BRISTOL: The small, fleshy fin on the back of all salmon and trout species, assumed to be non-functional and removed by commercial hatcheries, may be vital for the detection and response to stimuli such as touch, sound and changes in pressure.
Millions of fish in North America have this fin – called the adipose fin – and until now it has been considered to be a vestige of a larger dorsal fin and removed each year as a means of distinguishing hatchery reared salmon and trout from their wild counterparts.
Canadian researchers have now identified a neural network in the fin that indicates it likely has a sensory function, which means removing it may have consequences.
“If this is impacting young fish in terms of their survival, hatcheries need to find another method that is less invasive,” said John Buckland-Nicks, senior research professor at St Francis Xavier University in Nova Scotia, Canada, and the lead author of the study in the Proceedings of the Royal Society B.
Nerves prove elusive
In the late 1970s, the removal of the adipose fin as part of his research spurred Tom Reimchen, senior instructor at the University of Victoria, Canada and senior author of the study, to investigate its possible function.
“I couldn’t understand why the fish jerked so much when the adipose fin was clipped. If this is a useless appendage, why would a nerve go into it?” he said.
Although other researchers had looked at the fin, nerves had never been found. “People thought it was a benign flap of tissue and didn’t expect there to be nerves,” said Buckland-Nicks. “I was only confident they were there when silver stain showed nervous tissue was present.”
In 2004, Reimchen showed that the swimming behaviour of rainbow trout became less efficient after removal of the fin. Although a sensory function was suspected, it couldn’t be proven at the time.
What’s in a name?
In this new study, the researchers examined the ultrastructure of the adipose fin. The results showed that despite its name, the fin contains no adipose tissue. Below the skin layers of the fin, in what is called the subdermal space, the researchers found loose connective tissue bridged by collagen cables.
They also detected an extensive neural network throughout the fin, which proves that the circuitry exists to support the adipose fin acting as some form of hydrodynamic sensor.
Vestigial, but functional, organs
Vestigial traits can provide insight into the evolutionary history of an organism. The remnant pelvis in dolphins and whales provides evidence that they were once land-dwelling mammals.
The wings of ostriches and emus, though not used for flight, show their ancestors once flew. In humans, the coccyx is a remnant tailbone and our appendix is a vestige of the cecum in our herbivorous ancestors.
However, further research into these rudimentary features sometimes proves that these traits aren’t merely vestiges of more functional organs in our ancestors. They have been maintained in our evolutionary history because they continue to serve a purpose.
The importance of the adipose fin
Research has shown that the appendix may have immune functions and also hosts beneficial bacteria important to the digestive system. Our remnant tailbone, the coccyx, is an important point of attachment for various muscles, tendons and ligaments. Now the adipose fin is another trait to be taken off the vestigial list.
In terms of evolution, the benefits of keeping these vestigial traits must outweigh any costs. “Because drag is the major component of any aquatic organism, maintaining a structure that would create drag without a benefit is unlikely based on first principles,” said Reimchen.
“Overall this confirms what many of us have known, which is that fish are smart about using environmental flows,” said George Lauder, professor of organismic and evolutionary biology at Harvard University in the U.S. who has a separate research program that specifically looks at the hydrodynamics of locomotion in fishes.
“This research gives one new mechanism by which fish can sense flows from the environment as well as those generated by the main dorsal fin.”
Original paper in Proceedings of the Royal Society B