Browsing by Author "Johnsen, Sönke"

Many animals can sense the earth’s magnetic field and use it to perform incredible feats of navigation; however, studying this phenomenon in the lab is difficult because behavioral responses to magnetic cues can be highly variable. My Ph.D. research attempts to fill this knowledge gap in the following ways: we first explore potential sources for this variability, including both natural and artificial sources of noise. We then examine the ways in which these natural sources of noise could be used to study magnetoreception in animals that are not feasible to study in the laboratory. Finally, we propose a possible solution for how navigating animals may overcome noise to still accomplish highly accurate migrations. Chapter 1 contains the relevant background and introduction. In Chapter 2, we conduct a synthetic review of natural and anthropogenic sources of radio frequency electromagnetic noise (RF) and its effects on magnetoreception. Anthropogenic RF has been shown to disrupt magnetic orientation behavior in some animals. Two sources of natural RF might also have the potential to disturb magnetic orientation behavior under some conditions: solar RF and atmospheric RF. In this review, we outline the frequency ranges and electric/magnetic field magnitudes of RF that have been shown to disturb magnetoreceptive behavior in laboratory studies and compare these to the ranges of solar and atmospheric RF. Frequencies shown to be disruptive in laboratory studies range from 0.1 to 10 MHz, with magnetic magnitudes as low as 1 nT reported to have effects. Based on these values, it appears unlikely that solar RF alone routinely disrupts magnetic orientation. In contrast, atmospheric RF does sometimes exceed the levels known to disrupt magnetic orientation in laboratory studies. We provide a reference for when and where atmospheric RF can be expected to reach these levels, as well as a guide for quantifying RF measurements.

In Chapter 3, we explore how these natural sources of noise may allow us to study magnetoreception in animals that are not feasible to study in the laboratory. Although it is difficult to perform behavioral experiments on baleen whales, it may be possible to use live stranding data (strandings that indicate the whale may have made a navigational error, rather than those having died at sea and washed ashore) as a tool to investigate the cues they use while navigating. Here we show that there is a 2.1-fold increase in the likelihood of a live gray whale (Eschrichtius robustus) stranding (n=186) on days with a high sunspot count than on low sunspot days (p<0.0001). Increased sunspot count is strongly correlated with solar storms – sudden releases of high-energy particles from the sun which have the potential to disrupt magnetic orientation behavior when they interact with earth’s magnetosphere. We further explore this relationship by examining portions of earth’s electromagnetic spectrum that are affected by solar storms and found a 3.7-fold increase in the likelihood of a live stranding on days with high solar radio flux (RF) as measured from earth (p<0.0001). One hypothesized mechanism for magnetoreception, the radical-pair theory, predicts that magnetoreception can be disrupted by RF radiation, and RF noise has been shown to disrupt magnetic orientation in certain species. To our knowledge, this is the first evidence that provides support for a specific magnetoreception mechanism in whales.

Finally, in Chapter 4, we propose a mechanism for how magnetoreceptive animals may overcome noise to perform incredibly accurate migrations. Many animals use the geomagnetic field to migrate long distances with high accuracy; however, research has shown that individual responses to magnetic cues in the laboratory can be highly variable. Thus, it has been hypothesized that magnetoreception alone is insufficient for accurate migrations and animals must either switch to a more accurate sensory cue or integrate their magnetic sense over time. Here we suggest that magnetoreceptive migrators could also use collective navigation strategies. Using agent-based models, we compare agents utilizing collective navigation to both the use of a secondary sensory system and time-integration. In our models, by using collective navigation alone, over 70% of the group is still able to successfully reach their goal even as their ability to navigate becomes extremely noisy. To reach the same success rates, in our models, a secondary sensory system must provide perfect navigation for over 73% of the migratory route, and time integration must integrate over 50 time-steps, indicating that magnetoreceptive animals could benefit from using collective navigation. Finally, we explore the impact of population loss on animals relying on collective navigation. We show that as population density decreases, a greater proportion of individuals fail to reach their destination and, in our models, a 50% population reduction resulted in up to a 37% decrease in the proportion of individuals completing their migration. We additionally show that this process is compounding, eventually resulting in complete population collapse.

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.

