Joel Sartore / Photo Ark / Nudibranch (Hypselodoris apolegma), Variable neon slug (Nembrotha kubaryana), Golden Nembrotha (Nembrotha aurea), Nudibranch (Hypselodoris bullockii), Anna?s chromodoris (Chromodoris annae) and Willan...

Linda Pitkin / Green turtle (Chelonia mydas) resting on seabed feeding on Seagrass (Halophila stipulacea) with a Remora (Echeneis naucrates) on its back and shoals of small fish swimming around, Marsa Alam, Egypt, R...

Linda Pitkin / Barrel sponge (Xestospongia testudinaria) on reef with shoal of Barracuda (Sphyraena sp.) in background, Tubbataha Reefs Natural Park, Palawan, Philippines, Sulu Sea.

Linda Pitkin / Anthias (Pseudanthias sp.) and Damselfish (Chromis sp.) swimming over coral reef with Acropora (Acropora sp.) corals and Fire coral (Millepora sp.), Tubbataha Reefs Natural Park, Palawan, Philippines,...

Pal Hermansen / Group of male Toads (Bufo bufo) trying to mate with female, underwater in small lake, Akershus, Norway. May.

Tui De Roy / Green turtle (Chelonia mydas) swimming with school of Barracuda (Sphyraena sp.) behind, Cousins Rock, near Santiago Island, Galapagos Islands. Endangered.

Joel Sartore / Photo Ark / African upside-down jellyfish (Cassiopea depressa) moving 'bell', Oklahoma Aquarium. Captive.

Joel Sartore / Photo Ark / Bigclaw crayfish (Faxonius placidus) female oxygenating eggs by fanning them, upper Duck River, Coffee County, Tennessee, USA. Controlled conditions.

Joel Sartore / Photo Ark / Spanish dancer nudibranch (Hexabranchus sanguineus) with an Emperor shrimp (Zenopontonia rex) moving on top of it, Farglory Ocean Park, Hualien. Captive.

Joel Sartore / Photo Ark / Close up of a Common topknot (Zeugopterus punctatus) head. The animal is looking at the camera. Aquazoo Loebbecke Museum, Dusseldorf. Captive.

Joel Sartore / Photo Ark / Close up of a Coral grouper (Cephalopholis miniata) head. The animal looks around. Oklahoma Aquarium. Captive.

Joel Sartore / Photo Ark / Bigmouth chub (Nocomis platyrhynchus) looking around, Aquatic Wildlife Conservation Center, Marion, Virginia. Captive.

Joel Sartore / Photo Ark / Buffalo darter (Ehtheostoma bison), Brush Creek, Lewis County, Tennessee, USA. Controlled conditions.

Jack Perks / Three-spined stickleback (Gasterosteus aculeatus) male making a nest. The fish repeatedly swims in and out of the frame with vegetation in its mouth, constructing the nest with this material. UK, June...

Jack Perks / Shoal of Roach (Rutilus rutilus) and Perch (Perca fluviatilis). A Brown trout (salmo trutta) swims into the frame, startling the shoal, as it looks for prey. The fish swims out of the frame. Rutland,...

Jack Perks / Sea trout (Salmo trutta) pair courting, in preparation to spawning. The male quivers near the female and then she uses her tail fin to dig a shallow depression (a redd). Perthshire, UK, November.

Jack Perks / Sea trout (Salmo trutta) pair courting, in preparation to spawning. The female uses her tail fin to dig a shallow depression (a redd) and then the male quivers near her. Perthshire, UK, November.

Jack Perks / Atlantic salmon (Salmo salar) pair courting. The female is digging a redd with her tail fin. The pair display courtship as they prepare to spawn. Perthshire, UK, November.

Jack Perks / Ruffe (Gymnocephalus cernua) shoal swimming in deep water, Oxfordshire, UK.

Jack Perks / Pike (Esox lucius) catching a Mayfly (Ephemeroptera sp.) as it emerges, Hampshire, UK, May.

Jack Perks / Common carp (Cyprinus carpio) swimming in a river, feeding on Zebra mussels (Dreissena polymorpha). The animal swims out of the frame. London, UK.

Jack Perks / Chubs (Squalius cephalus) gathering as they prepare to spawn. A shoal of Minnows (Phoxinus phoxinus) are swimming nearby. Nottinghamshire UK , May.

Jack Perks / Mirror carp (Cyprinus carpio) swimming into frame and past the camera. The animal disappears into the murky water. Shropshire, UK.

Jack Perks / Barbel (Barbus barbus) males pushing each other as they prepare to spawn, Nottinghamshire, UK. May.

Jack Perks / Brown trout (Salmo trutta) pair courting. The male quivers near the female and then she uses her tail fin to dig a shallow depression (a redd). Chalk stream, Cambridgeshire, UK. January.

Jack Perks / Common bream (Abramis brama) shoal swimming past, in a shallow river. The fish swim off into the background. Hampshire, UK.

Jack Perks / Common barbel (Barbus barbus) appearing and feeding at the bottom of the river alongisde Common dace (Leuciscus leuciscus). The Barbel swims out of the frame. Nottinghamshire, UK.

Tony Wu / Elkhorn sculpin (Alcichthys elongatus) male, guarding its brood of bright yellow eggs in a rock crevice, Hokkaido, Japan, Pacific Ocean.

