Altered, Visual function leads to Increased Mortality

Decreases in visual functions can have a strong impact on behavior, leading to changes in individual response and abilities in the environment, including, for example, perception of food or avoidance of predators. Variation in the visual system can also influence learning tasks when visual stimuli are used (Corral-López et al., 2017). 

Sensory drive has been implicated in speciation in various taxa, largely based on phenotype-environment correlations and signatures of selection in sensory genes, including vision (Maan et al, 2017). 

It is biologically plausible that an animal which has difficulties in finding food and avoiding predators will have lower survival chances in wildlife.

Only very few studies are available in which it was demonstrated that decreased visual capacities lead to reduced survival of the organism. In general, mortality is rarely assessed but survival-reducing factors (feeding, predation) are mainly investigated. Here we consider the work about different toxicants that disrupt complex fish behaviors, such as predator avoidance, reproductive, and social behaviors. Toxicant exposure often completely eliminates the performance of behaviors that are essential to fitness and survival in natural ecosystems, frequently after exposures of lesser magnitude than those causing significant mortality (Brown et al., 2004).

• Fuiman et al. (2006) specifically investigated the importance of several putative survival skills for escaping a predator. They first analysed routine swimming, acoustic startle stimulus and visual startle stimulus of red drum larvae and subsequently performed a predation experiment using the same larvae in the presence of a live predator. The authors found that the effectiveness of escape responses was almost 100% and thus responsiveness determined survival under predation. Of the different putative survival skills, only visual responsiveness was significantly correlated to escape potential, while others such as acoustic responsiveness were not significantly contributing to escape potential. Further investigation showed that only visual responsiveness differed significantly between poorly responding larvae and better responders.
• Dehnert et al., 2019: In zebrafish, 2, 4-Dichlorophenoxyacetic acid exposure during eye development impaired visual behavior, i.e. reduced prey capture. Additionally, exposed fish showed reduced neural activity within the optic tectum following prey exposure.
• Besson et al., 2020 exposed metamorphosing convict surgeonfish to pharmacological treatments. They performed a 10⁻⁶ M NH3 treatment (a TH antagonist) to achieve TH signal disruption and they observed an adverse outcome on retinal layer level. Repressed retinal development at both day 2 and day 5 with a 10-25 % decrease of bipolar cell density was detected. They followed up with a behavior test at day 2 with blacktail snapper as a predator and got the following results: 

1. In the test using chemical cues of the predator the NH3-treated fish did not discriminate between water sources, while control fish clearly avoided predator cues.

2. In the visual cues test the NH3-treated fish showed no preference and spent 25 % more time in visual stimulus compared to controls.

3. In a survival predation test in an in situ arena they observed that day 2 NH3 treated fish exhibited 30% lower survival than d2 control fish.

• Furthermore Besson et al., 2020 conducted a Chlorpyrifos (CPF) treatment 30 μg L−1 and observed a significant reduction (25%) in T4 levels at day 2 in CPF30 fish, as well as significantly reduced T3 levels in CPF30 fish (28%) compared with control fish. CPF30 fish also exhibited reduced densities of bipolar cell (10%) of retinal layer and CPF30 fish experienced lower survival.
• Flamarique et al. (2013) showed that thyroid hormone treatment impacted the development of the visual system in rainbow trout and reduced the distances and angles at which prey were located (variables that are known indicators of foraging performance). Using optical measurements and photon-catch calculations, the study showed that control rainbow trouts perceived prey (Daphnia) with greater contrast compared to thyroid-hormone-treated fish. Reduced foraging performance is likely to reduce survival in the wild.
• Heijlen et al. (2014) showed that knockdown of Type 3 Iodothyronine Deiodinase, known to disrupt eye development (Houbrechts et al. (2016), causes embryos to spend significantly less time moving, and perturbs the escape response after a tactile stimulus. An inability to escape predators likely reduces survival in the wild.

It is obvious that impaired vision leads to higher mortality, as the sense of sight is important for survival, and if it is impaired, feeding or escape becomes more difficult. However, the number of studies investigating this connection is limited. It is often unclear to what extent this relationship is determined by altered visual function versus other pathways such as alterations in muscle development or other factors contributing to these types of behaviour. Also, the natural conditions, which depend on many variables, are difficult to reproduce in the laboratory or to compare between different laboratories.