Humans have a notoriously weak sense of smell. Although we can sniff out strong odours, we tend to rely on our eyes to locate most things; however, many fish follow their metaphorical noses to track down a meal. But a fish's sense of smell can be affected by the quantity of calcium salts and sodium (from regular salt) in their water; which is fine if you spend your life cruising the oceans and your body is in harmony with salty water. But what about fish that switch back and forth between the sea and brackish water in estuary feeding grounds? How is their sense of smell affected by changes in salinity when sniffing out the next meal? Knowing that ocean-going European seabass (Dicentrarchus labrax) often visit brackish estuaries to feed, Zélia Velez, Peter Hubbard, Pedro Guerreiro and colleagues from the Centro de Ciências do Mar, Portugal, decided to find out how the different levels of salinity and the presence of other salts (including potassium, calcium and magnesium salts) affects the seabass's sense of smell.
Rearing a school of young seabass in massive tanks of full-strength seawater [35 parts per thousand (‰) salt water] until fully grown, Guerreiro then gently reduced the salinity for some of the fish, to adjust them to more brackish conditions (5‰ salt). Once the fish had adapted to their new home, Velez and Hubbard checked whether living in a different strength of salt water had altered the fish's olfactory rosettes – the structures where the sensory cells that detect scents are located. Scrutinising thin slices of the sensory structure under a microscope, Velez noticed that the fish adapted to the brackish water had developed more cells that produce mucus to coat the scent-sensitive structure. But how would the transition to fresher water affect the seawater fish's sense of smell?
First, Alexandra Alves and Rita Costa tested the fish's sense of smell in various versions of artificial seawater, including potassium and magnesium salts, and with and without calcium chloride and regular salt. The fish that had been kept in seawater could smell the change in the level of calcium in the water as it declined, while the fish that had adjusted to life in brackish water could smell an increase in the level of calcium. However, both groups of fish could smell increases in sodium in their surroundings. The level of different salts in the water altered the fish's sense of smell.
Then, Velez and Hubbard dissolved four different amino acids – the building blocks of proteins – in seawater and freshwater, to find out how transitioning between the two forms of water would impact the fish's ability to sniff out prey. This time, the seawater residents lost their sense of smell for amino acids dissolved in freshwater, even though the fish could smell the amino acids strongly when in seawater; transitioning from their ocean homes to life in an estuary seriously disrupted the fish's ability to smell their prey. However, the fish that had been living in brackish water for 3 weeks seemed to have recovered their sense of smell; they were equally sensitive to the scent of amino acids in freshwater and seawater. Also, the seawater residents’ sense of smell for the amino acid alanine was fine-tuned by the addition of calcium salts to the water, which restored the fish's sense of smell that had been lost in freshwater.
‘The olfactory system of seabass is able to adapt to low salinity’, says Velez, explaining that although the ocean-going fish are likely to lose their sense of smell when venturing into river estuaries, it probably recovers after a period in brackish water.