Fish

Marine fish are divided into three main groups: jawless fish (lampreys and hagfish), cartilaginous fish (sharks, rays and chimaeras) and bony fish (the vast majority of species). Bony fish range from the tiny pygmy goby — one of the world's smallest vertebrates at 8mm — to the ocean sunfish, which can weigh over 2 tonnes. Fish breathe through gills, extracting dissolved oxygen from water. Most are ectothermic, though some species like tuna and swordfish can partially regulate their body temperature. The lateral line system detects vibrations and pressure changes, providing a sensory capability unique to fish and amphibians.

Coral reefs support the greatest diversity of fish species of any marine habitat. A single reef system may host hundreds of species, each occupying a specific ecological niche. Parrotfish graze on coral, excreting the calcium carbonate as the white sand found on tropical beaches. Cleaner wrasse establish 'cleaning stations' where larger fish queue to have parasites removed. Clownfish live symbiotically within the stinging tentacles of sea anemones, immune to their host's venom. The complex interactions between reef fish species create intricate food webs that are highly sensitive to disturbance.

The deep sea — below 200 metres — is the largest habitat on Earth, and its fish have evolved extraordinary adaptations to survive in conditions of crushing pressure, near-freezing temperatures and total darkness. Anglerfish use bioluminescent lures to attract prey in the pitch-black depths. Viperfish have hinged skulls that allow them to swallow prey larger than their own heads. The barreleye fish has transparent heads and tubular eyes that can rotate to look upward through its skull. Many deep-sea fish produce their own light through bioluminescence, using it for communication, camouflage and predation.

Fish populations worldwide are under severe pressure from overfishing, habitat destruction, pollution and climate change. The Food and Agriculture Organization estimates that over 35% of global fish stocks are overfished. Destructive fishing practices such as bottom trawling devastate seafloor habitats. Coral reef degradation eliminates habitat for thousands of reef fish species. Ocean acidification — caused by CO₂ absorption — affects fish behaviour, sensory systems and the calcification of reef-building species. Sustainable fisheries management, marine protected areas and consumer choices all play important roles in protecting fish populations for future generations.