Alex Mustard / Grooved brain coral (Diploria labyrinthiformis) at night with polyps extended to feed, Grand Cayman, Cayman Islands, British West Indies. Caribbean Sea.

Alex Mustard / Yellow tube sponge (Aplysina fistularis) on coral reef, diver exploring in background. Black Rock Drop Off, East End, Grand Cayman, Cayman Islands, British West Indies. Model released.

Gary Bell/ Oceanwide / Sea Goldie (Pseudanthias squamipinnis) school in coral reef with feather stars, Great Barrier Reef, Queensland, Australia

Gary Bell/ Oceanwide / Scarlet Cleaner Shrimp (Lysmata amboinensis) cleaning mouth of Tomato Grouper (Cephalopholis sonnerati), Great Barrier Reef, Australia

Gary Bell/ Oceanwide / Scarlet Cleaner Shrimp (Lysmata amboinensis) cleaning mouth of Tomato Grouper (Cephalopholis sonnerati), Great Barrier Reef, Australia

Alex Mustard / Split level image of Leather corals (Sarcophyton sp.) flourishing in shallow water close to uninhabited coral islands. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean...

Alex Mustard / Commensal emperor shrimp (Periclimenes imperator) moves across the colourful red patterned surface of its host Candycane sea cucumber (Thelenota rubralineata). Misool, Raja Ampat, West Papua, Indonesi...

Alex Mustard / Three Pink anemonefish (Amphiprion perideraion) living in a purple skirted magnificent sea anemone (Heteractis magnifica) on a coral reef, below a small island. Misool, Raja Ampat, West Papua, Indones...

Alex Mustard / Tawny nurse sharks (Nebris ferrungineus) at night. Vavuu Atoll, Maldives. Indian Ocean

Alex Mustard / Midnight snapper (Macolor macularis) shelters in a cavern on a coral reef with white soft corals (Scleronephthya sp.) and solderfish. Vavuu Atoll, Maldives. Indian Ocean

Alex Mustard / Pair of Western clown anemonefish (Amphiprion ocellaris) spawning orange eggs on the rock beneath their Magnificent sea anemone (Heteractis magnifica) home on a coral reef. This photo shows the larger...

Alex Mustard / Portrait of a Giant frogfish (Antennarius commersoni) on a large Yellow elephant ear sponge (Ianthella basta). Bitung, North Sulawesi, Indonesia. Lembeh Strait, Molucca Sea.

Alex Mustard / Portrait of a tarpon (Megalops atlanticus) on a coral reef. East End, Grand Cayman, Cayman Islands, British West Indies. Caribbean Sea.

Alex Mustard / Forster's hawkfish (Paracirrhites forsteri) on a coral reef. Jackson Reef, Tiran, Sinai, Egypt. Strait of Tiran, Gulf of Aqaba, Egypt.

Alex Mustard / Coral (Lobophytum sp.) on a reef flat, with Hawksbill turtle (Eretmochelys imbricata) in background. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) unusually feeding on hard coral polyps. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean.

Alex Mustard / Blacktip reef shark (Carcharhinus melanopterus) swims through schools of fish. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean.

Alex Mustard / Green turtle (Chelonia mydas) descending after breathing at the surface. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean.

Shane Gross / Pygmy seahorse (Hippocampus bargibanti) holding on to a branch of a sea fan coral. Malapascua, Philippines.

Shane Gross / Green sea turtle (Chelonia mydas) resting in a coral reef. Eleuthera, Bahamas.

Shane Gross / Elkhorn corals (Acropora palmata) at night silhouetted by a scientist's light during the annual coral spawn. Abaco, Bahamas.

SCOTLAND: The Big Picture / Orange jewel anemones (Corynactis viridis) Sula Sgeir, Scotland, UK, February.

SCOTLAND: The Big Picture / Dead man's fingers (Alcyonium digitatum) on a rocky reef with view of the sky, Shetland, Scotland, UK, August.

SCOTLAND: The Big Picture / Close up of a colony of Jewel anemones (Corynactis viridis) Scotland, UK, October.

Pascal Kobeh / Trumpetfish (Aulostomus chinensis) on the coral drop off, Sulu Sea, Philippines.

David Fleetham / Bargibant's pygmy seahorse (Hippocampus bargibanti) wrapped around Eye fan coral (Cnidaria sp). Philippines.

Alex Mustard / Yellowtail tang (Zebrasoma xanthurum) male courting a female prior to spawning at dusk on a coral reef. Temple, Ras Umm Sid, Sharm El Sheikh, Sinai, Egypt. Red Sea.

Alex Mustard / Galapagos green turtle (Chelonia mydas agassizii) swims over a coral reef. Darwin Island, Galapagos National Park, Galapagos Islands. East Pacific Ocean.

Juergen Freund / Timelapse of Giant sea cucumbers (Thelenota ananas) moving underwater, with a Giant clam (Tridacna gigas) in the background, Great Barrier Reef, Queensland, Australia, 2015.

