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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).
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| 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.
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| 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.
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