Jordi Chias / Schoolmaster snapper (Lutjanus apodus) school in the reef, Little Cayman island, Cayman Islands, Caribbean.

Jordi Chias / Nassau grouper (Epinephelus striatus) on a coral reef, Little Cayman Island, Cayman Islands, Caribbean.

Alex Mustard / Nassau grouper (Epinephelus striatus) yawning on a coral reef drop off. San Salvador island, Bahamas. Bahamas Sea, Tropical West Atlantic Ocean.

Alex Mustard / Maldivian anemonefish (Amphiprion nigripes) shelters against a closed magnificent sea anemone (Heteractis magnifica). Male Atoll, Maldives. Indian Ocean.

Alex Mustard / Maldive anemonefish (Amphiprion nigripes) shoal swimming in the current flowing over a Magnificent sea anemone (Heteractis magnifica). Baa Atoll, Maldives. Indian Ocean.

Tony Wu / Twinspot snapper (Lutjanus bohar) shoal rushing up to spawn, releasing a cloud of milky white sperm and eggs in the water column. Caroline Islands, Palau, November.

Tony Wu / Twinspot snapper (Lutjanus bohar) engaged in a mass spawning aggregation early in the morning. Caroline Islands, Palau. April. Winner of Underwater category, Wildlife Photographer of the Year 2016.

Alex Mustard / Nassau grouper (Epinephelus striatus) watched by a diver on a coral reef. San Salvador island, Bahamas. Bahamas Sea, Tropical West Atlantic Ocean.

Doug Perrine / Barjacks (Caranx ruber), Yellowtail snapper (Ocyurus chrysurus), and Sergeant major damselfish (Abudefduf saxatilis) swimming around wreck of bow gun on C-58 shipwreck off Puerto Morelos, near Cancun,...

Michael Pitts / Maldives clownfish (Amphiprion nigripes) chasing juveniles, with anemones (Actiniaria) in the background, Maldives, Indian Ocean.

Michael Pitts / Anemones (Actiniaria) with Maldives anemonefish (Amphiprion nigripes) and Surgeonfish (Acanthurus) moving in a strong current, Maldives, Indian Ocean.

Michael Pitts / Maldives anemonefish (Amphiprion nigripes) and Surgeonfish (Acanthurus) swimming in a strong current, with moving anemones (Actiniaria), Maldives, Indian Ocean.