Piscine wonders v. Darwin.

"Happy Salmon" and Other Wonders of the Fish World's Migrating Marvel

Evolution News & Views

Salmon may not be happy when we eat them, but we're happy learning about them. So in a symbiotic relationship, we should take care of them so that future observers of these masters of migration can continue to inspire future generations of nature lovers. In Living Waters , Illustra Media tells the story of the salmon's amazing life cycle. What's new about these fish that swim thousands of miles at sea, yet find their native freshwater streams years later? Several discoveries have come to light since the film was released.

Drugged Salmon

One news article  says that "Happy salmon swim better." Like people, salmon can get anxious. "Current research from Umeå University shows that the young salmon's desire to migrate can partly be limited by anxiety," this article says. Fear of the unknown downstream slows down the young migrants. But is this experiment ethical?

The research team studied how salmon migration was affected both in a lab, where salmon migrated in a large artificial stream, and in a natural stream outside of Umeå in Northern Sweden. In both environments, researchers found that salmon treated with anxiety medication migrated nearly twice as fast as salmon who had not been subjected to treatment. Several billion animals migrate yearly and the results presented here, i.e. that anxiety limits migration intensity, is not only important for understanding salmon migration but also for understanding migration in general. [Emphasis added.]

Well, maybe these salmon got a little too happy! The scientists may have only discovered that whatever they gave them made them reckless, like snowboarders on stimulants. Natural anxiety might serve to protect salmon from unnecessary risks. In any case, we do not recommend letting your kid give Ritalin to your goldfish as a science project.

Daredevil Salmon

You can imagine the stress on a salmon in this next story. Look at the video of 9-to-10 pound chum salmon swimming across a Washington state highway, right in front of an oncoming car. Why did the salmon cross the road? Because the scent of its natal stream took a shortcut over the highway after heavy rain, National Geographic explains. In the article you can also watch a bobcat take advantage of the opportunity.

Drowning Salmon

The last story was about too little water; this one on Phys.org is about too much. "How will salmon survive in a flooded future?" Fishery scientists, realizing how important salmon fishing is to the northwest economy (it's a $1 billion industry in Alaska), are worried that flood conditions in spawning grounds might scour the delicate salmon eggs out of their nests and wash them away downstream. The key to preserving their breeding grounds, they found, is keeping the area's rivers and floodplains pristine.

"Flood plains essentially act as pressure release valves that can dissipate the energy of large floods," says Sloat. "In fact, most salmon prefer to spawn in stretches of river with intact floodplains, which is probably no coincidence because these features of the landscape help protect salmon eggs from flood events."

Thermoregulation and Osmoregulation

The salmon's ability to change its gill physiology when going from freshwater to salt water and back is called osmoregulation (see how that's a great design story, here ). Now, researchers at Oregon State University  have found that northern sockeye salmon can regulate their temperature as well, "despite evolutionary inexperience." Imagine that! Maybe they took a class in fish school.

Sockeye salmon that evolved in the generally colder waters of the far north still know how to cool off if necessary, an important factor in the species' potential for dealing with global climate change....

Research by Oregon State University revealed that sockeyes at the northern edge of that range, despite lacking their southern counterparts' evolutionary history of dealing with heat stress, nevertheless have an innate ability to "thermoregulate."

The salmon regulate their body heat by finding water just right for their needs. Sounds simple, doesn't it?

While it may seem obvious that any fish would move around to find the water temperature it needed, prior research has shown thermoregulation is far from automatic -- even among populations living where heat stress is a regular occurrence.

By monitoring tagged fish, the researchers found that the salmon knew how to cool off at tributary plumes or in deeper water. It ends up saving them a lot of energy to stay at their optimum "Goldilocks" temperature -- not too hot, not too cold. The scientists never do explain how the sockeye salmon learned to do this despite "evolutionary inexperience."

