We can classify fish into two large groups that are quite different from each other: the chondrictes (cartilaginous fish) and the osteitis (bony fish). Despite some glaring differences, it's common to get confused by differentiating the two groups. Below we will highlight the main differences between a cartilaginous fish and a bony fish. It is worth noting that we can find some representatives that are outside the rules below.
First we can differentiate the two groups by skeleton. Cartilaginous fishes have a skeleton made up entirely of cartilage, while bony fishes have a skeleton made up basically of bones.
Another striking difference concerns the gills. Bony fish have a membrane that covers the gill slits, while cartilaginous fish have their gills exposed, without any protection.
Scales can also be used to differentiate these two groups. While cartilaginous fish have placoid scales and dermal and epidermal origin, bony fish have scales of exclusively dermal origin.
Looking at the mouth, it is also possible to see a difference. While cartilaginous fish have a ventral mouth, bony fish have their mouth in the anterior region of the body.
Bone fish have, among other characteristics, the presence of operculum
Reproduction is also an important factor. While bony fish have external fertilization, cartilaginous ones have a structure called clasper, which works by helping in internal reproduction. The clasper is a modified pelvic fin that aids in the introduction of sperm. In addition to this difference, we can highlight the fact that in cartilaginous fish there is no appearance of larvae, while in bony fish there is a larva that later develops and forms the fingerling.
We can also see that cartilaginous fish have a cloaca, unlike bony fish.
Another difference concerns the swim bladder, a structure that helps the fish float. This structure is found only in bony fish.
We can mention as examples of cartilaginous fish the shark, stingray and the dogfish. Among the bony fish, we can mention the catfish, painted and carp.
Source: Brazil School - https://brasilescola.uol.com.br/biologia/diferencas-entre-peixes-osseos-cartilaginosos.htm
