The opsin repertoire of syngnathids (pipefish & seahorses) and its expression changes throughout development, season and between sexes in Syngnathus typhle

Abstract Vision is an important sense for fish and, as various properties of the water column affect how well different light wavelengths are transmitted, fishes’ visual sensitivity spectra reflect the varying ambient light conditions across habitats. Syngnathids (pipefishes & seahorses) inhabit various salt- and freshwater habitats-many affected by geological, anthropogenic and seasonal changes, challenging these visual predators. Here, we explored their visual opsin gene repertoire and found that, while retaining most common visual opsin genes, syngnathids have lost the UV-sensitive SWS1, while the subfamily of Nerophinae likely evolved a second copy of RH2 and all seahorses lost SWS2b. We further tested if the widely distributed Syngnathus typhle shows flexibility in visual capabilities throughout development and across the breeding season in its habitat, the Baltic Sea. Adult opsin expression composition was established soon after birth, with SWS2b barely being expressed. Adults changed the composition of expressed opsins across seasons, likely in response to the changing sunlight intensity and food composition & availability. Being the choosier sex, males expressed more green-light sensitive RH2 than females, possibly improving their ability to assess mate quality. S. typhle therefore showed highly dynamic opsin expression, plastically responding to fluctuations in its specifically variable habitat.