Cuttlefish ink may overwhelm sharks’ sense of smell

A plume of ink can help hide a cuttlefish as it scuttles away from a predator. But that smoke screen’s stench may also warn sharks to stay away.

The ink’s primary component, melanin, strongly sticks to sharks’ smell sensors — more so than the odorant in mammalian blood, researchers report January 8 in G3 Genes|Genomes|Genetics. The finding hints that in the common cuttlefish (Sepia officinalis), ink evolved to exploit sharks’ keen sense of smell.

Nicknamed “swimming noses,” some sharks can sniff their way home from almost 10 kilometers away. But that remarkable sense spans a narrow range of scents. Researchers have sequenced the genomes of nearly 40 shark and ray species. On average, the animals possess genes for 43 odor-detecting molecules known as receptors. (Mammals have, on average, around 850 smell receptor genes.)

That limits the number of smells sharks can distinguish, although they may be fine-tuned for scents relevant to survival, says sensory biologist Colleen Lawless of the University of Massachusetts Amherst.

Past research has shown that after encountering cuttlefish ink, sharks dart away, she says. “It’s almost like if you walk into a room and someone’s farted really bad and you just want to completely run the opposite way.”

But scientists weren’t sure how the ink’s chemical compounds interact with sharks’ smell receptors.

So, while at University College Dublin, Lawless and colleagues created 3-D computer models of 146 smell receptors using genetic data from the cloudy catshark (Scyliorhinus torazame), small-spotted catshark (Scyliorhinus canicula) and great white shark (Carcharodon carcharias). The modeled shapes revealed that melanin, responsible for the dark color of a common cuttlefish’s ink, possesses a molecular structure and other properties that let it strongly stick to all the tested smell receptors. Melanin’s binding affinity for the receptors surpassed that of the compound responsible for mammal blood’s metallic odor.

The effect probably extends to all shark species, Lawless says. Sharks generally share the same core set of smell receptors, despite species’ different lifestyles and ecological niches.

“Sharks’ great sense of smell is also their Achilles’ heel,” Lawless says. “Cuttlefish ink has found a way to take advantage of their limited number of smell receptors and use it against them.”