Reefs cover under one percent of ocean, host a quarter of fish
Hard coral structures take up under one percent of the world ocean’s surface yet shelter roughly a quarter of all marine fish species. A small healthy patch can hold hundreds of kinds—an inverted abundance beside the open sea’s other ninety-nine percent.
Algae, invertebrates, and fish have evolved together on reefs over millions of years, and reefs hold the richest fish communities on Earth, perhaps 6,000 to 8,000 species. The Indo-Pacific, from the Red Sea across the Indian and Pacific Oceans, has an estimated 4,000 to 5,000 reef species; the greater Caribbean has only 500 to 700, and the two share none naturally. Why so many species crowd onto reefs is still argued after 50 years, with habitat complexity, varied food, and larval settlement all proposed.
Most reef fish are ray-finned, with spiny fins that can often lock upright or carry venom. Instead of the torpedo shape of open-ocean swimmers, many are built to dodge and turn; butterflyfish and angelfish are flattened sideways like pancakes. Triggerfish wedge themselves into crevices at night by raising their spines, and surgeonfish can slash rivals with blades near the tail.
Colour does many jobs. The foureye butterflyfish wears a false eyespot near its tail and a dark bar hiding its real eye, so predators lunge at the wrong end. The tasseled scorpionfish looks like crusted seabed while waiting for gobies, and the striated frogfish dangles a worm-like lure above its mouth. The clown triggerfish is countershaded, white-spotted below and yellow-and-black above.
Mouths match menus: butterflyfish nip coral polyps with forceps-like jaws, parrotfish scrape algae with beaks, and goatfish probe sand with chin barbels. Carnivores outnumber herbivores. Coral groupers hunt cooperatively with moray eels, and when a harem loses its male, the largest female turns male and can make sperm within ten days.
Source: Coral reef fish
