Dolphins don’t worry about pairing, but can fish smell?

Have you seen the ad for Snapple, where a dolphin decked out in pearls and furs tries to smell a perfume at a department store, and fails; then inside the Snapple cap you read that dolphins can’t smell?

Indeed, some Cetaceans, including dolphins, porpoises, and toothed whales, can’t smell much if anything because the genes that would normally code for odor receptors (ORs) have substitutions that make for nonfunctioning receptors for over some 95% of them. Furthermore, they lack an olfactory bulb.

Which led me to wonder, can fish smell? or is it in the nature of the water environment to make smelling unnecessary?

To explore the answer to this question, I searched for an article on fish odorant receptors, and found a very thorough study by Alioto and Ngai comparing fish genomes to mouse genomes, which gives strong clues concerning the evolution of our odorant receptors and those of Cetaceans.*

ORs were created in our vertebrate genomes through gene duplication, starting with the gene for melanocortin receptors. Melanocortins are hormones, all derived from slicing of a larger protein, proopiomelanocortin, in the brain and other tissues. As the name suggests, melanocortins are involved with functions as divergent as skin pigmentation and corticosteroid production, and also in appetite and satiety. 

Genes normally duplicate in the course of evolution. Over time the duplicates accumulate mutations, which change their functions; these mutated duplicates may in turn duplicate again and accumulate mutations. This process of duplication and mutation leads to the development of families of genes.

It turns out that families of ORs appear to have been present from the earliest development of vertebrates. According to Alioto and Ngai, lampreys have OR genes in two families, both of which are closer in sequence to the ancestral melanocortin receptor gene, and therefore more “primitive.” Zebrafish have somewhere between 6 and 8 families of OR genes, which (in theory at least) should enable them to sense a wider repertoire of scents than mice or humans, who only have 2 families.  However, we have a larger number of different sequences than do fish, about 1000 versus about 100 in fish, which may allow us to make finer differentiation among smells. It is thanks to this refinement within families that we can detect the difference between heptanal with its herbal note and octanal with its orange citrus note—these two compounds differ only by one carbon atom with its two hydrogens. 

This image, from http://www.nature.com/ng/journal/v45/n4/fig_tab/ng.2568_F1.html, shows the evolutionary relationships among lampreys, ray-finned fish including the zebrafish, and mammals.

So why did toothed Cetaceans lose their capacity to smell? It can be argued that it is because their nose is no longer in front of them. If you look at the fossil ancestors of Cetaceans, you will see that the nose moves progressively to the top of the head, where it is now, in the form of a blowhole. Blowholes are closed when they are under water, so any sniffing capacity would be useless there. 

The evolutionary tree of whales and porpoises. Note the position of the nasal opening, which is right in the middle of the top of the skull at the end of the evolution.

From http://phenomena.nationalgeographic.com/2014/03/12/seeing-the-ocean-with-a-buzzing-nose-2/

Baleen whales (aka Mysticetes) also have blowholes, but they do have olfactory bulbs and a set of ORs, about 51% of which appear to be functional, and all of which belong to only one of the two mammalian classes—they seem to have lost the other. Furthermore, their olfactory bulb represents 0.13% of their brain weight compared to 0.0008% of our brain weight. (Incidentally we also have about 50% of our ORs in the form of non-functional pseudogenes.) So their repertoire of scents is perhaps limited to one type, but definitely present.

The reason for the difference may lie in the way the two classes of cetaceans hunt prey. The toothed whales do so by echolocation—no sense of smell needed. By contrast, baleen whales need to find schools of krill, which do not give a distinct pattern with echolocation, but may emit specific scents.

* Tyler S Alioto and John Ngai. The odorant receptor repertoire of teleost fish. BMC Genomics, 2005, 6:173; DOI: 10.1186/1471-2164-6-173. 

** http://www.north-slope.org/assets/images/uploads/Thewissen%20et%20al%202011-MarMammSci-bowhead%20olfaction.pdf