This dissertation research focused on the ways in which clear crustaceans with complex bodies (i.e. with hard cuticles, thick muscles, and other internal organs) maintain transparency across their entire body volume. I used transparent crustacean species that had relatively large (> 25 mm long and > 2 mm thick) bodies and that occupied physically different (pelagic vs. benthic reef) habitats. Studying these transparent crustaceans and making comparisons with closely related opaque crustaceans provided some of the first insights into the puzzling problem of the physical basis of transparency in whole organisms.

First, I examined the ultrastructure of the cuticle of hyperiid amphipods, the first surface to interact with light, to understand what features may minimize reflectance. I investigated the cuticle surfaces of seven species of mostly transparent hyperiids using scanning electron microscopy and found two previously undocumented features that reduced reflectance. I found that the legs of Cystisoma spp. were covered with an ordered array of nanoprotuberances that functioned optically as a gradient refractive index material to reduce reflections. Additionally, I found that Cystisoma and six other species of hyperiids were covered with a monolayer of homogenous nanospheres (approximately 50 nm to 350 nm in diameter) that were most likely bacteria. Optical modeling demonstrated that both the nanoprotuberances and the monolayers reduced reflectance by as much as 250-fold. Even though the models only considered surface reflectance and not internal light scattering, these models showed that the nanoprotuberances and spheres could improve crypsis in a featureless habitat where the smallest reflection could render an animal vulnerable to visual predation.

Second, I took a morphological approach to investigate how light scattering may be minimized internally. Using bright field microscopy, I explored whether there were any gross anatomical differences in the abdominal muscles between a transparent species of shrimp, Ancylomenes pedersoni, and a similarly sized opaque shrimp species, Lysmata wurdemanni. I found no differences in muscle fiber size or any other features. Using transmission electron microscopy (TEM) to visualize muscle ultrastructure, I found that the myofibrils of the transparent species were twice the diameter of the opaque species (mean values of 2.2 μm compared to 1.0 μm). Over a given distance of muscle, light passes through fewer myofibrils due to their larger diameter, with fewer opportunities for light to be scattered at the interfaces between the high-index myofibrillar lattice and the surrounding lower-index fluid-filled sarcoplasmic reticulum (SR). Additionally, because transparency is not always a static trait and can sometimes be disrupted after exercise or physiological stress, I compared the ultrastructure of muscle in transparent A. pedersoni shrimp with the ultrastructure of muscle in A. pedersoni that had temporarily turned opaque after exercise. I found that in this opacified tissue, the fluid-filled space around myofibrils had an increased thickness of 360 nm as compared to a normal thickness of 20 nm. While this could have been a fixation artifact, this result still suggests that opacified tissue had some change in osmolarity or increase in fluid. Models of light scattering across a range of thicknesses and possible refractive indices showed that this observed increase in fluid-filled space dramatically reduced transparency.

Third, I further investigated how exertion or physiological stress may disrupt transparency, what may occur in the tissues to cause this disruption, and what may explain the increased fluid-filled SR interface. I hypothesized that increased perfusion, or an increase in blood volume between muscle fibers, can disrupt the normal organization of tissue, resulting in increased light scattering. I measured pre- and post-exercise perfusion via the injection of a specific fluorescent stain (Alexa Fluor 594-labeled wheat germ agglutinin) that labeled the sarcolemmal areas in contact with hemolymph and the endothelial cells of the blood vessels, and found more open vessels and greater hemolymph perfusion around fibers post-exercise. Changing salinity in the shrimps’ tanks, wounding the shrimp, and injecting proctolin (a vasodilator) were also associated with increased opacity and perfusion. To visualize the shrimps’ overall muscle morphology, I used Diffusible Iodine-based Contrast-Enhanced Computed Tomography (DICECT) to scan one control (transparent) and one experimental (opaque) A. pedersoni. The resulting images added further support to my hypothesis that hemolymph volume in the muscle increases in post-exercise opacified A. pedersoni.

