Abstract
Absorbance spectra of single rhabdoms were studied by microspectrophotometry (MSP) and spectral sensitivities of whole eyes by electroretinography (ERG) in three glacial-relict species of opossum shrimps (Mysis). Among eight populations from Fennoscandian fresh-water lakes (L) and seven populations from the brackish-water Baltic Sea (S), L spectra were systematically red-shifted by 20–30 nm compared with S spectra, save for one L and one S population. The difference holds across species and bears no consistent adaptive relation to the current light environments. In the most extensively studied L–S pair, two populations of M. relicta (Lp and Sp) separated for less than 10,000 years, no differences translating into amino acid substitutions have been found in the opsin genes, and the chromophore of the visual pigments as analyzed by HPLC is pure A1. However, MSP experiments with spectrally selective bleaching show the presence of two rhodopsins (λmax ≈ 525–530 nm, MWS, and 565–570 nm, LWS) expressed in different proportions. ERG recordings of responses to “red” and “blue” light linearly polarized at orthogonal angles indicate segregation of the pigments into different cells differing in polarization sensitivity. We propose that the pattern of development of LWS and MWS photoreceptors is governed by an ontogenetic switch responsive to some environmental signal(s) other than light that generally differ(s) between lakes and sea, and that this reaction norm is conserved from a common ancestor of all three species.
Highlights
The genus Mysis comprises a great number of species widely distributed over the arctic and temperate zones of the northern hemisphere
Molecular evidence indicates that the evolutionary radiation of this group dates back ≥2 Myr, the divergence of M. segerstralei and M. salemaai is more recent (Väinölä 1986; Audzijonyte et al 2005; Audzijonyte and Väinölä 2005, 2006)
This as well as the relation of the whole group to the wider genus Mysis is schematically shown in Fig. 1a
Summary
The genus Mysis (opossum shrimps) comprises a great number of species widely distributed over the arctic and temperate zones of the northern hemisphere. Molecular evidence indicates that the evolutionary radiation of this group (formerly regarded as a single species M. relicta, sensu lato) dates back ≥2 Myr, the divergence of M. segerstralei and M. salemaai is more recent (Väinölä 1986; Audzijonyte et al 2005; Audzijonyte and Väinölä 2005, 2006). This as well as the relation of the whole group to the wider genus Mysis is schematically shown in Fig. 1a (cf Audzijonyte et al 2012). These switches have often been associated with significant changes in light conditions
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