Browsing by Subject "Eye size"
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Item Open Access Behavioral Measures and Ecological Correlates of Vision in Poeciliid Fishes(2022) Solie, SarahUnderstanding 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.
Item Open Access Vision and Bioluminescence in Cephalopods(2018) Thomas, Kate NicoleIn the deep pelagic ocean, there are no structures to serve as hiding spots, and visual interactions among animals are potentially continuous. The light environment in the midwater habitat is highly structured due to light scattering and absorption. Downwelling sunlight becomes exponentially dimmer, bluer, and more diffuse with depth. This optical structure means that an animal’s depth and viewing direction greatly affect the distances at which it can see visual targets such as potential prey or approaching predators. Additionally, this light environment mediates the visibility of bioluminescent camouflage and signals. My dissertation examines how the midwater light environment affects the ecology and evolution of vision and bioluminescence through an examination of cephalopods, a highly visual group that exhibits a broad diversity of eye adaptations and multiple evolutions of bioluminescence. My research investigates (1) vision and behavior in a deep-sea squid with dimorphic eyes, (2) depth-dependent patterns in cephalopod eye size and visual range, and (3) evolutionary dynamics in bioluminescent cephalopods.
First, I examined the function of differently sized and shaped left and right eyes in midwater “cockeyed” squids (Histioteuthis and Stigmatoteuthis) by using in-situ video footage from remotely operated vehicles at the Monterey Bay Aquarium Research Institute to quantify eye and body orientations. I found evidence that the larger left eye orients upward toward downwelling sunlight and may be useful for spotting silhouettes of potential prey, while the smaller right eye orients slightly downward into darkness, where it may be specialized for detecting bioluminescent flashes. I also found that 65% of adult squids had a yellow pigment in the lens of the larger left eye, which may be used to break the counterillumination camouflage of their prey. Visual modeling showed that the visual gains provided by increasing eye size were much higher for an upward-oriented eye than for a downward-oriented eye, which may explain the evolution of this unique visual strategy.
Second, I examined the effects of depth and optical habitat on eye scaling across cephalopod species by collecting morphological measurements from 120 species at the Smithsonian National Museum of Natural History and constructing a corresponding database of species depth distributions and light habitats from the literature. I then compared absolute eye sizes and relative eye scaling to species light habitats, and found that cephalopods occupying dim light habitats had significantly larger eyes than those occupying bright or dark (abyssal) habitats. My results provide evidence for increased investment in eye size with depth through the midwater habitat until dim, downwelling sunlight disappears.
Finally, I examined the potential effects of the midwater light habitat on evolutionary dynamics among bioluminescent cephalopods using comparative evolutionary methods. I constructed a database of cephalopod daytime depths (as a proxy for light level), body sizes, eye investment, and bioluminescence from published records, then used a published phylogeny and Brownian Motion and Ornstein-Uhlenbeck likelihood models of continuous character evolution in discrete selective categories to determine a best-fit model of evolution. I found evidence that bioluminescence and non-bioluminescent cephalopods are under different selective regimes with different trait optima for depth, body size, and eye investment. Together, this work shows that the structured, directional light environment of the pelagic midwater realm has implications at organismal, macroecological, and macroevolutionary levels.