David Fleetham / Green sea turtles (Chelonia mydas) on corals, Hawaii.

David Fleetham / Necklace sea star (Fromia monilis) on gorgonian coral, Indonesia.

David Fleetham / Pair of squat lobsters (Galathea sp) on Gorgonian coral, Komodo, Indonesia.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) pair circling each other above coral reef. Seven Mile Beach, Grand Cayman, Cayman Islands, British West Indies. Caribbean Sea.

Jordi Chias / Goliath grouper (Epinephelus itajara) inside a coral cave. Bloody Bay Wall, Little Cayman, Cayman Islands. British West Indies. Caribbean Sea.

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.

Jordi Chias / Wave breaking over the reef, Table coral (Acropora) Safaga, Egypt, Red Sea.

Jordi Chias / Hawksbill turtle (Eretmochelys imbricata) feeding on coral, South Ari Atoll, Maldives Islands, Indian ocean.

Jordi Chias / Hawksbill turtle (Eretmochelys imbricata) Port El Ghalib, Egypt, Red Sea.

Jordi Chias / Red Sea anemonefish (Amphiprion bicinctus) in their home, a Sea anemone (Heteractis magnifica), on a coral reef. Big Brother island, Red Sea. Egypt.

Franco Banfi / Three False clownfish (Amphiprion ocellaris) in Sea anemone (Heteractis magnifica) Lighthouse Reef, Cabilao Island, Bohol, Central Visayas, Philippines, Pacific Ocean.

Alex Mustard / Striped mackerel (Rastrelliger kanagurta) mouth wide open as it swims through the water, filtering zoopankton with its gill rakers. Marsa Shouna, Port Ghalib, Marsa Alam, Egypt. Red Sea

Ingo Arndt / Aerial view of Hardy Reef, Great Barrier Reef, Queensland, Australia, December 2010.

Alex Mustard / Warty / Clown frogfish (Antennarius maculatus) male portrait, Anilao, Batangas, Luzon, Philippines, March

Alex Mustard / Yellow tube sponge (Aplysina fistularis) on a coral reef, with the camera rotated during the long exposure. East End, Grand Cayman, Cayman Islands. British West Indies. Caribbean Sea.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) male chomps on soft coral. Jackson Reef, Sinai, Egypt. Strait of Tiran, Red Sea.

Alex Mustard / A stand of Pillar coral (Dendrogyra cylindrus) growing on a coral reef, East End, Grand Cayman, Cayman Islands

Alex Mustard / Boulder brain coral (Colpophyllia natans) photographed with a long exposure with camera rotation. Taken with circular fisheye lens. Grand Cayman, Cayman Islands. Caribbean Sea.

Alex Mustard / Spotlight goby (Gobiosoma louisae) looking out from inside a Yellow tube sponge (Aplysina fistularis), growing on a coral reef. Bloody Bay Wall, Little Cayman, Cayman Islands, British West Indies. Car...

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 / Green sea turtle (Chelonia mydas) female resting on a coral reef. Rock Islands, Palau, Mirconesia. Tropical west Pacific Ocean

Jordi Chias / Spotted catshark (Scyliorhinus canicula) just hatched from egg case that is on Seafan (Paramuricea clavata), Ullastres seamount, Costa Brava, Llafranch, Catalunya, Spain

Alex Mustard / Colourful reef scene with Leather corals (Sarcophyton sp.), Soft corals (Dendronephthya sp.) Seafans (Melithea sp.) and fish, including Magenta slender anthias (Luzonichthys waitei) and Scalefin anthi...

Franco Banfi / Sea star (Hacelia attenuata) on coral covered with mucilage, a symptom of rising sea temperature, Vis Island, Croatia, Adriatic Sea, Mediterranean.

Franco Banfi / Green moray (Gymnothorax funebris) Santa Lucia, Camaguey, Cuba, Caribbean Sea, Atlantic Ocean

Franco Banfi / Corals attached to structure of bio-rock, a method of enhancing the growth of corals and aquatic organisms, Karang Lestari Pemuteran project, Desa Pemuteran, Bali Island, Indonesia, Pacific Ocean.

Claudio Contreras / Aerial view of Red mangrove (Rhizophora mangle) coastal lagoon, Sian Ka'an Biosphere Reserve, Caribbean Sea, Mexico, January

Claudio Contreras / Aerial view of Contoy island from the north, Contoy Island National Park, Mesoamerican Reef System, near Cancun, Caribbean Sea, Mexico, January

Claudio Contreras / Giant barrel sponge (Xestospongia muta) Cozumel Reefs National Park, Cozumel Island, Caribbean Sea, Mexico, February

Alex Mustard / Backlit detail of branching fire coral (Millepora alcicornis) showing the hair like polyps, filled with stinging nematocysts. East End, Grand Cayman, Cayman Islands. Caribbean Sea.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) cruising along the drop off of a coral reef. Bloody Bay Wall, Little Cayman, Cayman Islands. Caribbean Sea. NB. Turtle has two identification tags in its fron...

