fish visual system responds to complex light environments.

4. Diversity of the Fish Non-Visual Photosensory System and Physiological Roles of Melanopsin

4.1. Overview of Melanopsin

In addition to the photoreceptive opsins associated with the visual system, fish also possess a unique class of non-image-forming photoreceptor proteins within their non-visual systems—melanopsin (typically encoded by the opn4 gene) [8]. Melanopsin was first discovered in the skin melanophores of the African clawed frog (Xenopus laevis) [73] and was later widely reported in the retinal ganglion cells of vertebrates [74]. Studies on zebrafish (Danio rerio) have shown that eyeless and pinealectomized larvae can still exhibit tracking behavior toward a moving light source, indicating that melanopsin located in deep brain regions mediates non-image-forming photosensation [75]. With the discovery of melanopsin, it has progressively been recognized as a crucial peripheral circadian photoreceptor in fish, playing a significant role in various physiological processes.

Like other opsins, melanopsin belongs to the G protein-coupled receptor family, sharing common structural features such as seven transmembrane α-helices, an intracellular carboxyl terminus, an extracellular amino terminus, and a lysine residue in the seventh transmembrane domain. The key differences lie in the presence of a “DRY” (Asp-Arg-Tyr) tripeptide motif at the top of the third helix in melanopsin and its relatively low amino acid sequence similarity to other opsins [73,75]. Based on evolutionary relationships, melanopsin has diverged into two distinct paralogous gene types, opn4m and opn4x [76,77]. In fish, melanopsin is not only present in the intrinsically photosensitive retinal ganglion cells (ipRGCs) of the retina [76], but its expression is also detected in various tissues such as the skin, brain tissue, and pineal gland [61].

4.2. Diversity and Classification of Non-Visual Opsins

Non-visual opsins encompass a variety of classes. Beyond melanopsin, they can be classified into three main categories based on their tissue localization: deep-brain non-visual opsins, pineal non-visual opsins, and teleost-multiple-tissue (TMT) opsins in bony fish. It should be noted that these non-visual opsins and their expression patterns are species-specific. Among these, pineal non-visual opsins include exo-rhodopsin (exrh) [78] and parapinopsin [79]. Deep-brain non-visual opsins comprise the earliest discovered vertebrate ancient (VA) opsin and its variant, VA-long (VAL) opsin [80], as well as encephalopsin/panopsin (Opsin3), melanopsin (Opsin4), and neuropsin (Opsin5) [73,78,81,82]. Similarly to visual opsins, non-visual opsins also belong to the G protein-coupled receptor superfamily. To initiate the phototransduction process, they require 11-cis retinal to commence light absorption [12].

4.3. Photoreceptive Properties and Signal Transduction Mechanism of Melanopsin

Melanopsin, like teleost multiple-tissue opsin (TMT), acts as a backup photoreceptor for sensing blue-wavelength light in aquatic environments and is an indispensable component of the circadian phototransduction pathway in teleost fish; both proteins can be activated by blue light and green light [83,84,85]. Melanopsin primarily functions by absorbing external photons, causing its chromophore, retinal, to isomerize from the cis to the trans configuration [86], and subsequently activating phospholipase C (PLC) by binding to the Gq/11 class of G proteins. PLC then hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to generate diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) [87,88]. This process triggers the influx of extracellular Ca²⁺ through TRP (Transient Receptor Potential) ion channels, leading to membrane depolarization and the activation of voltage-operated calcium channels (VOCCs) [89,90]. This signaling pathway enables melanopsin to directly convert light energy into biological signals, thereby participating in the regulation of circadian rhythms.

4.4. Non-Image-Forming Visual Functions of Melanopsin

Unlike visual opsins, which primarily mediate image formation, melanopsin in fish serves not only a photoreceptive role but also performs multiple related physiological functions. I. Regulation of Fish Circadian Rhythms: Melanopsin detects external light signals and triggers regulatory changes in circadian rhythms, thereby synchronizing the organism’s internal clock. It can also maintain rhythmicity in the absence of light. For example, the cave-dwelling fish (Phreatichthys andruzzii), which inhabits subterranean reef environments over the long term, has completely regressed eyes yet retains a functional circadian system. Studies on dark-adapted cavefish also indicate that melanopsin can mediate the regulation of peripheral circadian rhythms [83]. II. Influencing Fish Embryonic Development: Melanopsin also regulates the hatching of fish embryos. Immunocolocalization studies of melanopsin and melatonin in the pineal photoreceptor cells of Atlantic halibut (Hippoglossus hippoglossus) embryos revealed that, in addition to diffusing through the cerebrospinal fluid to multiple brain regions, melatonin can directly regulate deep brain areas via specific neuronal pathways (from the anterior diencephalon to the hypothalamus) [91,92]. Studies have found that light spectrum can influence fish embryonic development in aspects such as hatching rate and malformation rate [93], and it is highly likely that melanopsin acts as a photosensory element involved in fish hatching. III. Regulation of Fish Body Color Change: In experiments with Nile tilapia (Oreochromis niloticus), under different light conditions, the expression levels of opsins and melanopsin were altered, as well as the responsiveness of skin chromatophores [94]. Recent studies indicate that Opn5m, a common subtype of melanopsin, is located in the pituitary gland of the Japanese medaka (Oryzias latipes). Upon ultraviolet light stimulation, it secretes melanocyte-stimulating hormone (MSH), thereby stimulating pigment production [95]. IV. Influencing Fish Hormone Secretion: The light perception mediated by melanopsin also exerts broad effects on the endocrine system of fish. Melatonin synthesis is indirectly regulated by melanopsin, while melatonin, in turn, acts on fish growth, development, and reproduction through downstream hormonal pathways. Studies show that under a 12-h dark/12-h light cycle, when brief light exposure treatments of different spectra (blue: 435–475 nm, green: 495–565 nm, red: 607–647 nm) are administered at night, blue light causes the greatest suppression of melatonin levels in the retina [95].

The discovery of melanopsin has extended beyond the traditional image-forming scope of opsins, establishing its role as a key mediator of non-image-forming vision in fish. From its signal transduction mechanism to its involvement in diverse physiological pathways, melanopsin not only assists fish in maintaining physiological rhythms and ecological behaviors within complex light environments but may also play a pivotal role in adaptive evolution.

5. Conclusions

The impact of the light environment on fish vision and physiological regulation is systemic. This review synthesizes the adaptive strategies employed by fish to cope with complex photic conditions, spanning retinal structure, the molecular mechanisms of opsins, and the non-visual functions mediated by melanopsin. These multi-layered mechanisms—from macro to micro, and from image-forming to non-image-forming—collectively constitute a flexible and efficient photosensory system, serving as a crucial foundation for fish survival and reproduction. While current research has laid a solid foundation, the overall understanding remains fragmented. To address this limitation, future studies urgently require an integrated and systematic perspective, focusing on the following key directions: First, to elucidate the specific roles of melanopsin in non-image-forming vision and related physiological regulation in fish. Second, to analyze the coordinated adaptive strategies of the fish visual system under different light environments. Third, to conduct light environment regulation research on economically valuable fish species, elucidating the physiological roles of opsin genes diversity and expression, thereby deepening the understanding of fish biological behavior. Translating findings from these research directions into scientifically informed technologies for aquaculture and developing management strategies for optimal fish light environments will not only help optimize farming practices and enhance fish health and welfare but also be significant for promoting the sustainable development of the industry.