Whale shark rhodopsin evolution explained
How whale shark rhodopsin evolved to see, in the deep blue sea – a unique adaptation explains the secrets of whale shark vision
A research group including Professors Mitsumasa Koyanagi and Akihisa Terakita of the Osaka Metropolitan University Graduate School of Science has investigated both the genetic information and structure of the photoreceptor rhodopsin, responsible for detecting dim light, of whale sharks to investigate how they can see in the dim light at extreme depths.
The research group compared the whale sharks to zebra sharks, which are considered their closest relative, and brown-banded bamboo sharks, which are in the same group: the order orectolobiformes – commonly known as carpet sharks.
Koyanagi said: “This research used genetic information and molecular biological techniques to achieve stunning results – without harming whale sharks or their biology.
“Our research approach is to use these techniques to provide clues that reveal the mysteries of how these organisms live.
“The beautiful part is that it even works for species where information is limited, such as large or wild animals that are difficult to observe or follow in their natural habitat.”
See also: Breakthrough as endangered sharks’ genomes are sequenced
The research revealed that the whale sharks’ rhodopsin can efficiently detect blue light – the most common wavelength of light in the deep-sea – because two amino acid substitutions shifted the light spectra that rhodopsin detects, making it sensitive to blue wavelengths.
However, one of the amino acid substitutions defies conventional wisdom, as it corresponds to a mutation at a position known to cause congenital stationary night blindness in humans.
The researchers found that the amino acid substitutions make the whale shark rhodopsin less thermally stable, it decays rapidly at 37°C, compared to human or other sharks’ rhodopsin without the substitution.
However, at deep-sea temperatures – well below 37°C – the functionality of the whale shark rhodopsin can be maintained, suggesting that this unique adaptation evolved for life in the low-temperature low-light deep-sea environment.
The whale shark is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of 18.8m (61.7ft). The whale shark holds many records for size in the animal kingdom, most notably being by far the largest living non-mammalian vertebrate.
It is found in open waters of the tropical oceans and has an estimated lifespan of 80–130 years. They are filter feeders and feed almost exclusively on plankton and small fishes, posing no threat to humans.
The species is considered ‘endangered’ on the IUCN Red List due to the impacts of fisheries, by-catch losses, and vessel strikes, combined with its long lifespan and late maturation.
Image: New study reveals that the photoreceptor rhodopsin of whale sharks (Rhincodon typus), pictured here, evolved to improve sight for the low-light low-temperature deep-sea environment in a unique way. © Mitsumasa Koyanagi, OMU.