Sturgeon are aquatic animals known for their unique appearance and ancient lineage. People often wonder whether these large fish possess bones like most other fish species. Understanding a sturgeon’s skeleton reveals its evolutionary journey and biological characteristics.
A Cartilaginous Skeleton
Sturgeon have a skeleton composed of cartilage, a flexible and resilient connective tissue. This material, similar to gristle in human noses and ears, provides structural support without the rigidity of true bone. Unlike most modern fish, sturgeon cartilage does not undergo significant ossification, the process where cartilage is replaced by bone during development. Their internal support also includes a persistent notochord, a rod-like structure that runs along their body, offering axial support throughout their lives. While largely cartilaginous, sturgeon retain some bony elements, particularly in their skulls, pectoral fin spines, and the armor-like plates on their skin.
Ancient Lineage and Unique Traits
The cartilaginous skeleton of sturgeon is linked to their deep evolutionary history, marking them as “living fossils.” They belong to the subclass Chondrostei, an ancient group of ray-finned fish. This cartilaginous nature is considered a derived trait; their early ancestors possessed bony skeletons. Sturgeon evolved to reduce bone in their internal framework, an adaptation contributing to their survival through environmental changes.
Another trait reflecting their ancient origins is their covering of ganoid scales, often called scutes. Unlike the overlapping scales of most fish, sturgeon scutes are large, bony, diamond-shaped plates arranged in five longitudinal rows along their bodies. These scutes provide tough, armor-like protection and can sometimes be mistaken for an external bony skeleton. This combination of internal cartilage and external bony scutes highlights the sturgeon’s unique position.
Distinguishing Sturgeon from Other Fish
Sturgeon occupy a unique evolutionary branch, distinct from both bony fish (Osteichthyes) and cartilaginous fish like sharks and rays (Chondrichthyes). Most modern fish belong to the Osteichthyes, characterized by fully ossified skeletons. While sturgeon share the cartilaginous skeleton with Chondrichthyes, they are not closely related to sharks or rays. Unlike sharks, sturgeon possess a swim bladder and a single gill opening covered by an operculum, unlike multiple exposed gill slits.
Their classification as ray-finned fish (Actinopterygii) places them within bony fish, despite their predominantly cartilaginous internal structure. This distinction underscores their unique evolutionary path, where they diverged before the widespread development of bony skeletons seen in most fish today. Sturgeon represent an example of how different lineages have adapted and retained distinct skeletal features over millions of years.