Diving Deeper into the Salmon Nose

Fans of Living Waters probably remember the dramatic animated dive into a salmon's nostrils (see it here). Recently, we added new information about turbines in the nose. Now, we can learn about another wonder at the molecular level. Salmon and other fish, as well as mammals, have a molecular amplifier involving chloride ions. Stephan Frings, a molecular biologist at Heidelberg University, talks about the discovery in the Proceedings of the National Academy of Sciences. First, let's hear him wax ecstatic about olfaction in general.

The sense of smell and its astonishing performance pose biologists with ever new riddles. How can the system smell almost anything that gets into the nose, distinguish it from countless other odors, memorize it forever, and trigger reliably adequate behavior? Among the senses, the olfactory system always seems to do things differently. The olfactory sensory neurons (OSNs) in the nose were suggested to use an unusual way of signal amplification to help them in responding to weak stimuli. This chloride-based mechanism is somewhat enigmatic and controversial. A team of sensory physiologists from The Johns Hopkins University School of Medicine has now developed a method to study this process in detail. Li et al. demonstrate how OSNs amplify their electrical response to odor stimulation using chloride currents.

The mammalian olfactory system seems to have the capacity to detect an unlimited number of odorants. To date, nobody has proposed a testable limit to the extent of a dog´s olfactory universe. Huge numbers from 1012 to 1018 of detectable odorants emerge from calculations and estimations, but these are basically metaphorical substitutes for the lack of visible limits to chemical variety among odorous compounds. Dogs can cope with their odor world by using just 800 different odorant receptor proteins, a comparably tiny set of chemical sensors, expressed -- one receptor type per cell -- in 100 million OSNs in the olfactory epithelium. Olfactory research has revealed how it is possible to distinguish 1018 odorants with 800 receptors. To do this, the receptors have to be tolerant with respect to odorant structure. After all, the huge numbers suggest that an average receptor must be able to bind millions of different odorants. Low-selectivity odorant receptors are, therefore, indispensable for olfaction. The olfactory system nevertheless extracts high-precision information from an array of low-precision receptors by looking at the activity of all its OSNs simultaneously. The combined activity pattern of all neurons together provides the precise information about odor quality that each individual OSN cannot deliver. Thus, combinatorial coding is the solution to the problem of low-selectivity receptors.

If you are not sufficiently boggled by that, consider that the incoming signals are very weak. A typical OSN (the only neuron exposed to the environment) has only a millisecond to sense an odorant. Because that is too short to trigger the receptor, it has to integrate 35 sensations in 50 milliseconds. To increase their sensitivity, the cilia at the tips of the OSNs -- where the action takes place -- charge their receptors with chloride ions. These ions boost depolarization and promote electrical excitation, amplifying the output signal. Here's where salmon come in:

Interestingly, the components of this mechanism were discovered in freshwater fish, amphibian, reptiles, birds, and mammals, indicating that the interplay of cation currents and chloride currents is important for OSN function throughout the animal kingdom.

A recent study appears to confirm this hypothesis in some cases. You, too, may be "smelling better with chloride." (Here, have some salt on your salmon fillet.) But Frings admits, "The relation between OSN activity at the onset and odor perception at the conclusion of signal processing is far from being understood." The olfactory system is "very different in virtually all respects" from the other senses, like vision and hearing.

First, thousands of OSN axons -- all with the same odorant receptor protein -- converge onto a common projection neuron in the olfactory bulb. This extreme convergence shapes the signal that enters the brain, and we still have to find out how ORN electrical amplification contributes to this process. Second, when the olfactory information enters the piriform cortex, the largest cortical area in the olfactory system, it enters a world quite different from the primary visual cortex. Extensive horizontal communication between the principal neurons and continuous exchange with multiple other brain regions turn the original afferent signal into highly processed information. Finally, the way to perception leads through brain regions that establish, evaluate, and use olfactory memory. Thus, much signal processing has to take place before a mouse [or a salmon, for that matter] performs in an operant conditioning experiment.

Next time you go fishing, take a second to look into the eyes and nose of your catch. Our of reverence, you may just want to throw it back.