The answer to whether a sturgeon is a shark is definitively no, despite some shared superficial characteristics. Both fish groups possess ancient lineages and certain primitive physical traits, such as an elongated body shape and a heterocercal tail. However, these similarities are a result of convergent evolution, where unrelated species develop similar features, rather than close biological kinship. The fundamental differences in their internal and external anatomy place them in entirely separate classes of vertebrates.
The Defining Characteristics of Sharks
Sharks belong to the class Chondrichthyes, defined by having a skeleton composed entirely of cartilage, rather than bone. This cartilaginous endoskeleton is lightweight and flexible, often strengthened by mineral deposits in areas like the jaw and vertebrae. Unlike most other fish, sharks lack a swim bladder for buoyancy, instead relying on a large, oil-filled liver and the low density of their cartilage to maintain depth.
The outer skin of a shark is covered in placoid scales, which are tiny, tooth-like structures also known as dermal denticles. These denticles reduce drag, contributing to the shark’s streamlined movement. A defining feature is the presence of five to seven exposed gill slits on each side of the head, which are not covered by a protective flap.
The Unique Anatomy of the Sturgeon
Sturgeons are classified within the class Actinopterygii, meaning they are ray-finned fish, placing them with the bony fish, even though their skeleton is largely cartilaginous. Sturgeons fall into the subclass Chondrostei, a group of primitive bony fish that have secondarily lost much of their original bony skeleton. Their anatomy includes bone elements, particularly in the skull and fins, and the presence of a notochord that persists throughout their lives.
Instead of the dermal denticles found on sharks, the sturgeon’s body is protected by five distinct rows of bony plates called scutes. These scutes are a type of modified scale that act as armor along the sides and back. The sturgeon’s mouth is located ventrally, is toothless, and is highly protrusible, adapted for sucking up food from the bottom sediment. Four sensory barbels hang in front of the mouth, which the sturgeon uses to detect prey.
Why Sturgeons and Sharks Are Not Related
The most significant biological separation lies in their skeletal structure and evolutionary history. Sharks are true cartilaginous fish (Chondrichthyes). Sturgeons are technically bony fish (Actinopterygii) that reverted to a largely cartilaginous skeleton over time, meaning their ancestors had bony skeletons, while the ancestors of sharks were always cartilaginous.
Another major difference is in their respiratory systems. Sharks breathe through multiple exposed gill slits, but sturgeons possess a single, bony operculum, or gill cover, which is characteristic of bony fish. Sturgeons also have a swim bladder, a feature absent in all sharks. These fundamental differences confirm that sturgeons and sharks diverged hundreds of millions of years ago, making their similar appearance an example of parallel evolution.