Julián Torres-Dowdall

Nonlethal age determination is a priority in fish biology contexts where sacrifice is restricted, but estimates derived from nonlethally attainable structures tend to suffer from poor precision and accuracy relative to necessarily lethal, otolith‐derived estimates. Dorsal spines, while less accurate than otoliths, have shown higher precision compared to other alternative structures, making them a useful candidate for testing reader‐based approaches to improved age estimation. Here, we test whether small, lake‐specific training sets derived from paired otolith‐spine images, derived from incidental mortalities can provide useful calibration to spine‐based age estimates in four centrarchid populations from the northern Midwest, including two largemouth bass ( Micropterus nigricans ) and two bluegill ( Lepomis macrochirus ) populations. Five moderately experienced readers estimated ages from spine images before and after calibration with paired otolith‐spine images. Calibration consistently resulted in increased estimated ages, particularly for older fish, and reduced variability among readers in both individual age estimates and predicted length‐at‐age trajectories. Specifically, we find that these structured training sets are useful in both reducing traditional underestimation biases and reducing reader‐based variance effects at both the level of individual estimates and in growth modelling contexts. Therefore, they may offer a practical means of leveraging available incidental mortalities in the improvement of otherwise nonlethal aging programs. Accordingly, we suggest that where available, readers first be familiarized with lake‐specific paired otolith‐spine images before independently estimating spine ages.

Littoral habitat is an important organizing force in lacustrine food webs. Modification of littoral habitat directly impacts aquatic macroinvertebrates, which are important vectors between basal resources and higher trophic levels. Lakeshore residential development is a pervasive phenomenon often characterized by the loss of coarse woody habitat (CWH) from lakes, and the construction of artificial wooden structures (docks and ramps). To characterize the effects of artificial wooden structures on macroinvertebrate spatial distributions, we surveyed assemblages across a lightly developed lake in New York’s Adirondack Park. Using benthic sweeps and Hester-Dendy samplers, we sampled assemblages at both natural CWH and artificial wooden structures (boat ramps), and compared them to those in adjacent, non-woody areas. We found that both CWH and artificial wooden structures facilitated larger, more complex assemblages than non-woody areas. We additionally considered general shoreline characteristics, and found only compositional effects on assemblages. We suggest that impacts on aquatic macroinvertebrates arise at a fine spatial scale, and find evidence that development-associated CWH loss drives macroinvertebrate depletion. Although CWH and artificial wooden structures are unlikely to be equivalent at whole-lake scales, macroinvertebrate depletion may be functionally alleviated at fine scales by the construction of artificial wooden structures such as ramps.

Phenotypic plasticity in visual opsin gene expression enables teleost fish to adjust their spectral sensitivity in response to environmental variability, yet the magnitude and drivers of this plasticity remain unresolved. We conducted a meta-analysis of 573 effect sizes from 36 studies to assess the prevalence and strength of opsin gene expression plasticity across teleost fishes, considering stimulus type (internal versus external) and timing of exposure (acute versus developmental). Plasticity of opsin gene expression was widespread and generally strong, with internal stimuli (e.g. hormonal changes) eliciting larger and more consistent responses than external stimuli (e.g. light conditions), particularly in the red-sensitive lws and UV-sensitive sws1 opsin genes. Phylogenetic analysis revealed no significant evolutionary constraints on opsin plasticity, suggesting that the capacity for visual system modulation is broadly distributed across teleost lineages. Our findings highlight the need for future studies to integrate behavioral, molecular and ecological data to evaluate the adaptive significance of opsin plasticity and its role in shaping visual performance under changing environmental conditions.