Scallops, a family of swimming bivalve mollusks, have dozens of eyes arrayed along the edges of their valves. Relatively little is known about the form and function of these unusual eyes. To learn more about them, we studied the visually influenced behavior of scallops, as well as the morphology and spectral sensitivity of their eyes. Of particular interest was whether or not the simple neural architecture of these animals constrains the number of visually-influenced behaviors they can perform. We were also interested to learn whether scallop eyes, despite providing relatively poor visual acuity, show optical refinements, such as corrections for spherical and chromatic aberration, that are known from the eyes of animals with better vision. In the following dissertation, Chapter 2 discusses the visually-influenced behaviors of scallops. It has been argued that bivalve mantle eyes only act as predator-detectors, but the behavioral trials described in this chapter suggest that vision may serve additional purposes in scallops. For example, it was found that visual cues relating to flow conditions may influence scallop feeding behavior. Chapter 3 presents a comparative study of scallop eye morphology. Here, it is found that eye morphology varies considerably between scallop species and that highly mobile scallops have better vision than less mobile or immobile species. Evidence is also presented that one of the two scallop retinas may perform tasks of similar importance to all species, such as predator detection, while the other retina may perform tasks more important to mobile species, such as those associated with the visual detection of preferred habitats. Chapter 4 investigates the spectral sensitivity of the two retinas in the mantle eyes of two scallop species. It is found that there is both inter- and intra-specific variation in scallop spectral sensitivity and that color perception in scallops may be influenced by both environmental light conditions and chromatic aberration caused by their lens. The research in this dissertation provides insight into how vision functions in animals that, like scallops, have a vast number of eyes, but a limited capacity for neural processing. Despite such limitations, it is evident that scallops display a wide range of visual behaviors and have eyes with highly-refined optics.

The goal of this dissertation was to better clarify the sensory and cognitive capabilities and limitations of a size-constrained animal. Because visually-guided behaviors are more experimentally tractable than behaviors guided by other sensory modalities, I chose to study a small animal with an unusually good visual system and a suite of apparently visually-guided behaviors, the jumping spider Lyssomanes viridis (Salticidae). Jumping spiders' principal eyes, which are adapted for the perception of shape and pattern, have the highest measured acuity of any arthropod, but also the narrowest field of view, making salticids a particularly interesting study system for measuring the capabilities and limitations of a tiny animal with small yet apparently highly functional eyes. For my dissertation, I examined the amount and type of visual information gathered in high-stakes encounters; i.e. species and sex recognition and male-male contests over females. In salticids, the wrong assessment of species and sex or fighting ability carries with it the risk of injury or even death. Thus, more information, and especially high-resolution information, should be particularly adaptive in such encounters, and should provide us with a good proxy of the perceptual and cognitive capabilities and limitations of this small animal.

In chapter two, I assayed the amount and type of visual information gathered in the context of species and sex recognition, and tested for crossmodal interactions between pheromones and visual cues. Using computer-animated stimuli, I found that, although males took the time necessary to visually scan both the face and legs of other spiders before deciding whether to threaten, court, or ignore them, their conspecific visual recognition templates were fairly coarse, and resulted in them making numerous misidentifications and frequently courting heterospecific salticids. This was especially true in the presence of conspecific female pheromones. Pheromones appeared to exert further top-down effects on visual recognition of conspecifics by bringing visual recognition templates into working memory, as was inferred from the fact that males spent less time examining conspecific images in the presence of conspecific female pheromones. Pheromones also increased the probability that a non-conspecific spider bearing even a slight resemblance to a conspecific female spider would be recognized and courted as a conspecific female. However, pheromones usually did not hasten the recognition of non-conspecific images; this indicates that males' poorer recognition accuracy in the presence of pheromones was not a result of males' spending less time visually examining non-conspecific images.

In chapter three, I looked for correlations between various visual features and contest success in order to determine what types of visual information opponents could theoretically use to assess their opponents' resource holding potential in contests over females. I found that all measured size-related traits correlated strongly with contest success, but that coloration did not, except in the rare cases in which a smaller male won a contest. In these encounters, males who won, despite being smaller, had less red chelicerae than their opponents. Finally, in chapter four, I used the results of chapter three to begin assessing whether the traits that correlate with contest success are actually assessed by males, and in particular, whether they are assessed visually. To do this, I presented males with various sizes of computer-animated opponents, and found that males were less likely to threaten larger opponents. Thus, males seem to be using visual cues to gather information about the size of their opponents. Whether they evaluate overall size, or more specifically, the size of their opponents' weapons, will be addressed in future work.

