Sharks, scent and the battle of chemical warfare

Chemical communication is a fundamental means of survival in the ocean. Both predators and prey use chemical signals to navigate their environment, find food, find mates and avoid danger. But smell is particularly intriguing in its dual role as both an asset and a weakness. For example, barnacles secrete glycoproteins as a mechanical defense against predation. However, predators such as starfish and snails have evolved olfactory receptors specially tuned to detect these compounds, turning the barnacles’ defense mechanism into a local beacon. And this dynamic chemical interaction is not unique to barnacles! California sea hare (Aplysia californica) uses paint as a defense mechanism against spiny lobsters, using a strategy that combines sensory disruption and mimicry. The ink confuses the predator’s sense of smell, allowing the sea hare to escape.

Nature is an evolutionary arms race driven by olfactory communication, where adaptation and counter-adaptation play out in the chemical realm.

Smell operates through specialized proteins known as olfactory receptors, which are part of the G protein-coupled receptor, or GPCR, family. These receptors, embedded in the cell membranes of the nasal epithelium in vertebrates, detect specific molecules in the environment. When a molecule binds to a receptor, it triggers a signaling cascade that the brain interprets as smell. Vertebrate olfactory systems are commonly classified into four receptor families: odorant receptors, trace amine-coupled receptors, class A olfactory receptors, and type 2 vomeronasal receptors. These receptor systems, originally characterized in mammals, have since been identified in a variety of vertebrates, including birds, amphibians and even sharks.

Sharks are often described as “swimming noses” because of their well-developed olfactory bulbs, which contribute to their reputation for an acute sense of smell. However, recent genetic studies challenge this assumption. Despite their iconic status as olfactory specialists, sharks possess fewer olfactory receptor genes than other vertebrates! While ray-finned fish have an average of over 200 such genes and mammals about 850, sharks have an average of 43. This apparent paradox suggests that the olfactory abilities of sharks are not simply about the number of receptors, but about functionality and ecological specialization theirs.

Enter a fascinating predator-prey relationship involving sniffing between sharks and cephalopods (like cuttlefish). Cuttlefish have evolved a sophisticated defense strategy involving ink. The ink, a dark mixture rich in melanin and amino acids such as taurine, acts as more than just a visual smokescreen. For predators like sharks, which rely heavily on smell, the ink can interfere with their olfactory receptors, disrupting their ability to detect prey effectively! By modeling the three-dimensional structures of the shark’s olfactory receptors, the researchers examined how different chemical compounds—including those in cuttlefish ink—bind to these receptors. The results reveal that components such as taurine and melanin can engage multiple types of olfactory receptors, overwhelming the shark’s sensory system. Interestingly, compounds such as pavoninin-4 (a natural shark repellent) and cadaverine (associated with decomposition) also showed high binding affinities. Pavoninin-4’s effectiveness as a natural deterrent suggests that it could potentially help prey species avoid shark predation; Meanwhile, cadaverine’s strong response may be related to its role in signaling the presence of fungi, a potential food source, or may act as a warning signal to avoid disturbed or dangerous areas.

These findings highlight the nuanced role of olfaction in predator-prey dynamics. Sharks, with their limited but highly specialized repertoire of olfactory genes, have evolved to interpret a complex chemical landscape, balancing cues associated with hunting, avoiding danger, and navigating their ecosystems. At the same time, prey species such as cuttlefish have developed chemical defenses that exploit this support. Thus, understanding the molecular basis of these interactions has broader implications. Studying olfactory adaptations in marine species can inform us about the evolutionary trade-offs between sensory specialization and ecological versatility. Similarly, it could lead to scientists developing shark repellents based on natural flavorings, which could help reduce bycatch in fisheries.

Understanding the complex olfactory mechanisms of sharks not only deepens our appreciation of these magnificent predators, but also opens the door to innovative applications in conservation, marine management, and human-shark coexistence.