Phenotypic plasticity enables organisms to adjust their traits in response to environmental changes, potentially enhancing survival under fluctuating conditions. While plastic capacity—the range of phenotypic change—has been extensively studied, the rate of plastic responses remains unexplored. The rate of plasticity is crucial, as prolonged mismatches between phenotype and environment reduce fitness. Nonetheless, evolutionary models typically do not investigate rates of plasticity. Here, we measure opsin gene expression to estimate the temporal changes in predicted visual sensitivity of Nicaraguan convict cichlids (Amatitlania siquia) from populations exposed to changes in light conditions. We show that rates of plasticity in single cone predicted sensitivity vary significantly among populations, developmental stages, and experimental light treatments, demonstrating that rates of plastic change are shaped by both genetic and context-dependent factors. Notably, great lake cichlids—native to predominantly turbid environments—responded more rapidly to red-shifted photic conditions than crater lake fish, but more slowly when returned to clear conditions. Additionally, younger individuals exhibited faster changes in opsin gene expression than older ones, highlighting the role of developmental stage in modulating plasticity. These findings challenge the assumption of constants rates of plasticity and suggest that selection could act not only on plastic capacity but also on the rate of plastic responses. Our results demonstrate that rates of plasticity are themselves evolvable traits. Incorporating this temporal dimension into models of plasticity will improve our understanding of how organisms respond to environmental heterogeneity, with broad implications for evolutionary biology and ecology.

Visual communication in fish is often shaped by their light environment, which influences both sensory (e.g., eye size, opsin gene expression) and signalling traits (e.g., body reflectance). This study explores the phenotypic variation in the visual communication traits of six species of centrarchids (Centrarchidae) inhabiting two contrasting light environments. We measured morphological, molecular and signalling traits to determine their variation across photic conditions. Our findings reveal significant interspecific variation in sensory traits but no consistent phenotypic variation between light environments. Centrarchids showed robust visual systems with green‐sensitive rh2 and red‐sensitive lws opsin genes representing the main chromatic channels, with their expression remaining largely unaffected between distinct light habitats. We also found significant molecular evolution in the visual opsin genes, although these changes were not associated with environmental conditions. However, body reflectance displayed species‐specific responses to environmental conditions, suggesting that signalling traits may be more flexible than sensory traits. Overall, our results challenge the generality of the current paradigm in visual ecology, which portrays visual systems in fish as highly tunable owing to photic conditions. Our study highlights the potential evolutionary or developmental constraints on centrarchid visual systems and their implications for adaptability to various habitats and novel environmental threats.

The wide-ranging photic conditions found across aquatic habitats may act as selective pressures that potentially drive the rapid evolution and diversity of the visual system in teleost fish. Teleost fish fine-tune their visual sensitivities by regulating the two components of visual pigments, the opsin protein and the chromophore. Compared with opsin protein variation, chromophore usage across photic habitats has received little attention. The Nicaraguan Midas cichlid species complex, Amphilophus cf citrinellus [Günther 1864], has independently colonized seven isolated crater lakes with different photic conditions, resulting in several recent adaptive radiations. Here, we investigate variation in cyp27c1, the main enzyme modulating chromophore exchange. We measured cyp27c1 expression in photic environments in the wild, its genetic component in laboratory-reared fish, and its response to different light conditions during development. We found that photic environments significantly predict variation in cyp27c1 expression in wild populations and that this variation seems to be genetically assimilated in two populations. Furthermore, light-induced cyp27c1 expression exhibited genotype-by-environment interactions in our manipulative experiments. Overall, within-lake variation in cyp27c1 expression was higher and inversely related to variation in opsin gene expression along the photic gradient, emphasizing the key role of cyp27c1 in the visual ecology of cichlid fish.

Determining how internal and external stimuli interact to determine developmental trajectories of traits is a challenge that requires the integration of different subfields of biology. Internal stimuli, such as hormones, control developmental patterns of phenotypic changes, which might be modified by external environmental cues (e.g. plasticity). Thyroid hormone (TH) modulates the timing of opsin gene expression in developing Midas cichlid fish (Amphilophus citrinellus). Moreover, fish reared in red light accelerate this developmental timing compared to fish reared in white light. Hence, we hypothesized that plasticity caused by variation in light conditions has coopted the TH signaling pathway to induce changes in opsin gene expression. We treated Midas cichlids with TH and crossed this treatment with two light conditions, white and red. We observed that not only opsin expression responded similarly to TH and red light but also that, at high TH levels, there is limited capacity for light-induced plasticity. Transcriptomic analysis of the eye showed that genes in the TH pathway were affected by TH, but not by light treatments. Coexpression network analyses further suggested that response to light was independent of the response to TH manipulations. Taken together, our results suggest independent mechanisms mediating development and plasticity during development of opsin gene expression, and that responses to environmental stimuli may vary depending on internal stimuli. This conditional developmental response to external factors depending on internal ones (e.g. hormones) might play a fundamental role in the patterns of phenotypic divergence observed in Midas cichlids and potentially other organisms.