Animals use color to hide from predators, signal to mates, and communicate, among other functions. Some animals, such as birds, butterflies, and spiders, have evolved extreme forms of coloration that push the limits of absorption and reflectance. In these animals, pigments and structural elements are combined to produce high absorption or high reflectance. Compared to more typical pigmentary or structural colors, relatively little work has been done to examine the physical basis of extreme coloration across taxa. In this dissertation, I use butterflies, deep-sea fishes, and cleaner shrimp to further explore the presence of extreme coloration in different taxa and the underlying color production mechanisms. In Chapter 2 I began my investigation of extreme coloration with butterflies. First, I identified several species of butterflies from four subfamilies that all have a reflectance < 0.5%. After identifying a set of particularly black butterflies, I used scanning electron microscopy (SEM) to visualize the morphology of the black scales for each species. I found that hole shape and size varied dramatically across species, with no correlation to reflectance. However, two structural features were consistently found in all species - steep ridges and expanded trabeculae compared to dark brown butterflies. Using finite-difference time-domain (FDTD) modeling, I discovered that those two conserved features reduce reflectance from the scales by up to 16-fold. Furthermore, additional modeling demonstrated that the ridges and trabeculae create more scattering, leading to more absorption by melanin embedded within the scales. Given that ultra-black scales in butterflies are often found adjacent to bright, colored scales, ultra-black may be used to increase the contrast of color signals. After analyzing the underlying basis of ultra-black coloration in butterflies, I turned to deep-sea fishes. Many deep-sea fishes exist in a world where the primary source of light is bioluminescence. Predators with sensitive eyes can detect even dim reflections from potential prey. I hypothesized that deep-sea fishes have evolved structural features or pigments to minimize reflectance from the skin. Similar to my approach in Chapter 1, I began by identifying 16 species of fishes with exceptionally low reflectance. I found that in all 16 species there was a continuous layer of melanosomes in the skin. My FDTD modeling showed that the melanosomes in the skin had the optimal size and shape to minimize reflectance. Furthermore, the melanosomes in deep-sea fishes are larger and more elongated for a given size than what is typical in other ectothermic vertebrates. In this case, unlike other ultra-black animals, where scattering is created by a keratin or chitin matrix, fish melanosomes provide both scattering and absorption. Ultra-black skin reduces sighting distance by predators by more than 6-fold compared to typical black fishes, making it a highly effective form of camouflage in the deep-sea. Between birds, butterflies, fishes, and spiders, the mechanisms underlying ultra-black coloration are relatively well understood. However, less attention has been paid to bright white coloration. In Chapter 4, I used similar methods to Chapters 2 and 3 to explore the physical basis of bright white coloration in cleaner shrimp antennae. Cleaner shrimp engage in a mutualistic relationship with client fishes. Some species of cleaner shrimp use long white body parts to signal to these client fish and advertise their services. Two such species are Ancylomenes pedersoni and Lysmata amboinensis, which signal to client fish using their white antennae. Using SEM, I found that the antennae in both species contain a 1-3µm thick layer of non-pigmented nanospheres. With FDTD modeling, I showed that these nanospheres are the optimal size to maximize reflectance and that they can increase reflectance by up to 19-fold compared to antennae without these nanospheres. Similar to what I found in deep-sea fishes, the nanoparticles create a highly scattering structure that, in the absence of absorption from melanin, forms a bright white color instead of ultra-black. Collectively, the three chapters presented here build upon a small, but growing, body of research into the physical basis of extreme animal coloration. This work provides a foundation for new investigations into the functional effects and the evolution of extreme coloration and has the potential to inspire novel man-made materials.

The dissertation work discussed here focuses on the behavioral and physiological aspects of visual sexual signaling in the blue crab, Callinectes sapidus. The blue crab has a pair of apposition compound eyes that are relatively acute (1.5 ° resolution) for an arthropod. The eyes have two photopigments sensitive to blue (λ_max = 440 nm) and green (λ _max=500 nm) light, allowing for simple color vision. Visual cues and signals are used during antagonistic and sexual communication and primarily involve claw-waving motions. A primary feature of the blue crab morphology is its sexually dimorphic claw coloration; males have blue and white claws and females have red claws. However, despite the potential for interesting color signaling, visual cues have typically been considered non-important, particularly in sexual communication where chemical cues have dominated blue crab signaling studies.

In a series of experiments designed to simultaneously test the role of visual cues in mating behavior and blue crab color vision, I tested males' responses to photographs of females with differently colored claws. I found that photographs of females elicited male courting behaviors. I also found that males preferred females with red claws over those with white or isoluminant (i.e. matched brightness) gray claws. The discrimination of red from isoluminant grey showed the use of color vision in male mate choice.

