Sticklebacks-You need new eyes

by Matthew Norton

Sight is a common sense in animals that live in the sea with their eyes, or equivalent visual organ, being modified in various ways to suit a particular species. These modifications can evolve in response to the light levels in their environment as well as their lifestyle, which can influence how well they need to see. This is well demonstrated in threespined sticklebacks, Gasterosteus aculeatus.

Threespined sticklebacks are small, carnivorous fish which have three spines (usually) in their dorsal fins and are found in both seawater and freshwater (rivers, lakes etc) environments. Their eyes also possess an uncommon type of light detecting cell (called cone cells) that allow them to see ultraviolet light.

Unsurprisingly, there have been a number of studies to determine why these fish need to see ultraviolet light, with the most likely explanation being that they reflect ultraviolet light with their silvery bodies and use this to communicate with each other through swimming displays. Threespined sticklebacks communicate with each other in various scenarios, such as cooperating with each other to share food and inspect potential predators, while also be aggressive to strangers and known cheaters. Breeding also involves various displays from the males to entice females to lay eggs in their nest, these include zig-zag swimming, leading females to the nest and fanning the nest with their tail.

The problem with being so expressive is that these sticklebacks risk attracting their predators. However, this is where seeing and reflecting ultraviolet light is advantageous because it does not travel far in water before it is absorbed and scattered by the water and any material and organisms suspended in it. This short range is sufficient for communication between threespined sticklebacks while minimising the risk of detection from eavesdropping predators.

However, freshwater populations of threespined sticklebacks use a different strategy, being less sensitive to ultraviolet light, apparently in favour of increased sensitivity to green light. This suggests that the range of ultraviolet light is too short in freshwater and instead rely on other wavelengths of light for communication. This shift in light sensitivity evolved since the last ice age, approximately 12,000 years ago, when the freshwater populations were separated from their seawater ancestors, a remarkable feat considering that normally such modifications would take millions of years to evolve.

How any creature sees the world is of major importance to how they interact with it, especially given that light is theoretically the fastest way that information can reach anyone in the world. This article has only touched on the sensitivity of stickleback eyes to different wavelengths of light, but there can be other variations. Some species are blind or can only ‘see’ light and dark, while others have complex eyes structures that can be use to build a detailed picture of the world and a select few can even see better than us.

From a human perspective

Sticklebacks are a popular group to use in scientific research, being small, tame and easy to keep and observe in aquariums while also possessing some interesting behaviours and evolutionary features. As a result they have played a role in some major scientific advances.

In particular the Dutch behavioural scientist Niko Tinbergen studied stickleback behaviour extensively from the 1930s, even claiming they were more reliable than popular alternatives, including bees and aquatic insects. This work, as well as that of two Austrian scientists, Konrad Lorenz and Karl von Frisch, was important during the early development of Ethology, a scientific discipline focusing on the evolution of animal behaviour.

Tinbergen himself introduced a guide for understanding an animal’s behaviour by considering all its different components, this was called “Tinbergen’s four questions”. Theoretically this can even by extended to any evolved trait, making these four questions a valuable tool for biologists.

Sticklebacks have been relevant to other fields of research, such as genetics where threespined sticklebacks were among the first species of fish to have their genome (i.e. all their genes) sequenced. This was probably because the recent separation, in geological time, between the seawater and freshwater populations provides a useful opportunity to study how an animal’s genome adapts to an environmental change. This is a topic of particular interest in evolutionary biology.

Both the areas of research I’ve mentioned can also be relevant to us. Human ethology has emerged from its ‘parent discipline’ as a way of understanding human behaviour and psychology in an evolutionary context. Meanwhile, human genomics can tell us more about certain diseases, such as heart disease, asthma and diabetes, which are influenced by multiple genes and their interactions with each other and their environment.

In scientific research there will always be some species that are better understood than others due to practicality and interest in the species. Nonetheless, these species can help us to better answer fundamental biological questions, which in some cases can be applied to better understand and help humanity.

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Image sources

Fulvio314. 2016. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. https://commons.wikimedia.org/wiki/File:Light_spectrum_(precise_colors).svg

Max Planck Gesellschaft. 1978. [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]. https://commons.wikimedia.org/wiki/File:Lorenz_and_Tinbergen2.jpg

All other images are public domain and do not require attribution