In this article, we will be taking a look at how pheromone olfaction influences predator-prey interactions in a marine environment as well as how ocean acidification disrupts this interspecific relationship.
Written by: Shreya
What’s the strongest scent you can smell right now? Some coffee brewing in the kitchen, or maybe the petrichor of fresh rain? What about a spider clinging to a wall two miles away? While your olfaction, or sense of smell, might not be quite powerful enough to succeed in the latter, several species including insects, fish, and marine mammals amazingly can do just that and more. While many of the proximal smells that humans perceive are produced by chemicals called odorants that are received by the olfactory bulb, more distal communication and sensory reception can be achieved through pheromones. Pheromones are chemicals, similar to odorants, that produce an innate response in the receiving organism. For instance, a prey species registering the scent of a predator would instinctively deploy anti-predator responses such as hiding, running away, terminating feeding behavior, etc. In this article, we will accordingly take a look at the pheromones involved in marine predator-prey interactions before diving into the impacts of ocean acidification, as caused by carbon dioxide emissions, on these interactions.
The Various Types of Predator-Prey Pheromones
Predator-prey interactions in the ocean environment heavily depend on a panoply of pheromones, some of which benefit the predator and others that assist prey. Through conditioning, species learn to detect the odors of other species and can even release their own odors to send out warning signals to their conspecifics, or others of their own species. Some of the crucial pheromones involved in these interspecific and specific communications are predator kairomones, alarm cues, disturbance cues, and prey odors.
Predator Kairomones:
Kairomones are cues that benefit the receiver and not the sender. There are two types of predator kairomones: predator odors and diet cues, both of which are emitted by predators and alert prey of predator presence. Predator odors are scents unique to each predator species that, when coupled with vision and conditioning, help an organism identify danger. They pair well with diet cues, which are metabolites in the faeces of predators that were left over after the digestion of prey. While prey can recognize cues from other closely-related prey species (or heterospecifics), their anti-predator behavior is strongest when these cues are generated from conspecific consumption. By registering that another organism has been consumed in their vicinity, these prey species know to hide or run away (3).
Alarm Cues:
Alarm or damage cues are chemicals released by mechanical damage to a conspecific, which acts as an indicator of a nearby prey organism in danger. These cues indicate to their conspecifics the presence of a predator while simultaneously attracting more predators to the area. Thus, these types of cues serve two purposes and benefit both ends of the interaction (8).
Disturbance Cues:
Disturbance cues, or alarm signals, are odors released by prey when in a state of stress or disturbance.These cues enable both the sender and conspecific receivers to get away from a potential threat since no one has been palpably harmed just yet.
Prey Odors:
Prey odors are scents released by prey species and received by predators as an indication of nearby food sources. These cues detail information such as whether prey is present, how far away it is, and how much is present. By traveling long-distance through ocean currents, they can create a “trail” for the predator to follow and thus their strength depends on the fluid dynamics of their local ocean, release rate, and amount released.
Impact of Ocean Acidification on Pheromone Communication
Clearly, the predator-prey dynamic is heavily reliant on a complex pheromone interaction that encompasses the use of various odors and cues. However, ocean acidification– and its particular effect on coral reefs– has been found to decrease the acuity of signal reception in both predators and prey. Ocean acidification is the ocean’s gradual decrease in pH due to higher absorption rates of atmospheric carbon dioxide. CO2 emissions from vehicle exhaust, factory production, transportation, and numerous other sources increase the concentration of this gas absorbed by the ocean, which in turn severely harms its marine life (9). This phenomenon is particularly dangerous in coral reefs, an ecosystem that harbors some of the greatest biodiversity on the planet: Newborn prey larvae and incoming settlers both must learn to recognize predator cues as signals of danger in order to survive since around 60% die within the first 48 hours of settling; unless they can register these odors and learn to associate them with predation risk soon, they face an intensified danger of dying out. Although the exact, biomolecular interaction between acidification and marine olfaction is not known, the overlying impact has been recorded in various studies.
Much research centering around this field has used damselfish species as its model organism and P. Fuscus as its predator. One study showed that when P. Fuscus were put in various channels containing extracts from damselfish that released alarm cues, organisms in tanks with a higher carbon dioxide concentration spent less time in chambers with these skin extracts. This could potentially indicate that ocean acidification makes certain predators less receptive to cues that would normally indicate the presence of food (2). Similarly, it was found that some prey species could not detect alarm cues under acidified conditions while the same could not be concluded for others. Overall, these studies have shown that some species appear to be immune to ocean acidification with regard to pheromone communication while others can no longer distinguish cues with the same acuity, leading to a survival advantage for certain species and presenting the risk of biodiversity reductions.
Implications especially arise in reef ecosystems where reefs, which assist in transducing odor cues, bleach and die off due to ocean acidification. Ocean acidification and rising atmospheric temperatures cause the warming of ocean waters, which causes corals to expel the symbiotic algae that give them their vibrant colors. Without the algae, which provide corals with 90% of their energy, the coral polyps are left in an extremely weak state (4). Coral degradation has been shown to decrease the ability of damselfish to recognize predator odors of P. Fuscus to recognize damage cues, and of prey species to register diet cues. When damselfish were conditioned to recognize predator cues in healthy coral environments and then placed into degraded coral, their acuity for predator detection by predator odor (which was artificially injected into their environment) decreased by nearly half (5). Similarly, another study found that damselfish could recognize heterospecific alarm cues in both live and degraded coral environments but were unable to use conspecific diet cues to incur anti-predator behavior in degraded habitats (6).
Conclusions:
Pheromones equip marine organisms with the amazing ability to discern the positions of their friends and foe with a several mile radius. Through associating particular odors with particular behavior, they can swiftly evade predation or sneak up on vulnerable prey. However, the existence of this unique sensory technique is at risk due to increased ocean acidification and carbon dioxide emissions. Although not much is known about the exact mechanics of marine olfaction, it is clear that continued anthropogenic pollution will severely impact predator-prey interactions and the biodiversity of our planet’s delicate coral reef ecosystems.
Sources:
Ocean acidification affects prey detection by a predatory reef fish
Coral degradation alters predator odour signatures and influences prey learning and survival
Diet cues and their utility for risk assessment in degraded habitats
The Evolution of Chemical Alarm Signals: Attracting Predators Benefits Alarm Signal Senders