In natural populations, the claws of sexually mature females vary from light orange to deep red. To determine males' abilities discriminate between similar colors, I tested male color preferences for red against several shades of orange varying in brightness. Overall, males showed an innate preference for red-clawed females over those with variations of orange claws. However, in tests between red and orange shades similar in both brightness and hue, male blue crabs did not show a distinct preference, suggesting that males are either not able or not motivated to discriminate between these shades. Further, my results suggest that male blue crabs may use a mixture of chromatic and achromatic cues to discriminate between long-wavelength colors.

After confirming the use of color in mate choice, I focused on the role of claw color in intraspecific communication. To quantify claw coloration, I measured spectral reflectance of claws of a blue crab population in North Carolina. In both sexes, the color of the claw varied with reproductive maturity and may act as a cue of reproductive readiness. Additionally, there was individual variation in claw color which could indicate individual quality. I have modeled the appearance of claw coloration to the blue crab eye and found that these color differences are visible to the blue crab eye and potentially signal gender, reproductive readiness, and/or individual quality.

After investigating male mate choice, I began investigating visual aspects of female mating behavior. In the blue crab, like many crustaceans, courtship occurs during the female molting cycle and copulation takes place after the female has shed her exoskeleton. In crustaceans and other arthropods with compound eyes, the corneal lens of each facet is part of the exoskeleton and thus shed during molting. I used optomotor assays to evaluate the impact of molting on visual acuity (as measured by the minimum resolvable angle &alpha _min) in the female blue crab. I found that visual acuity decreases substantially in the days prior to molting and is gradually recovered after molting. Prior to molting, &alpha_min was 1.8 °, a value approximating the best possible acuity in this species. In the 24 hours before molting, &alpha _minincreased to a median of 15.0 ° (N=12), an eight-fold drop in visual acuity. Six days after molting, &alpha _minreturned to the pre-molting value. Micrographs of C. sapidus eyes showed that a gap between the corneal lens and the crystalline cone appeared approximately five days prior to shedding and increased in width the process progressed. This separation was likely responsible for the loss of visual acuity observed in behavioral tests. Since mating is limited to the female's pubertal molt, a reduction in acuity during this time may have an effect on the sensory cues used in female mate choice. These results may be broadly applicable to all arthropods that molt and have particular importance for crustaceans that molt multiple times in their lifetime or have mating cycles paired with molting.

Cleaner shrimp are small, brightly-colored tropical crustaceans that attract reef fish clients to set locations on a reef called cleaning stations and then "clean" them by removing ectoparasites from their scales, gills, and mouths. Because clients benefit from the removal of potentially harmful parasites, and shrimp benefit from a meal, this interaction is considered mutualistic. The evolution of cleaner-client relationships is especially paradoxical, however, given that crustaceans make an easy meal for many reef fish. How did such an interaction arise and evolve? Additionally, how do cleaner shrimp and clients find and recognize each other, and why doesn't the client eat the cleaner? One hypothesis is that mutualistic partners evolve signals that identify them as beneficial partners. For the cleaner shrimp-client fish mutualism, it has been suggested that these signals are visual, and function to identify cleaners as helpers, rather than food, and perhaps to identify clients as seeking cleaning rather than a meal.

The broad goal of this dissertation was to examine the cleaner shrimp-client fish mutualism by combining visual physiology, visual ecology, and animal behavior. Throughout, I focus on visual acuity-the ability to perceive detail-an underexplored aspect of visual capability which should represent an important selective force on signals, as it determines what visual information can and cannot be resolved. To begin, in Chapter 2, I characterized the visual capabilities of cleaner shrimp for the first time, in particular examining spectral sensitivity, visual acuity, and temporal resolution of three species from the three primary genera where cleaning has arisen. This revealed that these cleaner shrimp have monochromatic, coarse vision, and thus that their color patterns likely do not serve an intraspecific signaling purpose, but rather may be part of signals directed at client fish. Thus, Chapter 3 examined visual acuity in fish by performing a literature synthesis of known visual acuity values across fishes and examining how acuity relates to certain aspects of morphology and ecology. Here, I found that acuity is higher in fish with larger eyes and in fish that live in spatially complex habitats.

Having examined the visual capabilities of both parties in the cleaner-client mutualism, I then explored signaling between cleaners and clients. In chapter (4), I used sequential analysis to demonstrate that certain stereotyped motions by cleaner shrimp are signals directed at clients, and provide the first evidence of potential signals on the part of client fish as well. Additionally, I developed an R package, AcuityView (Appendix A) which displays images with only the spatial information available to a receiver of given acuity from a given distance, and used it to show that cleaner and client signals are visible to their intended receivers. Lastly, in Chapter (5), I outlined what we know about acuity across species, provide primers on the anatomical basis for acuity and what factors can make acuity higher, and put forward specific predictions and hypotheses about how receiver acuity may influence signal form.

