selachians

Selachians are a subclass of cartilaginous fish, commonly known as sharks, skates, and rays. They are characterized by their skeletons made of cartilage rather than bone. This ancient lineage dates back over 400 million years, predating even the dinosaurs. Selachians possess a streamlined body shape, typically with five to seven gill slits on each side of their head. They have multiple rows of teeth that are continuously replaced throughout their lives. Their skin is covered in placoid scales, which are tiny, tooth-like structures that give them a rough texture and help reduce drag in the water.

Unlike bony fish, selachians lack a swim bladder. Instead, they maintain buoyancy through their large, oil-filled livers and the low density of their cartilaginous skeletons. They are found in all the world's oceans, from shallow coastal waters to the deepest abyssal plains, and some species inhabit freshwater environments. Selachians exhibit a wide range of diets, from filter-feeding on plankton to preying on large marine mammals. Their reproductive strategies vary, with some species laying eggs and others giving birth to live young. Despite their formidable reputation, most selachians are not dangerous to humans, and many face threats from overfishing and habitat destruction.