Whale sharks and cloudy catsharks have visual systems tuned towards blue light, and fewer genes associated with sense of smell, reports a study published online this week in Nature Ecology & Evolution. The authors also identify shared regulatory and reproductive genes between sharks and mammals.
Cartilaginous fish, including sharks, diverged from the rest of the jawed vertebrates around 450 million years ago - and have unique reproductive and sensory traits. However, genomic resources for cartilaginous fish are scarce, hindering our understanding of their unique lifestyles.
Shigehiro Kuraku and colleagues sequenced the genomes and transcriptomes of the brownbanded bamboo shark and cloudy catshark, and improved the genome assembly of the whale shark, the largest living fish. By comparing the shark genomes with those of other vertebrates, the authors find that cartilaginous fish have a slower rate of evolution than bony fish. They find that two of the three sharks species studied (whale sharks and cloudy catsharks) have a reduced repertoire of light-sensitive opsin genes, lacking the short-wavelength and green/blue-sensitive opsins. The authors suggest that the limited number of opsin genes in whale sharks and cloudy catsharks is an adaptation for living at depths that only blue light reaches. Additionally, all three shark species have only three olfactory receptor family genes, suggesting that they rely on an unconventional molecular mechanism to smell.
In the sharks, the authors also identify hormone and receptor genes similar to those implicated in appetite, digestion, fertility and sleep, in mammals - suggesting these molecular mechanisms trace back to before the last common ancestor of living jawed vertebrates.