Researchers have long assumed that black widow coloration functions as a warning signal to avian predators. However adult female black widow coloration does not resemble typical warning coloration in two distinct ways. First, black widows are less colorful than most other documented aposematic species. Second, the hourglass shape of an adult female varies both between species and within a site. Here I examine the ultimate causes and consequences of North American black widow coloration.

In chapter two I present data that suggest that black widow coloration not only functions as an aposematic signal to avian predators, but has also been selected to be inconspicuous to insect prey. In choice experiments with wild birds, I found that the red-and-black coloration of black widows deters potential predators: wild birds were ~3 times less likely to attack a black widow model with a red hourglass than one without. Using visual-system appropriate models, I also found that a black widow's red-and-black color combo is more apparent to a typical bird than typical insect (Euclidean color distance ~2.2 times greater for birds than insects). Additionally, an ancestral reconstruction revealed that red dorsal coloration is ancestral in black widows and that at some point some North American black widows lost their red dorsal coloration (while maintaining the ventral hourglass). Behaviorally, differences in red dorsal coloration between two North American species are accompanied by differences in microhabitat that affects how often a bird will view a black widow's dorsal region. All observations are consistent with a cost-benefit tradeoff of being conspicuous to potential predators while being inconspicuous to prey. I suggest that avoiding detection by prey --- combined with Müllerian mimicry --- may help explain why red-and-black aposematic signals occur frequently in nature.

In chapter three, I examine the variation in hourglass shape. Classical aposematic theory predicts near uniformity in warning signal appearance because a uniform signal is easier to learn to avoid than a variable signal. However the shape of the hourglass of North American black widows appears to vary both within and between sites in ways that are inconsistent with classical aposematic theory. Using 133 black widows of three different species from nine sites across the United States, I quantified the variation in hourglass shape and examined how Müllerian mimicry, species type, and condition each influenced hourglass shape. A principle components analysis revealed that 84.5% of the variation in hourglass shape can be explained by principle components 1, 2, and 3, which corresponded to hourglass size (PC1), the separation between hourglass parts (PC2), and the slenderness of the hourglass (PC3). Both a black widow's condition and species significantly predicted hourglass shape; however I found no support for localized Müllerian mimicry within different geographical regions. My results suggest a relaxed role for selection on hourglass shape. I discuss several hypotheses that could explain the variation in hourglass morphology including that potential predators may avoid any red markings rather than an exact shape (categorical rather than continuous perception).

In chapter four I expand on my work from chapter two to examine the eavesdropper's perspective on private communication channels. Private communication may benefit signalers by reducing the costs imposed by potential eavesdroppers such as parasites, predators, prey, or rivals. It is likely that private communication channels are influenced by the evolution of signalers, intended receivers, and potential eavesdroppers, but most studies only examine how private communication benefits signalers. Here, I address this shortcoming by examining visual private communication from a potential eavesdropper's perspective. Specifically, I ask if a signaler would face fitness consequences if a potential eavesdropper could detect its signal more clearly. By integrating studies on private communication with those on the evolution of vision, I suggest that published studies find few taxon-based constraints that could keep potential eavesdroppers from detecting most hypothesized forms of visual private communication. However, I find that private signals may persist over evolutionary time if the benefits of detecting a particular signal do not outweigh the functional costs a potential eavesdropper would suffer from evolving the ability to detect it.

In 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.

Sea urchins and brittle stars lack eyes, yet nonetheless are capable of vision, or the detection and resolution of spatial images and detail. Their vision, according to what is known today, is mediated through a light-sensing system that extends across the body and is processed via a decentralized nervous system. This is different from two-eyed and even most multi-eyed animals, where light is collected via discrete organs (eyes or eye cups) and processed in a brain or central ganglion. As benthic marine invertebrates, vision may be useful to sea urchins and brittle stars for navigating, finding shelter, or identifying predators. Although photoreceptor cells have been identified in brittle stars, much remains unknown about vision and light responses in both groups and the echinoderms as a whole (sea urchins, brittle stars, sea stars, sea cucumbers, and feather stars). My dissertation examines some of the gaps in this field of inquiry. I investigate (1) the potential ecological correlates of a sea urchin trait thought to mediate spatial vision, (2) how various regions of the urchin body differ in their sensitivity to light, and (3) if brittle stars are capable learning to associate a darkness cue with the presentation of food.

