Browsing by Author "Cronin, T. W."
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Item Hiding opaque eyes in transparent organisms: a potential role for larval eyeshine in stomatopod crustaceans(The Company of Biologists Ltd, 2014-06-23) Feller, K. D.; Cronin, T. W.Opaque screening pigments are a fundamental requisite for preserving resolution in image-forming eyes. Possession of any type of image-forming eye in a transparent, pelagic animal will thus undermine the ability of that animal to be invisible in the water column. Transparent, pelagic animals must therefore deal with the trade-off between the ability to see and the ability of other animals to see them. Stomatopod larvae, like many transparent crustaceans, possess specialized optics in their compound eyes that minimize the volume of the opaque retina. Though the volumes of these retinas are reduced, their opacity remains conspicuous to an observer. The light reflected from structures overlying the retinas of stomatopod crustacean larval eyes, referred to here as eyeshine, is hypothesized to further reduce the visibility of opaque retinas. Blue or green wavelengths of light are most strongly reflected in stomatopod larval eyeshine, suggesting a putative spectral matching to the light environment against which the larval eyes are viewed. We tested the efficacy of stomatopod crustacean larval eyeshine as an ocular camouflaging mechanism by photographing larvae in their natural light environment and analysing the contrast of eyes with the background light. To test for spectral matching between stomatopod larval eyeshine and the background light environment, we characterized the spectrum of eyeshine and calculated its performance using radiometric measurements collected at the time of each photographic series. These results are the first to demonstrate an operative mirror camouflage matched in both spectrum and radiance to the pelagic background light environment.Item Photoreception in the planktonic larvae of two species of pullosquilla, a lysiosquilloid stomatopod crustacean(The Company of Biologists Ltd, 1998-09-01) Jutte, P. A.; Cronin, T. W.; Caldwell, R. L.Optical microscopy, electron microscopy and microspectrophotometry were used to characterize pigments in the eyes of planktonic larvae of two species of the lysiosquilloid stomatopod Pullosquilla, P. litoralis and P. thomassini, which live sympatrically in French Polynesia. In contrast to the adult retina, which contains a diverse assortment of visual pigments in the main rhabdoms, the principal photoreceptors throughout the larval eyes of both species were found to contain a single rhodopsin with an absorption maximum (max) close to 446 nm. The expression of this visual pigment may survive metamorphosis, since several adult rhodopsins occur at a similar spectral position. The retinas of these planktonic larvae also contain a novel yellow photostable pigment, which is arrayed in a regular pattern at the distal margin of the larval retina. The absorption spectrum of this pigment is well matched to the larval rhodopsin, suggesting that it acts to screen the rhabdoms from stray light. By replacing opaque, black screening pigment, the transparent yellow pigment may act together with a blue iridescent layer in the larval retina to reduce the visual contrast of the larval eye against downwelling and sidewelling light, while simultaneously acting as a retinal screen.Item Photosensitivity spectrum of crayfish rhodopsin measured using fluorescence of metarhodopsin(Rockefeller University Press, 1982-02-01) Cronin, T. W.; Goldsmith, T. H.Discrepancies exist among spectral measurements of sensitivity of crayfish photoreceptors, their absorption in situ, and the number and absorption spectra of crayfish photopigments that are extracted by digitonin solutions. We have determined the photosensitivity spectrum of crayfish rhodopsin in isolated rhabdoms using long wavelength fluorescence emission from crayfish metarhodopsin as an intrinsic probe. There is no measurable metarhodopsin in the dark-adapted receptor, so changes in the emission level are directly proportional to metarhodopsin concentration. We therefore used changes in metarhodopsin fluorescence to construct relaxation and saturation ("photoequilibrium") spectra, from which the photosensitivity spectrum of crayfish rhodopsin was calculated. This spectrum peaks at or approximately 530 nm and closely resembles the previously measured difference spectrum for total bleaches of dark-adapted rhabdoms. Measurements of the kinetics of changes in rhabdom fluorescence and in transmittance at 580 nm were compared with predictions derived from several model systems containing one or two photopigments. The comparison shows that only a single rhodopsin and its metarhodopsin are present in the main rhabdom of crayfish, and that other explanations must be sought for the multiple pigments seen in digitonin solution. The same analysis shows that there is no detectable formation of isorhodopsin in the rhabdom.Item Polarization contrast vision in Octopus(The Company of Biologists Ltd, 1996-04-01) Shashar, N.; Cronin, T. W.While the ability to analyze polarized light is widespread among animals, its contribution to form vision has not yet been documented. We tested the hypothesis that polarization vision can be used for object discrimination, by training octopuses to distinguish between targets on the basis of the presence or absence of a pattern produced by a 90 ° polarization contrast within the target. Octopuses recognized a 90 ° contrast pattern within a single target, when presented either on a horizontal/vertical axis or on a 45 °/135 ° axis. They were able to transfer their learning to new situations and to detect a polarization contrast when the orientations of the e-vector of light passing through the target center and background differed by as little as 20 °. Polarization vision may provide information similar to that available from color vision and thus serve to enhance the detection and recognition of objects.Item Shrimps that pay attention: saccadic eye movements in stomatopod crustaceans(The Royal Society, 2014-02-19) Marshall, N. J.; Land, M. F.; Cronin, T. W.Discovering that a shrimp can flick its eyes over to a fish and follow up by tracking it or flicking back to observe something else implies a ‘primate-like’ awareness of the immediate environment that we do not normally associate with crustaceans. For several reasons, stomatopods (mantis shrimp) do not fit the general mould of their subphylum, and here we add saccadic, acquisitional eye movements to their repertoire of unusual visual capabilities. Optically, their apposition compound eyes contain an area of heightened acuity, in some ways similar to the fovea of vertebrate eyes. Using rapid eye movements of up to several hundred degrees per second, objects of interest are placed under the scrutiny of this area. While other arthropod species, including insects and spiders, are known to possess and use acute zones in similar saccadic gaze relocations, stomatopods are the only crustacean known with such abilities. Differences among species exist, generally reflecting both the eye size and lifestyle of the animal, with the larger-eyed more sedentary species producing slower saccades than the smaller-eyed, more active species. Possessing the ability to rapidly look at and assess objects is ecologically important for mantis shrimps, as their lifestyle is, by any standards, fast, furious and deadly.