EPOW - Ecology Picture of the Week

This Is Not A Shark

White Sturgeon (Acipenser transmontanus), Family Acipenseridae
Columbia River, Pacific Northwest U.S.

Explanation:  Shark!  Well, maybe not ... but this is a shark's tail.  Notice the lengthier and pointed upper lobe, just like all sharks have.  

But ... this is not a shark.

This is a sturgeon -- a white sturgeon, to be precise, in the Columbia River bordering Washington and Oregon.  Along with paddlefishes (of family Polyodontidae), sturgeon belong to order Acipensiformes, which is characterized by these tails (the caudal fin) in which the backbone curves upward into the upper, elongated lobe.  This tail shape is known as heterocercal, unlike the symmetric homocercal tails of bony fishes in which both tail lobes are mostly of equal size and shape (with some minor asymmetries present in the tails of gar and bowfin fish).

Sturgeon and paddlefish, like sharks and their ray and skate cousins, are all primitive forms.  
  

So what is the purpose of this asymmetric tail shape?  Much speculation has suggested that the heterocercal tail imparts stability, thrust, and lift for bottom-dwelling fishes, which may be the case with sturgeon and some sharks.  But studies have revealed that, during active swimming, the shark's tail results in creation of angled vortex jets of water, where the sturgeon's tail does not.  The difference might relate to the shark being more maneuverable than the sturgeon.  

Why sturgeon, paddlefish, and sharks, rays, and skates all have heterocercal tails is unclear.  If the evolutionary lines of these "primitive" fishes have persisted for hundreds of millions of years -- the oldest known shark is from a specimen 409 millions old from the early Devonian period, and sturgeon date to at least 100 million years -- these tails must have imparted some adaptive advantage.  The mystery is, why didn't bony fishes also evolve them, if they are so advantageous?  

Sturgeon are amazing and immense creatures.  They are the largest freshwater fish in North America, growing to a maximum of 20 feet (6.1 meters).  For that, you're going to need a bigger boat!  
    

Information:
     Wilga, C.D., and G.V. Lauder.  2002.  Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal swimming and vertical maneuvering.  The Journal of Experimental Biology 205:2365-2374.