Fish

Fish represent the most diverse group of vertebrates on Earth, living in ponds, coral reefs, deep oceans, and even temporary desert streams. Most fish breathe by pulling water over feathery gills that extract dissolved oxygen, then push the water back out through gill slits. Their bodies are covered with protective scales and a slime coat that reduces friction as they swim. Muscle bands flex against the spine to create side-to-side waves, while fins act like brakes or rudders to change direction quickly. Some species, such as tuna, have stiff fins for long migrations, while others, such as seahorses, rely on tiny fin fluttering to hover in place.

Fish use their senses in surprising ways. A lateral line running along each side detects vibrations, alerting fish to predators and helping schools move as one shimmering cloud. Many fish see colors we cannot because reef life relies on bright patterns for camouflage, warnings, or courtship. Others communicate with sounds produced by grinding bones together or drumming muscles on their swim bladder, which doubles as a buoyancy organ that keeps them from sinking or floating away. Electric fish like knifefish and electric eels send pulses into the water to navigate murky environments.

Reproduction strategies vary widely. Salmon hatch in mountain streams, travel to the ocean, then return to their birthplace to spawn. Cichlids guard eggs in their mouths, while seahorse fathers carry fertilized eggs inside a pouch. Some species lay thousands of eggs that drift in the plankton, trusting that a few will survive to adulthood. Others, such as rays and sharks, develop their young internally before giving birth to fully-formed pups. These life cycles connect rivers, wetlands, and seas, making fish excellent indicators of ecosystem health.

Fish support people by providing protein, fertilizers, and even inspiration for technology. Engineers design submarines and swim fins by studying streamlined fish shapes. Aquaculture farms raise tilapia and carp to relieve pressure on wild populations. Indigenous communities manage fisheries with seasonal calendars, protecting spawning runs so lakes and rivers remain healthy for generations. Scientists tag fish with acoustic transmitters to discover migration routes and identify pollution hotspots.

Protecting fish means caring for water quality, wetlands, and reefs. Runoff from cities and farms can carry chemicals that fuel algal blooms, robbing fish of oxygen. Climate change warms oceans and pushes coral reef fish toward the poles. Marine protected areas, dam removals, and river restoration projects reconnect habitats so fish can complete their life cycles. When students learn how gills work, test salinity levels, or visit hatcheries, they gain appreciation for the watery world that keeps fish swimming and people fed.