Diversity of Opsins in Marine Zooplankton: A Survey of Copepods Across Different Environments

Opsins are photoreceptive proteins essential for vision and light detection in animals, with fundamental roles in species interactions and ecological processes. Planktonic copepods, the most abundant metazoans on Earth, display a wide range of visually mediated behaviors throughout their life cycle, yet the molecular basis of their photoreception remains poorly understood. Here, we used a bioinformatic workflow, followed by phylogenetic and protein structural domain analyses, to characterize putative transcripts encoding c-opsin, r-opsin (MWS and Rh7), and tetraopsin in calanoid copepods from Antarctic, Subarctic, and Mediterranean ecosystems. All opsin classes were identified across the 13 transcriptomes, except Rh7, which was restricted to polar species. Structural analysis confirmed the expected domains and the functional lysine residue in most transcripts, and phylogenetic analysis placed the sequences within their expected clades, with copepod sequences clustering separately from malacostracan ones. Differences in opsin representation among species from distinct environments suggest possible adaptive responses to habitat-specific light regimes and other environmental pressures. The dataset, comprising more than 100 new candidate opsin genes, is a valuable resource for investigating phototransduction in zooplankton and contributes to our understanding of the evolution of photoreception in marine metazoans.