Avoiding predation in the underwater world

‍

The majority of marine animals across the planet live under threat from predation. Animals have adapted to deal with this risk, and have evolved over evolutionary time to avoid it. The first step in avoiding the threat of predation is to be able to recognise it. Learning what is and isn’t a potential predator is a risky business – as it’s quite possible that a first encounter could also be the last. Innate recognition of visual and chemical cues has therefore evolved, and is the result of prolonged sympatry (organisms living together in the same environment). Supporting evidence for this includes rare examples of where a prolonged sympatry has not existed, for example when a species has been recently introduced into a foreign ecosystem. Introduced species often become a thriving predator as their prey are unable to recognise them as a threat. This is the exact reason why lionfish introduced into the Caribbean sea have become so problematic, outcompeting other, easily recognisable, predators within the ecosystem. It is the same reason why those lionfish species are non-problematic in the environments they are found to exist indigenously in.

Recognition of a threat is one thing, but avoiding it is an ultimate requirement for survival. Predation commonly follows a general sequence: 1. Detection 2. Attack  3. Capture  4. Consumption. Marine animals have evolved a number of strategies that help reduce the risk of predation, aimed at minimising one or several of these stages with the sequence. Some of the most common methods are reviewed below

Hiding

The first, and often easiest method of reducing the risk of predation is simply by hiding from a predator. This is one of the most effective ways of avoiding detection. Even in the unlikely event of an eventual detection, hiding is also likely to prevent the sequence of predation from evolving. Hiding is, however, a costly behaviour to perform. When hiding, there is a significant trade off between risk and reward. Hiding limits the time available for foraging, mate acquisition, courting etc. Individuals who hide therefore incur fitness costs,  and may in some cases be at a significant disadvantage when competing against other, ‘non-hiding’ species.

Crypsis

Another method of avoiding detection is by means of camouflage – commonly adopted amongst a range of marine phyla. Perhaps most commonly, this is achieved in the form of visual camouflage, where an individual attempts to deceive a predator by visually matching the background of a certain environment. When successfully achieved, this is a highly successful method of becoming undetectable. However, closely resembling a stationary background often also requires the animal to be stationary. Like hiding, this too may limit the time spent finding food or a mate, ultimately resulting in fitness costs.  

‍

Crypsis within the marine world is not limited to visual camouflage. Some marine animals have the ability to mask certain olfactory cues they normally secrete, as can be see in species of parrotfish that produce a mucus cocoon at night, reducing their risk of being detected via olfactory cues. A species of filefish has even been found to secrete a smell similar to that of the coral they feed on, demonstrating an example of chemical camouflage!

Cryptic animals often develop small morphological features that mimic their surrounding environment. The barbel structures of this scorpionfish (Scorpaenopsis diabolus) resemble tufts of turf algae, helping it remain undetected.

Aposematic colouration

When unable to avoid detection, the next best option is to avoid attack. A conspicuous appearance – serving the complete opposite function of crypsis – also has certain benefits. Animals that have evolved vibrant colours, with highly contrasting patterns often use these as an advertisement of their toxicity – known as aposematic colouration. Aposematism offers a number of fitness advantages over hiding and crypsis. Perhaps one of the greatest of these being that its effectiveness is not limited to a specific environment (as is the case with animals attempting to become camouflaged), nor does it incur the same fitness costs associated with hiding. This method is therefore commonly found in smaller reef species, allowing them to move around more freely during the day.

‍

More commonly, it is the contrasting patterns rather than the colours themselves that are most important when advertising toxicity. Many predators, are unable to see in colour, yet would be unlikely to miss highly contrasting patterns like that of the venomous banded sea krait, or the elaborate patterns of nudibranchs. It is often the vibrant colours and patterns of aposematic creatures that make them such attractive subjects for divers and photographers.

Safety in numbers

A number of fish species form schools as a means of reducing the risk of predation. Despite an individual increasing its likelihood of being detected when joining a school, the reduced likelihood of being captured significantly outweighs the risk. This method is often the most effective means of avoiding predation when living in a pelagic environment, or when migrating. Schooling does however come with certain costs, often associated with increased competition from nearby conspecifics, particularly when predation threats are low. Furthermore, injured fish amongst the school may releases strong chemical cues, called ‘Schreckstoff’, which attract more predators and increase the likelihood of any individual being captured.

Physical defences

Many animals develop morphological features that prevent them from being consumed, examples of which include weapons, spines, or protective shells. Communicative behaviours have often co-evolved with the development of morphological features, allowing animals to produce signals that advertise their defensive potential against predators (e.g. crabs holding up their claws in order to signal the size of their weapons when feeling threatened – a communicative behaviour known as a ‘meral spread’). Although highly effective, such physical defences are often extremely costly to produce, and may require significant resources when being developed. Certain morphological structures may also affect locomotion, consequently affecting processes such as foraging or mate acquisition.

So which predatory avoidance method is best?

Most methods used to avoid predation come with both costs and benefits. Often the most effective method is heavily dependent on the ecological role in which an animal fulfils, as well as their size and relative position in a trophic web. Certain methods may offer additional benefits to life processes other than avoiding predation. Burrows created for hiding may provide an excellent environment for mate guarding, brooding, and parental care. Camouflaged animals avoiding detection may use crypsis to their advantage when trying to capture their own prey. Certain aposematic colours or patterns may be costly to produce, and may therefore be a desirable feature when trying to attract a mate. Schooling has shown to offer certain benefits to its members, particularly when locating areas of highest food availability, and certain physical defences may also help animals to acquire food (e.g. claws used to break open bivalve shells). Learning about how animals avoid predation can therefore often provide extremely useful information on the ecology of marine organisms, and the relationships they share with other members of their ecosystem.

-- Click here to see more of Piers' underwater photos of marine organisms. --

‍