What's The Skinny On The Adipose Fin?

By WSC Science Advisor Guy Fleischer

Every steelheader recognizes that small fin on the back of the fish just forward of the tail, the adipose fin. Adipose fins are only found in a few groups of fish, notably the Salmonidae, or salmon and trout family (including whitefishes and grayling), but also several other groups of fish that many of you have probably never heard of unless you are a fish geek, like me! These include smelt, percopsids (trout-perch), deep-sea lanternfishes (which, by the way, are essential in the diet of steelhead at sea), characins (piranhas and tetras), as well as some catfish.

Because the adipose fin is not rayed (those little radial supports between the webbing in all the other fins), one early hypothesis was that the fin stored fat or adipose tissue. Studies have generally confirmed that the fin does not hold adipose, yet the name stuck. Ichthyologists have determined that the adipose fin has evolved repeatedly in separate fish lineages, meaning they do not show a common ancestor. Still, this structure has emerged through "convergent evolution," or a structure that has evolved repeatedly, coming about in different species at different times. It must, therefore, not just be some vestige but an excellent thing to have!

Here's some early life history fish geek information that might help shed light on what is happening. The adipose fin can develop in larval fish in two different ways. One is the salmon-and-trout-type way, where the adipose fin develops from the larval-fin fold at the same time and in the same direct manner as the other median fins. The other is the characin-type way, where the adipose fin develops late after the larval-fin fold has diminished and the other median fins have developed. The existence of both types of development suggests the adipose fin is not "just a larval fin fold remainder, which is evidence that the adipose fin performs some critical functions. So, what does it do?

We all know this fin in that it is frequently clipped off to mark hatchery-raised fish. No big deal, right? Well, studies have found that trout with their adipose fin removed have an 8% higher tailbeat frequency. Additional research has suggested that this fin may be vital for the detection of and response to stimuli such as touch, sound, and changes in pressure. More recently, it has been found that this fin has extensive nervous tissue and an unusual subdermal complex of interconnected brain- and nerve-like cells. This suggests that the adipose fin may act as a precaudal flow sensor, where its removal can harm swimming efficiency in turbulent water.

Flow chamber experiments using steelhead confirm reduced swimming efficiency following adipose fin removal across multiple flow velocities. Such a flow sensor could detect the chaotic vortices before they enveloped the caudal fin, providing direct feedback to the central nervous system and subsequently improving tail fin motion during swimming. Looking across all groups of fish with adipose fins, there is the typical life history trait of having to navigate turbulent water or for sustained swimming speed.

But wait, there's more! The adipose fin is sexually dimorphic in salmonids and larger in males than females relative to body size. There is evidence, at least in brown trout and salmon, that females prefer males with larger adipose fins relative to their body size.

So next time you encounter one, you now know to show all the respect due to this fleshy little knob of a fin!