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Synopsis Sea urchins do not have eyes, yet they are capable of resolving simple 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 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.

More than 100 years ago, the biologist Jakob von Uexküll suggested that, because sensory systems are diverse, animals likely inhabit different sensory worlds (umwelten) than we do. Since von Uexküll, work across sensory modalities has confirmed that animals sometimes perceive sensory information that humans cannot, and it is now well-established that one must account for this fact when studying an animal’s behavior. We are less adept, however, at recognizing cases in which non-human animals may not detect or perceive stimuli the same way we do, which is our focus here. In particular, we discuss three ways in which our own perception can result in misinformed hypotheses about the function of various stimuli. In particular, we may (1) make untested assumptions about how sensory information is perceived, based on how we perceive or measure it, (2) attribute undue significance to stimuli that we perceive as complex or striking, and (3) assume that animals divide the sensory world in the same way that we as scientists do. We discuss each of these biases and provide examples of cases where animals cannot perceive or are not attending to stimuli in the same way that we do, and how this may lead us to mistaken assumptions. Because what an animal perceives affects its behavior, we argue that these biases are especially important for researchers in sensory ecology, cognition, and animal behavior and communication to consider. We suggest that studying animal umwelten requires integrative approaches that combine knowledge of sensory physiology with behavioral assays.

This is a pre-copyedited, author-produced version of an article accepted for publication in Integrative & Comparative Biology following peer review. The version of record Thomas W. Cronin, Sönke Johnsen; Extraocular, Non-Visual, and Simple Photoreceptors: An Introduction to the Symposium, Integrative and Comparative Biology, Volume 56, Issue 5, 1 November 2016, Pages 758–763, https://doi.org/10.1093/icb/icw106 is available online at: https://doi.org/10.1093/icb/icw106 or https://academic.oup.com/icb/article/56/5/758/2420636., It has been recognized for decades that animals sense light using photoreceptors besides those that are devoted strictly to vision. However, the nature of these receptors, their molecular components, their physiological responses, and their biological functions are often obscure. Only recently have researchers begun to learn how critical these non-visual or very simple visual responses are to organismal function; and new approaches, including high-throughput molecular genetic techniques, have led to a revolution in our understanding of the evolution, anatomical distribution, physiology, and – in some cases – function of non-visual photoreception in a diverse assemblage of organisms. Historically, these types of receptors have been described primarily among invertebrates, although they were recognized to be present in fishes, reptiles, and birds. Their presence in mammals was completely unsuspected until the end of the 20th century. In the following papers, we bring together specialists from throughout the field to review the current state of knowledge regarding extraocular, non-visual, and simple photoreceptors in a large diversity of organisms ranging from protists to advanced vertebrates and invertebrates.

The iconic red hourglass of the black widow spiders (genus Latrodectus) is traditionally considered an aposematic signal, yet experimental evidence is lacking. Here, we present data that suggest that black widow coloration may have evolved to be an aposematic signal that is more conspicuous to their vertebrate predators than to their insect prey. In choice experiments with wild birds, we found that the red-and-black coloration deters potential predators: Wild birds were ~3 times less likely to attack a black widow model with an hourglass than one without. Using visual-system appropriate models, we also found that a black widow’s red-and-black color combo is more apparent to a typical bird than a typical insect. Additionally, an ancestral reconstruction reveals that red dorsal coloration is ancestral in black widows and that at some point some North American widows lost their red dorsal coloration. Behaviorally, differences in red dorsal coloration between 2 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 trade-off of being more conspicuous to predators than to prey. We suggest that limiting detection by prey may help explain why red and black aposematic signals occur frequently in nature.

The colors of deep-sea species are generally assumed to be cryptic, but it is not known how cryptic they are and under what conditions. This study measured the color of approximately 70 deep-sea species, both pelagic and benthic, and compared the results with two sets of predictions: 1) optimal crypsis under ambient light, 2) optimal crypsis when viewed by bioluminescent "searchlights." The reflectances of the pelagic species at the blue-green wavelengths important for deep-sea vision were far lower than the predicted reflectances for crypsis under ambient light and closer to the zero reflectance prediction for crypsis under searchlights. This suggests that bioluminescence is more important than ambient light for the visual detection of pelagic species at mesopelagic depths. The reflectances of the benthic species were highly variable and a relatively poor match to the substrates on which they were found. However, estimates of the contrast sensitivity of deep-sea visual systems suggest that even approximate matches may be sufficient for crypsis in visually complex benthic habitats. Body coloration was generally uniform, but many crabs had striking patterns that may serve to disrupt the outlines of their bodies.

As the open ocean environment lacks points of refuge from visual predators, it has favored the evolution of extraordinary adaptations for optical concealment, such as vertical migration, trans- parency and counterillumination. Bioluminescent plankton, which respond to a mechanical disturb- ance with a flash of light, are ubiquitous in the ocean and potentially a threat to any organism seeking darkness as a means to evade detection. Estimating encounter probabilities for organisms maneu- vering through luminescent 'minefields' requires characterization of the three-dimensional distri- bution patterns of the potential light emitters. In order to measure nearest neighbor distances (NNDs) of bioluminescent biota, we have developed a spatial plankton analysis technique (SPLAT) for 3D reconstruction and statistical analysis of the spatial point patterns of identified bioluminescent displays. Analysis of aggregations of bioluminescent biota in the Gulf of Maine (Wilkinson Basin) revealed that encounter probabilities were highest in the temperature minimum zone (temperature