Browsing by Author "Cronin, Thomas W."
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- Spectral sensitivity of four species of fiddler crabs (Uca pugnax, Uca pugilator, Uca vomeris and Uca tangeri) measured by in situ microspectrophotometryPublication . Jordão, Joana; Cronin, Thomas W.; Oliveira, Rui FilipeFiddler crabs have compound eyes that are structurally fairly well understood. However, there has been much debate regarding their spectral sensitivity and capacity to enable colour discrimination. We examined the visual pigments of two North-American species (Uca pugnax and U. pugilator), one species from the Indo-West Pacific (U. vomeris) and the only Eastern-Atlantic species (U. tangeri) of fiddler crabs using in situ microspectrophotometry of frozen sections of dark-adapted eyes. Only one spectral class of visual receptor was found in the larger (R1–7) retinular cells of each species, with maximum absorption peaking between 508·nm and 530·nm (depending upon species). The R8 retinular cell, that might contain a shortwavelength sensitive photopigment and provide a basis for colour vision, was too small to analyze by these methods. Rhabdoms were lined with screening pigment which strongly influenced each species’ spectral sensitivity, sharpening the peak and shifting the maximum towards longer wavelengths, on occasion to as far as the 600·nm region. We hypothesize that sensitivity to longer wavelengths enhances contrast between background (blue sky or tall vegetation) and the male major claw during the waving display.
- Visual ecology of the fiddler crab, Uca tangeri: Effects of sex,viewer and background on conspicuousnessPublication . Cummmings, Molly E.; Cronin, Thomas W.; Jordão, Joana; Oliveira, Rui FilipeWe investigated the visual ecology of the coloration of the eastern Atlantic fiddler crab, Uca tangeri, with particular attention to predator (e.g. avian) and conspecific vision. Spectral reflectance measurements were made on different body parts used in possible intraspecific communication as well as background habitats including crab-made materials (e.g. mudballs). Avian-based and crab-based visual models were used to obtain different estimates of crab conspicuousness to potential predators and conspecifics. We found that male body parts (except for dorsal carapace) were significantly more conspicuous to conspecific viewers than female equivalent body parts, and showed greater within-body contrast estimates. Moreover, male major claw areas differed in reflectance properties, producing variation in conspicuousness that fit signalling predictions: areas visible during claw-waving events were most conspicuous against the background sky, whereas areas visible in nonwaving positions were more conspicuous against substrate backgrounds. For avian vision, sexually dimorphic coloration results in males being generally more conspicuous than females (in terms of brightness contrast) against all backgrounds, however, there was no sexual dimorphic conspicuousness of carapace coloration. Furthermore, one of the most conspicuous features of both male and female crabs is an area that is likely to be more visible to crabs (mouthparts) than to avian predators from above. While conspicuousness varies with background, the most conspicuous background for male signalling parts (major claw) is dark mudballs, suggesting that males may increase the conspicuousness of their signals by modifying their signalling environment.