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.
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.