When you cannot see, flee

Anyone diving in waters of Great Britain will be familiar with the sensation of occasionally being barely able to see their outstretched arm. Low visibility is common in waters containing large amounts of sediment, which is often one of the symptoms of human maritime activity on a fragile marine environment. While this is a nuisance for those of us attempting to appreciate the wonders of the underwater world, for many marine species the decrease in visibility can have an enormous effect on their chances of survival.

Increased deposition of sediment is among the many problems caused by humans in coral reefs, and its negative effects on coral have been well documented. As well as stifling the coral, all this floating sediment can also severely reduce visibility for the species that live on the reef. As the decreasing visibility in reefs make it extremely difficult for species to avoid predation using visual cues, these animals are now being forced to radically change the way they sense the world in order to survive.

While visual cues are the most accurate way of detecting a predator, many aquatic species also use chemical cues. For example, chemical alarm cues (or CACs) are released by several aquatic species automatically as they are wounded. These can give warnings to others that there is a predator in the area and that an attack is occurring, or has occurred in the recent past. However, chemical signals are less accurate than relying on one’s own vision, as the chemicals can linger in an environment long after the predator has left and cannot identify the type of predator. Though less accurate chemical cues do have the advantage of always being available to an animal, even in low visibility. Several species in environments of increasingly low visibility are predicted to change their behaviour to rely more on such chemical cues. This shift in behaviour is known as “Sensory Compensation”.

A recent study tested for sensory compensation in different levels of visibility using damselfish (Acanthochromis polyacanthus), a planktivorous fish found in great numbers on Australia’s Great Barrier Reef. The researchers exposed the fish to their own chemical alarm cues and observed the changes in their behaviour. They found that the lower the visibility, the greater the fish’s reaction to the CACs. This supports the sensory compensation idea and provides some interesting ideas on how increased reliance on chemical cues will affect marine animal’s behaviour in the wild.

The lack of visual information forces the use of the less accurate chemical cues, which in turn can lead to overly cautious decision making. The researchers suggest that in future this may have a detrimental effect on the fish in areas of low visibility, as they spend more time attempting to avoid mystery predators (who may not even be present at the time) and less time in other important activities such as feeding.

However, while sensory compensation may result in lower efficiency at other activities, the ability to avoid predators will increase survival rates. If visibility continues to decrease and animals continue to compensate by increasing reliance on chemical cues, we may expect senses to evolve to be able to interpret chemical cues more accurately. At its most extreme, this may result in visual cues becoming entirely useless in such environments, and cause species eventually evolve to lose their eyes as seen in some deep sea trench dwelling species. Sedimentation due to human activities can therefore have a significant effect on all species within that environment, for whom low visibility is more than just a nuisance!

Reference: To fear or to feed: the effects of turbidity on perception of risk by a marine fish. Biology Letters www.rsbl.royalsocietypublishing.org/content/early/2011/08/03/rsbl.2011.0645

Image Credits: SeaScape Studio, Australian Institute of Marine Science

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