Alex Mustard / Portrait of Bigfin squid (Sepioteuthis lessoniana) hovering in open water above a coral reef at night. Dampier Strait, Raja Ampat, West Papua, Indonesia. Tropical West Pacific Ocean

Alex Mustard / Yellow tube sponges (Aplysina fistularis) growing on a Caribbean coral reef. Cayman Brac, Cayman Islands. Caribbean Sea.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) male swimming in open water above a coral reef. Tank Rock, Fiabacet, Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea, Tropical West Pacific Ocean.

Alex Mustard / Common ghostgoby (Pleurosicya mossambica) with another fish in its mouth on soft coral (Dendronephthya sp.). Tulamben, Bali, Indonesia. Java Sea

Alex Mustard / Diver exploring the wreck of HMS Scylla, which is colonized by Dead man's fingers (Alcyonium digitatum). Whitsands Bay, Cornwall, England. UK, July.

Alex Mustard / Napoleon wrasse (Cheilinus undulatus) super male, patrolling coral, Ras Mohammed Marine Park, Sinai, Egypt. Red Sea

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

Alex Mustard / Caribbean reef squid (Sepioteuthis sepioidea) amongst gorgonians, on a shallow coral reef. Cayman Brac, Cayman Islands. Caribbean Sea.

Alex Mustard / Scissortail sergeants (Abudefduf sexfasciatus) swimming over hard corals (Acropora sp.) on shallow reef at sunset, split level view. Gubal Island, Egypt. Red Sea.

Alex Mustard / Group of predators hunting together, in an example of 'Nuclear foraging'. The Moray eel (Gymnothorax javanicus) flushes out prey, and is followed by a Napolean wrasse (Cheilinus undulatus) and a Bluef...

Alex Mustard / Large whip goby (Bryaninops amplus) with a parasitic copepod (with two trailing egg ribbons) perched on Red whip coral. Whale Rock. Fiabacet Islands, Misool, Raja Ampat, West Papua, Indonesia. Ceram S...

Alex Mustard / Colourful reef scene with Coral grouper (Cephalopholis miniata), map puffer (Arothron mappa) and Panda butterflyfish (Chaetodon adiergastos). Tank Reef, Fiabacet Islands, Misool, Raja Ampat, Indonesia...

Georgette Douwma / Erect rope sponge (Amphimedon compressa) Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Stove-pipe sponge (Aplysina archeri) Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Erect rope sponge (Amphimedon compressa) Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Row pore rope sponge (Aplysina cauliformis) Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Stove-pipe sponge (Aplysina archeri) and Orange elephant ear sponge (Agelas clathrodes) Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Green turtle (Chelonia mydas) with two large remoras (Echeneis naucrates) attached to its carapace, at rest on artificial reef. Mabul, Malaysia.

Georgette Douwma / Coral line nursery with Acropora corals grown to reintroduce to the Bonaire, Netherlands Antilles, Caribbean, Atlantic Ocean.

Georgette Douwma / Crinoid (Crinoidea) on artificial reef. Mabul, Malaysia.

Georgette Douwma / Granulated sea star (Choriaster granulatus) on artificial reef. Mabul, Malaysia.

Juergen Freund / Giant clam (Tridacna gigas) Raja Ampat, West Papua, Indonesia.

Pascal Kobeh / Divers are setting up concrete reef balls to build an artificial reef, Philippines, Sulu Sea. August 2014.

Richard Robinson / Featherstar (Crinoidea) on red coral (Errina novazelandiae) in Doubtful Sound, Fiordland National Park, New Zealand.

Tim Fitzharris / Green Sea Turtle (Chelonia mydas) and Orange-striped Triggerfish (Balistapus undulatus), Apo Island, Philippines

Michael Pitts / Green turtle (Chelonia mydas) swimming over a reef, Maldives, Indian Ocean.

Alex Mustard / Shoal of Bar jacks (Caranx ruber) hunting school of Silversides fish (Atherinidae) in coral cavern. East End, Grand Cayman, Cayman Islands, British West Indies. Caribbean Sea.

Alex Mustard / Close up of tiny (10mm) Pygmy seahorse (Hippocampus bargibanti) living disguised in Muricella sp. sea fan. Bitung, North Sulawesi, Indonesia. Lembeh Strait, Molucca Sea.

Alex Mustard / Hawksbill turtle (Eretmochelys imbricata) feeding on soft coral (Litophyton arboreum) Nuweiba, Sinai, Egypt. Gulf of Aqaba, Red Sea.

Alex Mustard / Busy coral reef community feeding on plankton, with Scalefin anthias (Pseudanthias squamipinnis), Soft corals (Dendronephthya sp.) and Fire corals (Millepora dichotoma), making up classic Red Sea scen...

Alex Mustard / Red Sea anemonefish (Amphiprion bicinctus) in Magnificent sea anemone (Heteractis magnifica), which has closed up in the late afternoon, on coral reef under stormy seas. St Johns Reef. Egypt. Red Sea.