Visual Pigment Absorbance Spectra and Spectral Sensitivity of Mysis relicta (Crustacea, Mysidae) in Two Light Environments

Abstract

Abstract: : Purpose: To study evolutionary adaptation to different spectral light environments of two populations of the opossum shrimp (Mysis relicta Lovén). One lives in the Baltic Sea (Pojoviken, Finland) and the other in the freshwater lake (Lake Pääjärvi; Finland) separated from the sea after the Ice Age (9000 years ago). The spectra of downwelling light in the two bodies of water peak at 575 nm and 670 nm, respectively. Methods: Absorbance spectra were recorded by single–cell microspectrophotometry (MSP) in single rhabdoms from one marine and one freshwater population. Quantum catches were calculated by convolution of the A2 templates for the respective λ_max values of the two populations with quantum spectra of the downwelling light. Results: Mean λ_max ± SEM of rhabdoms for the marine and the freshwater populations were 529.2 ± 1.3 nm and 554.9 ± 1.6 nm respectively. The shapes of the absorbance spectra indicated that the pigments were pure A2 porphyropsins. Convolution indicated that a 115% improvement in quantum catch would accrue in the Lake environment from the > 20 nm red–shift of the rhabdom pigment of the lake compared with the Baltic opossum shrimps. Conclusions: The red–shift (> 20 nm) in spectral absorbance in the lake population compared with the sea population correlates with the difference in light environments and gives very substantial improvement in quantum catch. It must be due to differences in the amino acid sequence of the opsin, as both had the same chromophore (A2). In itself the use of the A2 chromophore in both populations is an adaptation unusual in invertebrates.

Keywords: photoreceptors • protein structure/function