Fishes

Fishes are aquatic vertebrates that breathe with gills, move with fins, and typically live in water. They comprise the largest group of vertebrates, with thousands of described species in marine, freshwater, and brackish environments. Living fishes are divided into cartilaginous fishes (sharks, rays, and chimaeras) and bony fishes (Osteichthyes); a few jawless lineages, such as lampreys and hagfishes, survive as ancient representatives.

Anatomy and physiology: Most fishes are ectothermic and have streamlined bodies suited to life in water. They possess gills protected by an operculum, a two-chambered heart in many species, and a lateral line system to sense movement and vibration. Many bony fishes have a swim bladder for buoyancy and body scales made of bone; cartilaginous fishes lack a swim bladder and have placoid scales.

Reproduction and development: Reproductive strategies vary. Fertilization is often external, with eggs released into the water, but internal fertilization occurs in sharks and related groups. Development ranges from eggs that hatch into free-swimming larvae to live birth in some species.

Ecology and diversity: Fishes occupy nearly every aquatic habitat, from mountain streams to the open ocean and coral reefs. They play essential roles as predators, prey, herbivores, and detritivores. Many species migrate for spawning or feeding, and some are anadromous or catadromous.

Human interactions and conservation: Fishes are important for food, recreation, and research. They face threats from overfishing, bycatch, habitat degradation, pollution, and climate change. Sustainable fisheries, responsible aquaculture, and habitat protection are essential for preserving fish diversity and livelihoods.