Bony Fish vs. Cartilaginous Fish — What's the Difference?

Bony fish, classified under the class Osteichthyes, are characterized by their rigid skeletons made of bone. In contrast, cartilaginous fish, which belong to the class Chondrichthyes, have flexible skeletons composed entirely of cartilage, the same material that shapes our ears and noses. The skeletal structure significantly affects the biology and ecology of these fish, with bony fish typically having swim bladders that aid in buoyancy, while cartilaginous fish often lack this feature and must swim to keep from sinking.

Bony fish possess a variety of scales, including cycloid, ctenoid, or ganoid, which protect their bodies. Cartilaginous fish, such as sharks and rays, are covered with placoid scales, which have a tooth-like structure that provides a hydrodynamic advantage. This difference in scale structure contributes to the diverse habitats these fish can thrive in, with bony fish found in both freshwater and marine environments, and cartilaginous fish primarily dwelling in the ocean.

Reproduction in bony fish is mostly oviparous, with females laying eggs that are then fertilized outside the body. Cartilaginous fish, however, exhibit a range of reproductive strategies; many are ovoviviparous, where eggs hatch inside the mother's body, or viviparous, with the young developing within the uterus and born live. This variation in reproductive methods reflects their adaptability to different marine environments and ecological niches.

When it comes to feeding, bony fish exhibit a wide range of diets from plants to other fish, and they have well-developed jaws and teeth for capturing and processing their food. Cartilaginous fish, especially sharks, are often apex predators with powerful jaws and sharp, replaceable teeth, suited for a diet primarily consisting of meat, though some species like the whale shark feed on plankton filtered from the water.

In terms of sensory capabilities, bony fish generally have a lateral line system sensitive to vibrations and a swim bladder that can function in sound transmission. Cartilaginous fish lack a swim bladder but possess a highly developed lateral line system and, in some species, electroreceptors called ampullae of Lorenzini, which allow them to detect the electric fields produced by prey. The sensory adaptations of both bony and cartilaginous fish highlight their evolutionary divergence and specialization within aquatic environments.