First, I performed a comparative study on the density of spines on sea urchins. As stated previously, sea urchins do not have eyes yet they are capable of resolving coarse images. One suggestion as to the mechanism of this capability is that the spines shade off-axis light from reaching the photosensitive test (skeleton). Following this hypothesis, the density of spines across the body determines the resolution (or sharpness) of vision by restricting the incidence of light on the photosensitive skin of the animal, creating receptive areas of different minimum resolvable angles. Previous studies have shown that predicted resolutions in several species closely match behaviorally-determined resolutions, ranging from 10° to 33°. Here we present a comparative morphological survey of spine density with species representatives from 22 of the 24 families of regular sea urchins (Class Echinoidea) in order to better understand the relative influences of phylogenetic history and three visually-relevant environmental variables on this trait. We estimated predicted resolutions by calculating spine densities from photographs of spineless sea urchin tests (skeletons). Analyses showed a strong phylogenetic signal in spine density differences between species. Phylogenetically-corrected Generalized Least Squares (PGLS) models incorporating all habitat parameters were the most supported, and no particular parameter was significantly correlated with spine density. Spine density is subject to multiple, overlapping selective pressures and therefore it is possible that either: 1) spine density does not mediate spatial vision in echinoids, or 2) visual resolution via spine density is a downstream consequence of sea urchin morphology rather than a driving force of adaptation in these animals.

Second, I examined the sensitivity to light on different parts of the body of the urchin species Lytechinus variegatus. The sensitivity of an eye is important to understand because it not only determines the light levels under which an eye can function but also indirectly affects how sharp the vision can be. It is unknown how sensitivity maps across the body in urchins, which may have implications for how various parts of the body are used in visual tasks. I tested the behavioral sensitivity response of L. variegatus to light on different regions of the body, using positive or negative phototaxis as response criteria. I tested the ambulacral region first, because this has been shown to be more sensitive to light than the interambulacral region in other urchin species. Individuals of L. variegatus were negatively phototactic to the brightest light (10,000 lux) and exhibited positive phototaxis to any dimmer light, responding to as little as 10 lux (or about the amount of ambient light in late civil twilight). Next, I tested the relative sensitivity response of the ambulacrum and interambulacrum, the two regions of the body, and confirmed that the ambulacrum is the more sensitive of the two in L. variegatus. Finally, I tested the relative sensitivity response of different angular heights (elevations) on the urchin body, along the oral-aboral axis, as these may be ecologically meaningful to the animal. There was a behavioral shift as elevation increased. Bright (10,000 lux) light at 0° (the equator of the animal) caused positive phototaxis; at 30° above the equator, roughly an equal number of urchins moved towards and away from the light; and at 60° above the equator the light caused a negative phototaxis response. The negative phototaxis observed with the light at a 60° elevation on the animal may have ecological consequences or indicate that this region is less sensitive to light. The data from this study can inform which regions and structures future studies may want to target for sensitivity and vision studies in L. variegatus.

Third, I tested whether individuals of the brittle star species Ophiocoma echinata were able to associate a period of darkness with the presentation of a food reward. Like other members of Phylum Echinodermata, the ophiuroid nervous system is decentralized, consisting of five radially arranged ganglia joined by a central nerve ring. While operant and classical conditioning have been observed in asteroids in a limited number of studies, members of the other echinoderm classes remain relatively untested. A group of individually housed Ophiocoma in an experimental group were trained by only presenting food during a period of darkness, while control group animals were fed under regular daytime room lights many hours after a period of darkness of the same duration. After the training period, the experimental group demonstrated they had learned to associate the two cues by regularly emerging during the dark period even when no food was presented. The untrained control animals, as well as pre-training experimental animals, did not emerge during the dark periods, as no food was presented. There was, however, significant variation within the experimental group in terms of the number of times individuals displayed the learned behavior and how quickly animals learned the association. This study shows that classical conditioning is possible in a class of animals without centralized nervous systems.

These results contribute to greater understandings of resolution, sensitivity, and light-guided tasks in the echinoderms which have implications for the visual ecology of these species as well as the study of sensing and processing in decentralized systems.