Enigma of the Adipose Fin, by Don Orth

One of the many enduring mysteries of Ichthyology has been the purpose of the adipose fin.    This small fleshy fin on the dorsal surface between the dorsal and caudal fins is the least studied anatomical structure in fishes.   Other non-functional or vestigial traits persist in organisms, so perhaps this is the case for the adipose fin.    Freshwater sharks and rays have a vestigial rectal gland.  The rectal gland excretes monovalent ions in saltwater forms and yet it still remains in the sharks and rays that have adapted to freshwater.   Blind cavefish (Astyanax mexicanus) maintain a vestigial eye, even though it serves no function.    The “function of the adipose fin” is a great essay topic to assign to an Ichthyology student as it requires critical thought and reasoning and the answer is not in the textbook.   I have posted an essay on this topic that Sean Lusk wrote in April 2012.     

Millions of hatchery salmon are released each year with their adipose fin removed for easy identification of hatchery origins.     Most of us assume that adipose fin has little or no survival value and, based on that assumption, we further assume that its removal would have no deleterious effects.    Studies on the effects of fin removal indicate that removal of the adipose fin has less impact than removal of other fins.     Yet the undescribed function is a nagging uncertainty in a world of science.   

Adipose Fin of a Sea Trout Salmo trutta ©RLLord

Adipose fins are present only in the ray-finned fishes (Actinopterygii) and only  in some unrelated orders, including Argentiformes, Aulopiformes, Characiformes, Myctophiformes, Osmeriformes, Percopsiformes, Salmoniformes, Siluriformes,  and Stomiiformes.     The attempt to relate the presence of this vestigial organ with habitat proves difficult although there is some evidence within the catfishes that that the adipose fin is more likely in catfishes that inhabit flowing waters.     Some investigators have shown that male salmon and trout have slightly larger adipose fins than females.  Erick Petersson and associates did the only study I could find to see if female trout would choose a male based on adipose fin size (Petersson et al. Animal Behaviour 1999).     In that study male aggressiveness was a dominant influence on mate choice.  However, when they prevented males from interacting physically, they observed that 74% of the brown trout females preferred to dig their nests close to the male with the larger adipose fin.   The question of why remains unknown, yet the speculation is that adipose fin size may be an indicator of male quality.    The removal of the adipose fin in a hatchery salmon is not an issue since these males are stripped by human hands supplementing female mate choice.

Recently, John Buckland-Nicks and his associates used light and electron microscopy to describe the ultrastructure of the adipose fin.  For the first time, they showed evidence of interconnected nervous tissue in the adipose fin of a brown trout.   “Adipose” then is a misnomer as the fin did not contain adipose tissue, nor fin rays.   Rather the adipose fin with an extensive neural network has a mechanosensory function.      As Sean Lusk describes in his essay, the findings support a recent hypothesis that the adipose fin may act as a flow sensor.     The major mechanosensory system in the fishes is the acoustic-lateralis system, also known as the lateral line.     The lateral line and associated neuromast cells detect water flow and is oriented along the head and sides of the fishes.   The adipose fin would then detect water flow across the dorsal surface near the caudal region of the body.     

Now we are ready to speculate about the function of the adipose fin in the hydrodynamics of the fish.   In fact, the detection of the chaotic flow vortices in this region of the body might lead to improved swimming efficiency-- but only if the fish could moderate the influence of the turbulence.        If the adipose fin is only a passive sensor as maintained by Buckland-Nils, the trout would have to make adjustments actively.   Experimental data on hydrodynamics of the median fins in fishes indicate that these median fins maintain body stability by modifying the flow environment encountered by the tail.   The role of the adipose fin (active or passive) has been unstudied.     

However, a paper published last fall was the first description of the muscular linkage that controls adipose fin in a catfish.     Thispaper, by Thomas Stewart and Melina Hale from the University of Chicago, demonstrates the previously unknown functional potential of the enigmatic adipose fin.  The paper thoroughly describes the ultrastructure of the adipose fin and the muscular connections that permit the fish to actively control the position of the adipose fin.     This discovery is bound to lead to further physiological studies of passive and active fin movement in affecting drag and thrust, comparative studies of other adipose fin systems, and the role of adipose fin in communicating social status.     

Musculoskeletal linkage of the adipose fin in the sun catfish Horabagrus brachysoma

And why should anyone care?   Clearly the resolution of this perplexing question should suffice.   But for the pragmatist, I offer this.   If the structure and function of the adipose fin serves to improve the stability and efficiency of swimming in flowing waters, we have much to learn about applying this knowledge to the creation of autonomous underwater vehicles and robots.  

What is the Adipose Fin Good For? by Sean Lusk

Have you ever seen a picture of a trout and wondered why there is a flap of tissue between the dorsal and caudal fin? That little flap of tissue is known as an adipose fin and the purpose of this essay is to explore its purpose. The adipose fin has been present on fish ever since early Mesozoic period (250-65 MYA). Today the adipose fin is present in eight groups of extant fishes including the catfishes (Siluriformes), the salmonids (Salmoniformes), and the lanternfish (Myctophiformes). Fishes that possess an adipose fin come in all shapes and sizes and live in very diverse habitats.

For centuries, people believed the adipose fin was a vestigial structure simply because it is a seemingly worthless flap of tissue that lacks ‘true’ fin-like characteristics such as rays and spines. Hatchery technicians and fisheries biologists routinely cut of this ‘vestigial’ structure as a form of marking stocked fish. It is commonly believed that removing the adipose has no adverse effects on the fish (Hora 1969).

