The visual pigments of crabs - II. Environmental adaptations
1. Underwater downwelling quantal irradiance spectra were measured in estuarine and coastal areas under various tidal and rainfall conditions. At midday the available spectrum near the bottom has maximal irradiance in the region of about 570 to 700 nm in the estuary, whereas in offshore coastal areas greatest irradiance occurs between 500 and 570 nm. At twilight in an estuary, maximal underwater downwelling irradiance shifts to the 490-520 nm region. 2. The visual pigment absorption maxima of 27 species of benthic crustaceans from semi-terrestrial, estuarine and coastal areas have values ranging from 483 to 516 nm. There is no obvious shift in the λmax from long wavelengths in estuarine species to shorter wavelengths in coastal species. The only match between λmax and midday spectrum was for a continental shelf species, Geryon quinquedens. 3. The Sensitivity Hypothesis is predicted to account for the visual sensitivity of benthic crabs from estuarine and coastal areas. To assess the match between visual spectral sensitivity and environmental spectra, photon capture effectiveness was calculated for a range of idealized visual pigment absorption functions operating in the measured environmental spectra. 4. All crab species are poorly adapted for maximal photon capture at midday, since pigments having λmax longer than 540 nm function best under all daytime spectral conditions. Photon capture of visual pigments with λmax near 500 nm improves dramatically at twilight, particularly at lower visual pigment densities and shallow depths. However, pigments having λmax at wavelengths longer than those for the crabs are equally or more efficient at photon capture. Therefore the Sensitivity Hypothesis is not supported for crustaceans. © 1988 Springer-Verlag.
Forward, RB; Cronin, TW; Douglass, JK
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