Chondrichthyes vs. Osteichthyes — What's the Difference?

Chondrichthyes, including sharks, rays, and skates, have a skeletal structure made entirely of cartilage, which is lighter and more flexible than bone. This adaptation aids in their buoyancy and agility in the water. Osteichthyes, on the other hand, comprise the majority of fish species and are characterized by their bony skeletons, which provide more structural support but add to their weight.

While Chondrichthyes typically possess a series of gill slits for respiration, Osteichthyes have a bony operculum covering their gills, which aids in pumping water across the gills even when stationary. This difference in gill structure reflects their distinct evolutionary adaptations.

Chondrichthyes are known for having placoid scales, which are small, tooth-like structures that provide a rough, sandpaper-like texture to their skin. Osteichthyes, whereas, have a variety of scale types including cycloid, ctenoid, and ganoid, which generally offer a smoother surface and protection without compromising mobility.

Reproduction in Chondrichthyes can be quite diverse, with some species laying eggs (oviparous) and others giving live birth (viviparous). Osteichthyes mainly exhibit external fertilization where eggs are laid and then fertilized in the water, although there are exceptions with internal fertilization and live-bearing species.

Chondrichthyes often have a more streamlined and predatory design, reflecting their role as top predators in many aquatic ecosystems. Osteichthyes display a wider range of body shapes and sizes, from the tiny Paedocypris to the massive ocean sunfish, showcasing their extensive adaptability and ecological diversity.