The tiny adipose fin mounted atop many salmon and trout species may have a function after all, according to a research paper published online Monday in the research journal Proceedings of the Royal Society B.
Most fishery managers have long considered the adipose fin, which is located between the dorsal fin in the middle of the fishes’ backs and the tail caudal fin, as having little if any function. The adipose fin of millions of juvenile hatchery fish in the Columbia-Snake river basin alone are removed – clipped – before the fish are released into rivers, lakes and streams.
The clipped fin serves as an identifier for fishermen. Columbia River fisheries often require the release of unclipped fish, which are potentially of natural origin. The vast majority of hatchery salmon and steelhead released into the Columbia-Snake system are fin-clipped.
The adipose fin has “traditionally been regarded as vestigial without clear function,” according to the new paper, “Neural Network Detected in a Presumed Vestigial Trait: Ultrastructure of the Salmonid Adipose Fin.” In other words, most believe the adipose had lost all or most of its original function through evolution.
But those fleshy, non-rayed adipose fins may be alive and well and serving a good purpose.
“Our results show for the first time to our knowledge, unambiguous evidence of nervous tissue revealing a neural network throughout the fin which is consistent with a sensory function of the adipose fin,” the research paper says.
“We have presented, for the first time to our knowledge, unambiguous evidence that the salmonid adipose fin is innervated.” That means the flexible adipose may have, through that network, the means to signal the fish that trouble is coming.
“These results are consistent with a recent hypothesis that the adipose fin may act as a precaudal flow sensor, where its removal can be detrimental to swimming efficiency in turbulent water,” the paper says. “Our findings provide insight to the broader themes of function versus constraints in evolutionary biology and may have significance for fisheries science, as the adipose fin is routinely removed from millions of salmonids each year.”
Authors are John A. Buckland-Nicks and M. Gillis of the Biology Department at Saint Francis Xavier University in Antigonish, Nova Scotia, and Tom E. Reimchen of the Biology Department at the University of Victoria in British Columbia.
Proceeding B is the Royal Society's flagship biological research journal. The Royal Society is a fellowship of scientists and is the oldest scientific academy in continuous existence. It is headquartered in the United Kingdom.
The paper can be found at:
The research involved the close examination of brown trout adipose fins and attaching flesh with light and electron microscopy. A light microscope study has shown that the basic structure of the adipose fin is similar among different salmonids, which include salmon and trout species.
“Recent flow chamber experiments using steelhead trout (Oncorhynchus mykiss) indicate reduced swimming efficiency following adipose fin removal across multiple flow velocities leading to a hypothesis that the adipose fin may act as a precaudal flow sensor when swimming in turbulent water,” the research paper says. “Such a flow sensor could detect the chaotic vortices before they enveloped the caudal fin providing direct feedback to the central nervous system and subsequent improved caudal fin motion during swimming.”
“The adipose fin is removed as a routine marking technique applied extensively at hatcheries on millions of fish that are then used for stocking lakes and rivers,” the paper says. “Although extensive experimental evidence indicates that such removal has less impact than removal of other fins, our results indicate substantive caution in the removal of a sensory and functional trait on individuals already subject to major demographic and environmental impact.”
“If the adipose fin is in fact functioning as a precaudal flow sensor, allowing optimal manoeuvrability in turbulent water, … then it is logical that swimming in non-turbulent waters would be largely unaffected by removal of the adipose fin. This hypothesis could explain why previous studies found that clipping of the adipose fin had no effect on stamina, susceptibility to predation or growth, as these studies were conducted in non-turbulent water,” the new research paper says.
Other research, however, did find that growth was significantly affected by adipose fin clipping when fish were swimming in turbulent water, the paper says.
“There are also suggestions that the adipose fin has a potential role during courtship as males have a larger fin than females and the latter appear to prefer males with a relatively larger adipose fin,” the paper says. Past research has proposed “that both these factors may reflect a selective landscape in streams where males are subject to greater swimming demands than that of females. The occurrence of a larger adipose fin in males may relate to hydrodynamic functionality owing to the greater complexity and density of the nerve net in a larger fin.”