Sea (in)sight: from phylogeographical insights to visual local adaptation in marine gobies = (In)zicht op zee: van fylogeografische inzichten naar visuele lokale adaptatie bij mariene grondels
Exactly 150 years ago, Charles Darwin described natural selection as the motor of the evolution of life. Nevertheless, it is not yet clear how important natural selection is for the evolution of marine organisms.The genetic adaptation to local environmental conditions as a result of natural selection, a process known as local adaptation, will be reduced by the migration of organisms due to its homogeneous character. Because of the huge potential for migration in the ‘open’ sea, for a long time biologists declared that local adaptation is rare and even absent.Nevertheless, current research shows that the sea is not as ‘open’ as it may seem. Many marine organisms are able to occupy a permanent place and hence occur in distinct populations. Since migration seems limited, the possibility of local adaptation in marine species presents an important research question. The most recent studies showed that natural selection might be an important evolutionary force in the ocean, however without any good scientific evidence.The present thesis has the ambition to prove that marine species may indeed be genetically adapted to local conditions. A promising opportunity is the possibility for local adaptation to the light regime of the sea. The light that organisms perceive varies between seas due to the differences in turbidity and the colour of the water. The importance of sight for marine animals is obvious, especially to find food and mates, and to avoid predators. Therefore, the aim of the thesis was to study local adaptation at the rhodopsin gene - the gene of the visual pigment that determines the visual capacity in dim-light - of a marine goby, the sand goby (Pomatoschistus minutus). The sand goby is a small and abundant fish species that lives along the European coasts.The results showed strong evidence that sand goby populations are genetically adapted to their specific and local light environment. They are adapted to high turbidity in the Baltic Sea and the Mediterranean lagoons, and to the more blue light of the Bay of Biscay and along the coasts of Spain and Portugal. Moreover, the sand gobies of the North Sea reveal a strategy of adaptation to the unstable local light conditions. In the current state of science, the rhodopsin gene provides one of the strongest indications that local adaptation occurs in the marine environment. Therefore, they encourage analogous studies to find further evidence for l ocal adaptation to other marine environmental conditions such as salinity tolerance and temperature. Such studies will clarify the importance of natural selection as evolutionary force for marine life.To conclude, this study reveals that the sand goby is evolutionary adapted to its light environment. There are strong indications that if the light environment changes due to either pollution or climate change, marine fishes won’t likely be able to adapt rapidly to the new circumstances. Good management of the light conditions of the marine ecosystem will be essential to support a balanced ecosystem and healthy fish stocks