Gene duplication is one of the most important sources of novel genotypic diversity and the subsequent evolution of phenotypic diversity. Determining the evolutionary history and functional changes of duplicated genes is crucial for a comprehensive understanding of adaptive evolution. The evolutionary history of visual opsin genes is very dynamic, with repeated duplication events followed by sub- or neofunctionalization. While duplication of the green-sensitive opsins rh2 is common in teleost fish, fewer cases of multiple duplication events of the red-sensitive opsin lws are known. In this study, we investigate the visual opsin gene repertoire of the anabantoid fishes, focusing on the five lws opsin genes found in the genus Betta. We determine the evolutionary history of the lws opsin gene by taking advantage of whole-genome sequences of nine anabantoid species, including the newly assembled genome of Betta imbellis. Our results show that at least two independent duplications of lws occurred in the Betta lineage. The analysis of amino acid sequences of the lws paralogs of Betta revealed high levels of diversification in four of the seven transmembrane regions of the lws protein. Amino acid substitutions at two key-tuning sites are predicted to lead to differentiation of absorption maxima (λ_max) between the paralogs within Betta. Finally, eye transcriptomics of B. splendens at different developmental stages revealed expression shifts between paralogs for all cone opsin classes. The lws genes are expressed according to their relative position in the lws opsin cluster throughout ontogeny. We conclude that temporal collinearity of lws expression might have facilitated subfunctionalization of lws in Betta and teleost opsins in general.

Selection pressures differ along environmental gradients, and traits tightly linked to fitness (e.g., the visual system) are expected to track such variation. Along gradients, adaptation to local conditions might be due to heritable and nonheritable environmentally induced variation. Disentangling these sources of phenotypic variation requires studying closely related populations in nature and in the laboratory. The Nicaraguan lakes represent an environmental gradient in photic conditions from clear crater lakes to very turbid great lakes. From two old, turbid great lakes, Midas cichlid fish (Amphilophus cf. citrinellus) independently colonized seven isolated crater lakes of varying light conditions, resulting in a small adaptive radiation. We estimated variation in visual sensitivities along this photic gradient by measuring cone opsin gene expression among lake populations. Visual sensitivities observed in all seven derived crater lake populations shifted predictably in direction and magnitude, repeatedly mirroring changes in photic conditions. Comparing wild-caught and laboratory-reared fish revealed that 48% of this phenotypic variation is genetically determined and evolved rapidly. Decreasing intrapopulation variation as environments become spectrally narrower suggests that different selective landscapes operate along the gradient. We conclude that the power to predict phenotypic evolution along gradients depends on both the magnitude of environmental change and the selective landscape shape.

For fishes in temperate environments, the timing of seasonal reproduction has been correlated with changes in photoperiod and temperature. In tropical environments, seasonal hydrological patterns have been found to be drivers of the onset of reproduction. Despite these established relationships, data on reproductive seasonality for viviparous fish are limited. Here, we investigate aspects of reproduction in the viviparous fish Jenynsia tucumana Aguilera & Mirande, 2005 over the course of 12 months in the Salí River basin (Tucumán, Argentina), which is characterized by dry, short winters, and long, tropical summers. Size and reproductive data were collected each month. We found J. tucumana displayed marked sexual dimorphism in size, with females having higher length and weight when compared to males. Larger females have higher fecundities compared to smaller females. The breeding season corresponds with seasonal changes in photoperiod, air temperature, and rainfall, but ends with the arrival of heavy rainfall later in the season. Interestingly, J. tucumana produce and mature oocytes year-round, unlike congeneric species. This may be an adaptive strategy imposed by the irregular flooding of subtropical mountainous river stream habitats, which maintain high productivity well into early autumn.