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.