Breathless fish lose their sense of smell

Fish are often led by their noses. Well, maybe not their noses precisely, but their sense of smell is definitely a key player in life, guiding them to food, mates and safe places to live, while avoiding the scent of predators and toxic contamination. Many of the coastal locations where fish choose to make their homes are also running short of oxygen, thanks to climate change and human pollution, and a reduction in oxygen can affect some fish senses, such as their vision and hearing. But no one knew whether low oxygen might also impact a fish's sense of smell. Cosima Porteus (University of Toronto Scarborough, Canada) and Peter Hubbard (Centro do Ciéncias do Mar, Portugal), with other colleagues, had already discovered that high CO₂ levels affect the ability of the European sea bass (Dicentrarchus labrax) to sniff out food, so Porteus's graduate student, Liam Tigert, travelled to Hubbard's Algarve lab to test the sense of smell of young gilt-head seabream (Sparus aurata) to find out how much the sense might be affected by plummeting coastal oxygen levels.

Aware that fish sniff for single amino acids – the building blocks of proteins – when searching for food, the duo in Portugal dissolved tiny quantities of cysteine, which smells of rotten eggs, arginine, which smells bitter, and leucine, which smells sweet, in water with different oxygen levels, ranging from fully oxygenated (20 kPa O₂) to slightly reduced oxygen (12.5 kPa O₂) and low oxygen (5.7 kPa O₂). Then, they squirted puffs of the individual amino acid solutions into the nostrils of each fish's snout, which is where the sensory cells that pick up smells are located, while recording the electrical signals produced by the fish's sense of smell to find out how weakly or strongly the fish picked up the odours.

Sure enough, as each smelly amino acid became more dilute, the fish struggled to detect them, but when Tigert and Hubbard checked how much the declining oxygen levels affected the fish's sense of smell, it was clear that low oxygen levels impaired their ability to smell the strongest doses of rotten egg-scented cysteine. In contrast, sniffing arginine in low oxygen only affected the fish's sense of smell when the amino acid was extremely dilute, while the fish were equally able to pick up sweet-smelling leucine regardless of how much oxygen was around. Tigert explains that this indicates that nerves transmitting the fish's sense of smell are affected to different extents by low oxygen levels, as each amino acid is probably picked up and detected by the receptors on different sense-of-smell neurons. And after the team had tested the fish's sense of smell for rotten egg-scented cysteine at the lowest oxygen concentration, they returned the oxygen concentration back to normal before checking whether the fish could still smell the amino acid. Impressively, the animal's sense of smell recovered perfectly, suggesting that a reduction in the fish's energy supply, caused by the lack of oxygen, was responsible for the impaired sense of smell.

Although the sense can recover when oxygen is fully restored, it is concerning that low oxygen in coastal waters could make it troublesome for fish to track down food, likely affecting their health. However, the team points out that, long term, the fish may be able to adapt to declining oxygen in their water, allowing them to recover their sense of smell despite being more breathless.