 

Adipose fin removed from hatchery salmon (top) and adipose fin in wild salmon.  Photo: Washington Department of Fish and Wildlife wdfw.wa.gov

Although little is known about the true purpose of the adipose fin, a recent hypothesis by T.E. Reimchen and N.F. Temple has emerged suggesting the adipose fin acts as a precaudal sensing organ which aids a fish in navigating turbulent water. This hypothesis initiated a study which found that the adipose fin is comprised of a complex series of nerves, supportive tissue, and many star-shaped glial cells which are commonly found in the brain and spinal cord.

Reimchen and Temple, biologists from the University of Victoria, Victoria, British Columbia, Canada, believed that a structure that has persisted a couple hundred million years more than likely has a purpose; they devised an experiment to test the adipose fin for possible uses (Reimchen 2004). They hypothesized that the adipose might play a role as a precaudal sensory organ. They theorized that this precaudal sensory organ might detect water flow before it reaches the caudal fin allowing the fish to make very precise caudal adjustments based on fluctuations in turbulence. These precise caudal adjustments could allow fish to swim and navigate waters more efficiently. To test this hypothesis the biologists placed juvenile steelhead, Oncorhynchus mykiss, in a variable velocity flow chamber and measured the amplitude and frequency of movements of the caudal fin. After seven independent trials using several different velocities, Reimchen and Temple found that smolts, with their adipose fins removed, had an average 8% increase in caudal movements compared to other steelhead that had their adipose fin intact. J. A. Buckland-Nicks, M. Gillis and T. E. Reimchen teamed up and decided to take the original Reimchen and Temple experiment a step further and dissected the adipose to see what this ‘vestigial structure’ was made of.

Light micrograph of cross section of adipose fin

 in brown trout

 Nicks, Gillis, and Reimchen removed ten adipose fins from ten brown trout, Salmo trutta, and dissected the fins (Buckland-Nicks 2011). They discovered the fin was comprised of four layers: the epidermis (ED), dermis (D), hypodermis, and subdermal space (SD). A huge break came through when they further examined the subdermal space and found that this tissue was innervated and contained many star-like astrocyte cells (ALCs). The significance of this find was; why would a vestigial structure contain cells that are commonly found in the spinal cord and the brain? This discovery by Nicks, Gillis, and Reimchen strongly supported the Reimchen and Temple hypothesis that the adipose fin might play a major role as a precaudal sensor.

After Reimchen and Temple’s first experiment, they decided to expand their study by looking for a correlation between habitat and the presence of an adipose fin (Temple 2008). They noticed that three of the eight extant species of fishes that possess an adipose fin (Siluriformes, Characiformes, and Percopsiformes) also inhabit very turbid waters. They focused on the Siluriformes and found that of the 1,906 species of catfishes, the adipose fin was more common in habitats with flow (e.g. rivers) than habitats that did not have flow (e.g. lakes). This study further supports their hypothesis that the adipose fin does play a major role as a precaudal sensor.

To date, the most plausible explanation for the adipose fin is Reimchen and Temple’s hypothesis that it functions as a precaudal sensor. In addition to Reimchen and Temple’s hypothesis there are other hypotheses that suggest the adipose fin might be a similar structure to that of finlets on tuna which control water flow across the top and bottom of the caudal peduncle (Webb 1975). Another hypothesis suggests that the adipose fin plays a role in courtship with females selecting males with larger adipose fins to mate (Peter 2008).

Scientists have worked tirelessly to learn the purpose of the adipose fin and potential life threatening effects of its removal. Researchers are most interested in the effects of removing the adipose fin of anadromous salmon. Salmon, with no adipose fin, have to expend significantly more energy than fish with adipose fins because they have to move their caudal fin an estimated 8% more frequently. This expended energy could be used for growing, surviving, and reproducing. Leaving the adipose fin on stocked salmon could increase reproductive success, boost survival rates of threatened populations of anadromous salmon and be a pivotal factor for the survival of these species. Although the true function of the adipose fin is still unknown, researchers are making great efforts to understand its purpose. If it is found that the adipose fin does serve a valuable function, its removal will more than likely be prohibited as a form of marking fishes.

 

References

Buckland-Nicks, J. A., M., Gillis, and T. E., Reimchen. 2011. Neural network detected in a  presumed vestigial trait:  ultrastructure of the salmonid adipose fin, Proceedings of the Royal Society B, 279: 553-563

Hora, D.L. 1969. The Effect of Fin Removal on Stamina of Hatchery-Reared Rainbow Trout.   The Progressive Fish-Culturist, 31(4) 217-220

Peter, A. H., A.E. Westley, M., Stephanie, A. Carlson, P.Q. Thomas 2008. Among-population  variation in adipose fin size parallels the expression of other secondary sexual characteristics in sockeye salmon (Oncorhynchus nerka), Environmental Biology of Fishes, 81: 439–446

Reimchen T.E., and N.F. Temple. 2004. Hydrodynamic and phylogenetic aspects of the adipose  fin in fishes, Canadian Journal of Zoology, 82: 910–916

Temple, N.F., and T.E., Reimchen. 2008. Adipose fin condition and flow regime in catfishes,  Canadian Journal of Zoology, 86: 1079-1082

Webb, P.W. 1975. Hydrodynamics and the energetics of fish propulsion, Bulletin of the Fisheries  Research Board of Canada, 190: 1-159.