Chemical Warfare Between Cuttlefish and Sharks

（Image source：Lawless C， CC BY 4.0 ）

The release of ink as a defensive behavior is one of the most familiar traits of cephalopods. When threatened, cuttlefish expel a dense cloud of dark ink into the surrounding water. This black plume is often interpreted simply as a visual smokescreen that obscures the animal from a pursuing predator, buying valuable time for escape. For a long time, however, little was known about the precise chemical composition of this ink or the molecular effects it might exert on predators. Recent research has begun to reveal that the chemical components of cuttlefish ink can effectively deceive predators such as sharks that rely heavily on olfaction to locate prey.

Sharks possess highly developed olfactory bulbs in the brain, a feature that contributes to their remarkable sensitivity to odors. Yet genomic studies have uncovered a surprising pattern: sharks actually possess far fewer olfactory receptor genes than many other vertebrates. Mammals typically carry more than 800 olfactory receptor genes, and ray-finned fishes generally have over 200. Sharks, in contrast, average only a little more than forty. Despite this relatively small number, sharks still demonstrate extraordinary olfactory capabilities. One possible explanation is that their receptors are highly specialized and extremely sensitive. However, such specialization may also create a vulnerability. If particular environmental molecules bind strongly to these receptors, they could potentially disrupt the shark's entire olfactory system.

To investigate this possibility, researchers examined two shark species with distinct ecological lifestyles: the large coastal predator the great white shark (Carcharodon carcharias) and the smaller benthic species the cloudy catshark (Scyliorhinus torazame). Using computational modeling, scientists simulated the interaction between cuttlefish ink molecules and the sharks' olfactory receptor proteins. The receptors examined belonged to four major gene families responsible for vertebrate smell perception:

1. Odorant receptors (OR)

2. Trace amine-associated receptors (TAAR)

3. Class A olfactory receptors (ORA / V1R)

4. Vomeronasal type 2 receptors (V2R)

Representative predicted structures of the four major classes of olfactory receptors（Image source：Lawless C， CC BY 4.0 ）

Cuttlefish ink is composed primarily of melanin, the pigment responsible for its dark color, along with several free amino acids, among which taurine is particularly abundant. The molecular docking simulations revealed that melanin from cuttlefish ink binds very strongly to all tested olfactory receptors in both shark species. Taurine exhibited weaker binding than melanin but still demonstrated moderate affinity across multiple receptor types. These findings indicate that the chemical components of cuttlefish ink can effectively infiltrate the entire shark olfactory receptor system.

Olfactory receptor activation is not a simple binary process. Instead, the strength of receptor activation depends on the degree of molecular binding, producing nonlinear responses in the sensory system. Strongly binding molecules may generate prolonged or intensified neural signals. Because melanin binds so strongly to shark olfactory receptors, it may trigger powerful sensory signals in the shark brain, potentially overwhelming the normal processing of odor cues and leading to reflexive avoidance behavior.

Binding affinity of melanin and taurine from cuttlefish ink to shark olfactory receptors（Image source：Lawless C， CC BY 4.0 ）

Despite major differences in body size, ecology, and habitat, the great white shark and the cloudy catshark showed remarkably similar receptor binding patterns. These two evolutionary lineages diverged more than 150 million years ago, yet their olfactory receptors responded to cuttlefish ink molecules in almost identical ways. This suggests that the genetic sequences and three-dimensional structures of these receptors have remained highly conserved through evolutionary time. As a result, the defensive strategy evolved by cuttlefish can remain effective over very long evolutionary periods, providing a reliable biochemical defense against shark predators.

Author: Shui Ye-You

Reference:

Lawless C et al. (2025). Decoding deception: the binding affinity of cuttlefish ink on shark smell receptors. G3 (Bethesda).