Behavioral Measures and Ecological Correlates of Vision in Poeciliid Fishes

Understanding how animals see the world and how visual systems have evolved to meet the needs of particular animals are major goals of visual ecology research. The Poeciliidae are a diverse family of Neotropical freshwater fishes and are excellent models for visual ecology research given longstanding interest in visual signaling in this group. However, despite extensive research investigating the form and function of visual signals in the poeciliids, there remains a surprising paucity of research regarding poeciliid visual system function and evolution. To address this gap, my dissertation research sought to investigate: (1) how Trinidadian guppies (Poecilia reticulata) perceive visual stimuli that vary in spatial detail and contrast, (2) correlates of eye size and eye investment across P. reticulata populations that experience different threats from predation and, (3) visual signaling correlates of eye size across the family Poeciliidae.

The first chapter of this dissertation introduces the questions and the study system. In Chapter 2, I examine the ability of Trinidadian guppies (Poecilia reticulata) to perceive visual stimuli that vary in spatial frequency and contrast. Male P. reticulata bear complex body patterning made up of patches that vary in color, contrast, and size, and these visual signals that are known to be important in mate choice. However, the extent to which conspecifics are able to resolve the details of these patterns has historically been overlooked. I used an optomotor assay to measure the behavioral responses of eight individual P. reticulata (N = 4 males; 4 females) to rotating achromatic stimuli. Unsurprisingly, I found that P. reticulata are better able to perceive stimuli as they increase in contrast and decrease in spatial frequency. Moreover, I found that female P. reticulata may outperform males on optomotor tasks.

In Chapter 3, I investigate how predation environment contributes to eye size variation in P. reticulata. Eye size is an important predictor of visual abilities, and it varies widely across taxa. Moreover, eye size is known to be correlated with numerous ecological factors including habitat complexity, light availability, and predation risk. However, less is known about how differences in ecological parameters across populations influence variation in eye size within species. I measured the eye diameter and standard length of 45 females and 307 males from 21 populations of known geographic origin and predation environment. I found that eye diameter was correlated with predation environment after controlling for standard length, with fish from low-predation environments having eyes that are 5.5% - 7.9% larger in diameter than those from high-predation environments. I also found that sexual dimorphism in eye diameter appears to be driven by sexual dimorphism in standard length, as there was no significant effect of sex on eye diameter after accounting for standard length.

Finally, in Chapter 4 I examine variation in eye size across the Poeciliidae. The poeciliids are a diverse family of freshwater fishes to which Poecilia reticulata belongs, and the group exhibits substantial variation in the distribution and types of visual signals used in mate choice. I measured eye size and eye investment for 66 species of poeciliids and took a phylogenetic approach to test whether variation in eye morphology was correlated with aspects of visual signaling. I found that the presence of sexually selected visual signals was associated with greater eye investment and, in particular, that sexual dichromatism was associated with an approximately 6% increase in eye diameter investment compared to species without sexual dichromatism.