Sturgeon are large, ancient fish found across the Northern Hemisphere in rivers, lakes, and coastal waters. They are immediately recognizable by their unique, armored appearance, which leads many to wonder about their body covering. Sturgeon do not possess the typical, thin, flexible, and overlapping dermal scales found on most modern bony fish. Instead, their skin is largely smooth but features a distinct set of large, bony plates. These protective structures are properly called scutes.
The Unique Answer: Scutes, Not Scales
The integumentary covering of a sturgeon is fundamentally different from the cycloid or ctenoid scales seen on common fish like salmon or perch. Typical fish scales are small, light, and overlap like shingles, providing flexibility and a streamlined surface. Scutes, by contrast, are thick, hardened, and armor-like bony plates embedded in the skin.
These scutes are arranged in five distinct, longitudinal rows running the length of the sturgeon’s body: one row along the back (dorsal), two along the sides (lateral), and two along the belly (ventral). This arrangement offers a rigid, protective suit. The scutes are particularly pronounced and sharp on younger sturgeon, acting as defense against potential predators.
Anatomy and Composition of Sturgeon Scutes
Sturgeon scutes are classified as a modified form of ganoid scale, an ancient type of dermal armor found on primitive fish. While ganoid scales are often diamond-shaped and interlocking, sturgeon scutes are large, separate plates composed of bone tissue originating from the dermis.
The bony base of the scute is covered by ganoine, a hard, enamel-like substance. Ganoine is a mineralized tissue with a glassy finish, composed of apatite crystallites. This outer layer provides extreme hardness and wear resistance.
The scutes do not grow continuously or shed like the scales of many other fish; they are fixed skeletal structures that develop early in life. The number and size of these scutes can change as the fish ages, often becoming blunter or disappearing in the spaces between the five main rows in very old individuals.
An Evolutionary Relic: Why Sturgeon Retained This Armor
Sturgeon belong to the subclass Chondrostei, a group of ray-finned fish known for retaining many ancestral features. Their lineage traces back over 200 million years, earning them the nickname “living fossils.” The bony plates they wear are a direct inheritance from early, heavily armored fish ancestors.
The majority of modern bony fish, or Teleosts, evolved lighter, more flexible scales to allow for faster, more agile movement. Sturgeon, however, retained this heavy, protective armor, which is well-suited to their benthic lifestyle.
This ancient armor is retained alongside other primitive traits, such as a largely cartilaginous endoskeleton, a feature not typically found in modern bony fish. This combination of scutes and a cartilaginous skeleton represents characteristics passed down from the earliest ray-finned fish, highlighting the success of this armored design.