Adipose fin

i’m not from your home
a mystery contour is my crux
my place is not fixed


Redtail catfish (Phractocephalus hemioliopterus)

The adipose fin is located between a fish’s caudal (tail) fin and dorsal (back) fin.

Adipose tissue in you and I is another name for fat. Though some species of fishes do have squishy adipose fins, the majority of adipose fins don’t actually contain fat. The name is thought to be derived from a hypothesis of yesteryear – that adipose fins were simply fat storage. This fin, often described as enigmatic, is definitely a mysterious appendage. Persisting in ~6000 species of fishes since the Mesozoic era (~252 to 66 million years ago), researchers have pondered the origin and function of the adipose fin for some time.

In the 1980’s it was noted that in salmonids males consistently have larger adipose fins than females, but why? One hypothesis considered adipose fins as secondary sexual traits used to impress females, like antlers on male deer. Seems like a reasonable idea based on the following observations: 1) spawning females prefer to hang out near males with larger adipose fins, and 2) when males fight the adipose fins of losing, subordinate males appear to shrink over time!

Now what about swimming performance, after all most fish fins are important for movement. When the adipose fin is removed fish put their caudal fin to work, moving it side to side more vigorously to generate a larger amplitude. But this caudal fin workout may be energetically costly for a fish.

It is common practice in hatcheries to remove the adipose fin of salmonids so that fishers can easily distinguish between wild and hatchery-reared fish. Should there be concern for this established fisheries procedure? One study has found the number of fish that return home is the same regardless of whether they have their adipose fin clipped or not clipped.

The latest revelations of the adipose fin – they have appeared multiple times across fish lineages and contain specialized cells that may be able to sense water flow.

Adipose fins, the little fins that could…apparently do much more than originally thought.

redtail catfish (Lago Maximo, Brazil, 1 September 1865), Ernst Mayr Library

here and here