Kjernsmo, K.; Gronholm, M.; and S. Merilaita. 2016. “Adaptive constellations of protective marks: eyespots, eye stripes and diversion of attacks by fish”. Animal Behaviour 111: 189-195. doi:10.1016/j.anbehav.2015.10.028
Eyespots and Stripes Forever
I study African cichlids – an extremely diverse group of over two thousand fish species that live in Africa. These animals are extremely closely related, and one of the best ways to distinguish between them is by their strikingly different coloration patterns (Figure 1). Other fish are also readily identifiable by their coloration patterns, but there are some common themes in fish coloration, like eyespots and eye stripes.
Eyespots are a common coloration theme throughout the animal kingdom – they’re found on insects, fish, reptiles, and birds. This marking consists of concentric circles and is usually found on the tail end or the edge of the animal (far away from the actual eyes). Another common feature of fish coloration is eye stripes, or chunky lines that bisect the eyes. These two features are often found together on the same fish, even across distantly related species (Figure 2). Why do all these different fish share these distinctive markings? What is their purpose?
Predators are an ever-present fact of life for fish. Multiple factors determine whether a fish will survive an encounter with a predator (which are often other, larger fish), including how fast the potential prey is, whether the predator decides to attack, and if the prey is able to survive the damage inflicted by the predator. The predator may not attack – either because it isn’t hungry at that particular moment, or because it is intimidated by its prey. But should the predator attack the prey, it has to fatally wound its prey in order to be successful. The prey could escape if the predator misses during the attack or if it fails to damage a critical part of the prey such as the head or abdomen. It is thought that eyespots divert predator attacks to a less vulnerable and less critical part of the body – the tail. Since eyespots look like vertebrate eyes, the predator may be confusing the tail for the head. Or perhaps the eyespot is just a convenient, conspicuous point on an otherwise difficult-to-see background. The eye stripe, on the other hand, is thought to mask the eye. It runs straight through the true eye, distorting the shape and making it much more difficult to see. Eye stripes may therefore divert attention away from the head by hiding the true eye. This paper attempted to understand the influence of eyespots and stripes on the behavior of a common fish predator, the threespine stickleback (Figure 3).
These researchers set out to understand the following:
- Whether stickleback preferentially attack the side of the prey with the eyespot
- Whether the shape of the diverting mark matters (i.e., does a rectangle work as well as a circle in order to distract a predator)
- Whether a fish predator shows preference for attacking an eyespot or an eye masked by a stripe
In order to study these questions, the researchers set up an experimental aquarium like the one shown in Figure 4. First, stickleback were trained on unmarked artificial prey items to associate them with a food reward. Individual fish were acclimated to the aquarium in the starting zone, separated from the experimental arena by a removable, opaque divider. The artificial prey were placed on the feeding plate in the foraging zone. Artificial prey were printed on waterproof paper. Trained stickleback were introduced into the experimental aquarium which included only one of the four possible prey types (Figure 5, panels a,b,c, and d respectively):
- Prey with eyespot shape on one half of its body
- Prey with rectangular shape on one half of its body
- Prey with eyespot on one half and rectangle on other half of its body
- Prey with eyespot on one half and eye disrupted with stripe on other half of its body
The researchers then recorded the number of strikes the stickleback made to each side of the artificial prey item.
The results from all four experiments are presented in Figure 5. The researchers showed that stickleback attacked the half of the prey’s body with the eyespot significantly more often than the half of the body that didn’t have an eyespot. They also found that stickleback did not show a statistically relevant preference for attacking the rectangular marking over the half of the body without the marking – although they do show a slight trend for the conspicuous marking. With a larger sample size, that trend may prove to be statistically significant. These predators also did not show a significant preference for attacking the eyespot over the rectangular marking. They did show a very strong preference for attacking the eyespot over attacking an eye-like shape masked by a stripe.
Stickleback are definitely using coloration patterns to orient themselves along their prey’s body in a laboratory setting– the half of the prey with an eyespot was far more likely to be attacked than the half of the prey without such a marking. There was no such statistically significant preference for attacking a rectangular marking, although there was a trend in favor of the marked side of the artificial prey. When presented with both an eyespot and a rectangular marking, stickleback attacked the two halves of the prey item fairly evenly, suggesting that it is not the shape of the eyespot but rather the presence of a conspicuous marking that accounts for the protective nature of eyespots. Discovering why eyespots are shaped as they are will require further study. However, it is extremely apparent that the presence of an eyespot along with an eye that is masked by a stripe strongly diverts a predator’s attack towards the eyespot. It seems likely, then, that eyespots and eye stripes work together to protect prey from predators, and this is why they are both so common and so commonly found together.
Questions? Comments? Please sound off below! I’d love to hear from you :)
I am a doctoral candidate in the Organismic and Evolutionary Biology program at the University of Massachusetts Amherst. I’m interested in how an individual’s genes and the environment in which it grows come together to determine its physical traits. I study a group of closely related freshwater fish called cichlids which live in the African rift lakes like Victoria, Malawi, and Tanganyika.