Spectral sensitivity of four species of fiddler crabs (Uca pugnax, Uca pugilator, Uca vomeris and Uca tangeri) measured by in situ microspectrophotometry

摘要

Fiddler crabs have compound eyes that are structurallyfairly well understood. However, there has been muchdebate regarding their spectral sensitivity and capacity toenable colour discrimination. We examined the visualpigments of two North-American species (Uca pugnax andU. 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 offrozen sections of dark-adapted eyes. Only one spectralclass of visual receptor was found in the larger (R1–7)retinular cells of each species, with maximum absorptionpeaking between 508·nm and 530·nm (depending uponspecies). The R8 retinular cell, that might contain a shortwavelengthsensitive photopigment and provide a basis forcolour vision, was too small to analyze by these methods.Rhabdoms were lined with screening pigment whichstrongly influenced each species’ spectral sensitivity,sharpening the peak and shifting the maximum towardslonger wavelengths, on occasion to as far as the 600·nmregion. We hypothesize that sensitivity to longerwavelengths enhances contrast between background (bluesky or tall vegetation) and the male major claw during thewaving display.