An international team, among them researchers of the Senckenberg Biodiversity and Climate Research Centre, has discovered that despite having massive eyes, nocturnal fish have less brain tissue devoted to processing visual stimuli. The study sheds new light on how animals’ circadian rhythms have shaped vertebrate brains, and was published recently in the “Journal of Evolutionary Biology”.
When night falls on the world’s oceans, some fish are just about to get rest, whereas for others it is the start their day’s work. A group of researchers from Japan, the US and Germany have now scrutinized day-time and night-time active fish in order to find out how these contrasting lifestyles have affected fish brains.
They discovered that less brain tissue is dedicated to processing visual stimuli in nocturnal fish compared to fish that are mainly active during the day. “It’s unexpected because at the same time nocturnal fish have developed enlarged eyes, suggesting vision is very important for them, they devote less brain power to processing the information. Large eyes are better at gathering limited light but the smaller visual center suggests they are giving something up." says Dan Warren, senior researcher at the German Senckenberg Biodiversity and Climate Research Centre.
In the study, researchers analyzed day-time and night-time active fish species that were caught off the coast of Hawaii, Curaçao, and North Carolina, US. To investigate the size of parts of the fish brains the team made 3D images of them using the same CT-scanning technology used in human medicine. The measurements were then linked to evolutionary, ecological, morphological, and behavioral data of the fish collected.
In general, day-time active fishes had far more brain tissue available for processing visual information. In particular, reef-associated flatfish were equipped with more visual processing brain tissue than any other fish species. These fishes change color to match the complex color patterns of the reef, requiring the fish equivalent of a visual processing super-computer.
The team also found that fishes feeding in open water at night had similar amounts of brain tissue dedicated to processing visual stimuli as daytime active fishes. “This illustrates that beyond the difference between night and day time activity, the size of visual processing centers in fish has been really shaped by the need to avoid predators - both camouflage and early predator detection are examples where an enhanced vision might be particularly useful”, Warren and colleagues explain.
The team's research also raises a conservation concern. As coastal cities around the world continue to expand, urbanization is altering the amount of light entering the water at night. Artificial lightning has been proven to have the potential to alter animals’ circadian rhythms and thus their activity patterns. However marine food webs are based on these activity patterns. "Many studies have shown that rapid changes in food webs bring about a loss of biodiversity in an ecosystem. In addition, we now know from our study that the hithero circadian rhythms have been major players in shaping the fish brains. It stands to reason that besides short-term changes to food webs these disturbances may also drive neurological changes in the long run which consequences are hard to predict" concludes Warren.
Senckenberg Biodiversity and Climate Research Centre (SGN)
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