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The shape and relative size of an ocular lens affect the focal length of the eye, with consequences for visual acuity and sensitivity. Lenses are typically spherical in aquatic animals with camera-type eyes and axially flattened in terrestrial species to facilitate vision in optical media with different refractive indices. Frogs and toads (Amphibia: Anura) are ecologically diverse, with many species shifting from aquatic to terrestrial ecologies during metamorphosis. We quantified lens shape and relative size using 179 micro X-ray computed tomography scans of 126 biphasic anuran species and tested for correlations with life stage, environmental transitions, adult habits and adult activity patterns. Across broad phylogenetic diversity, tadpole lenses are more spherical than those of adults. Biphasic species with aquatic larvae and terrestrial adults typically undergo ontogenetic changes in lens shape, whereas species that remain aquatic as adults tend to retain more spherical lenses after metamorphosis. Further, adult lens shape is influenced by adult habit; notably, fossorial adults tend to retain spherical lenses following metamorphosis. Finally, lens size relative to eye size is smaller in aquatic and semiaquatic species than other adult ecologies. Our study demonstrates how ecology shapes visual systems, and the power of non-invasive imaging of museum specimens for studying sensory evolution. [ABSTRACT FROM AUTHOR]

For eyes to maintain optimal focus, precise coordination is required between lens optics and retina position, a mechanism that in vertebrates is governed by genetics, visual feedback, and possibly intraocular pressure (IOP). 1 While the underlying processes have been intensely studied in vertebrates, they remain elusive in arthropods, though visual feedback may be unimportant. 2 How do arthropod eyes remain functional while undergoing substantial growth? Here, we test whether a common physiological process, osmoregulation, 3 could regulate growth in the sophisticated camera-type eyes of the predatory larvae of Thermonectus marmoratus diving beetles. Upon molting, their eye tubes elongate in less than an hour, and osmotic pressure measurements reveal that this growth is preceded by a transient increase in hemolymph osmotic pressure. Histological evaluation of support cells that determine the lens-to-retina spacing reveals swelling rather than the addition of new cells. In addition, as expected, treating larvae with hyperosmotic media post-molt leads to far-sighted (hyperopic) eyes due to a failure of proper lengthening of the eye tube and results in impaired hunting success. This study suggests that osmoregulation could be of ubiquitous importance for properly focused eyes. [Display omitted] • Post-molt eye growth in insect larvae involves rapid support cell swelling • Support cell expansion coincides with endogenous hemolymph osmotic changes • Interfering with osmotic processes results in farsightedness and behavioral deficits • Osmoregulation may be of ubiquitous importance for properly focused eyes Rathore et al. find that the post-molt rapid growth of camera-type eyes of Thermonectus marmoratus larvae is driven by osmotic processes. Supporting evidence suggests that endogenous hemolymph osmotic changes correlate with support cell swelling. Manipulating the osmotic environment leads to far-sighted larvae with behavioral deficits. [ABSTRACT FROM AUTHOR]

Sudden disease outbreaks may not necessarily reflect a recent pathogen introduction but may instead arise from the disruption of a host-pathogen equilibrium. Together with invasive species, emerging pathogens pose significant threats to biodiversity. The dynamics of each stressor have been studied separately, yet rarely when interacting. Using a 40-year dataset, we tested the hypothesis that the introduction of an invasive fish leads to such a disruption, manifested by ranavirosis outbreaks on amphibian hosts. MCP sequencing revealed the historical presence of two major Ranavirus clades, with low prevalence. The introduction of fish was not followed by the emergence of new viruses, but rather by an increase in the prevalence of the strains already present, fitting the 'endemic pathogen hypothesis'. Two decades after the first die-offs, one amphibian species persists in extremely low numbers, but Ranavirus prevalence is closer to the enzootic phase that preceded the outbreaks. Models show that host population collapse and lack of recovery are best explained by the concerted interaction of Ranavirus and invasive fish. We provide robust evidence that invasive species can impact naïve communities by disrupting the host-pathogen balance, exacerbating health threats. This study emphasizes the importance of exploring the historical interactions between multiple stressors to understand population declines. • Historical presence of Ranavirus was not associated with signs of disease or population declines (EPH); • Introduction of invasive species was not followed by the emergence of new viruses; • Sudden disease emergence may arise from a disruption of a host-pathogen equilibrium; • Invasive species can impact native populations by disrupting host-pathogen equilibria, leading to disease-driven declines; • Removal of invasive species decreases the extinction risk of the host population even in the presence of the pathogen. [ABSTRACT FROM AUTHOR]