Tim Laman / Two Gobies (Gobiidae) resting on a Sea fan (Gorgonia sp.), Raja Ampat Marine Park, Raja Ampat Islands, West Papua, Indonesia.

Tim Laman / King penguins (Aptenodytes patagonicus) swimming underwater, Macquarie Island, Subantarctic Island of Australia, Pacific Ocean.

Henley Spiers / Green turtle (Chelonia mydas) juvenile, swimming in open ocean, with two Pilot fish (Naucrates ductor) and a Remora (Echeneidae) swimming alongside, Eastern Pacific Ocean. Endangered.

Emanuele Biggi / Common toad (Bufo bufo) male, resting on riverbed during breeding period, Viterbo, Italy.

Emanuele Biggi / Lemur leaf frog (Agalychnis lemur) eggs with developing embryos inside. The still developing tadpoles have external gills, visible as tiny branches on the side of the heads. Captive, occurs in South A...

Franco Banfi / RF - Dugong (Dugong dugon) swimming back to surface to breathe after feeding on seagrass, accompanied by Golden trevally (Gnathanodon speciosus) juvenile, Marsa Alam, Egypt, Red Sea. (This image may b...

Franco Banfi / Dugong (Dugong dugon) feeding on Seagrass (Halophila stipulacea) meadow accompanied by Golden trevally (Gnathanodon speciosus) juvenile, Marsa Alam, Egypt, Red Sea.

Alex Mustard / Green turtle (Chelonia mydas) female, swimming over coral reef with sunburst, Tubbataha Atolls, Palawan, Philippines, Sulu Sea. Endangered.

Alex Mustard / Sap-sucking slug / Shaun the sheep slug (Costasiella kuroshimae) crawling across green algae (Avrainvillea sp.) on seabed, Bitung, North Sulawesi, Indonesia. Lembeh Strait, Molucca Sea.

Alex Mustard / RF - Green turtle (Chelonia mydas) male, silhouetted in Snell's window with sunburst, Tubbataha Atolls, Palawan, Philippines, Sulu Sea. Endangered. (This image may be licensed either as rights ma...

Alex Mustard / Mimic octopus (Thaumoctopus mimicus) moving over sandy seabed, Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Diver swimming through the Silfra fissure, a crack in the Earth's crust between the North American and Eurasian continental plates, which is filled with glacial melt water, Thingvellir National P...

Franco Banfi / Spinner dolphin (Stenella longirostris) pod swimming over seabed in shallow, sandy lagoon, Marsa Alam, Egypt, Red Sea.

Alex Mustard / Coleman shrimp (Periclimenes colemani) female, with smaller male on left, living in a Fire urchin (Asthenosoma varium), Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Three White-eyed moray eels (Gymnothorax thyrsoideus) and Palechin moray eel (Gymnothorax herrei) with Cleaner shrimps (Decapoda) resting in a hollow log on seabed, Bitung, North Sulawesi, Indonesia,...

Alex Mustard / Wonderpus (Wunderpus photogenicus) on seabed flaring its arms, curling the ends and shows a contrasting colour pattern as a warning, Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Yellow pygmy gobies (Lubricogobius exiguus) pair living inside a discarded toothpaste tube on the seabed, Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Painted frogfish (Antennarius pictus) camouflaged among a Sponge on seabed, Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Nudibranch (Flabellina exoptata) crawling over an rock covered by orange sponge. Bitung, North Sulawesi, Indonesia, Lembeh Strait, Molucca Sea.

Alex Mustard / Conch (Strombus sp.) eye detail, West Bay, Grand Cayman, Cayman Islands, Caribbean Sea.

Alex Mustard / Spinyhead blenny (Acanthemblemaria spinosa) juvenile, looking out from a hole in coral reef, surrounded by brown Boring sponge (Cliona sp.) and Sponge zoanthids (Parazoanthus parasiticus), East End, G...

David Fleetham / Gorgonian gobies (Bryaninops amplus) pair, resting next to their eggs on Wire coral (Cirrhipathes sp.), Hawaii, Pacific Ocean.

Harry Collins / Brown bear (Ursus arctos) hunting Salmon (Salmonidae sp.) using the 'snorkeling' technique. The bear looks for fish by immersing its head in water. Katmai National Park, Alaska. August.

Harry Collins / Brown bear (Ursus arctos) hunting Salmon (Salmonidae sp.) using the 'snorkeling' technique. The bear looks for fish by immersing its head in water. Katmai National Park, Alaska. August.

Franco Banfi / Giant oceanic manta ray (Manta birostris) at a cleaning station. Small fish swarm the underside of the ray as it swims over, removing parasites from its skin. Maldives, Indian Ocean.

Alex Mustard / Common ghostgoby (Pleurosicya mossambica) hiding within branches of Soft coral (Dendronephthya sp.) on coral reef. Gubal Island, Egypt, Gulf of Suez, Red Sea.

Tony Wu / Copperstriped cardinalfish (Ostorhinchus holotaenia) male releasing newly hatched fry from mouth whilst swimming, Kagoshima Prefecture, Japan, Pacific Ocean.

Tony Wu / Spotted moray eel (Gymnothorax meleagris) pair coming together within a colony of Leaf coral (Pavona decussata) to spawn, Kagoshima Prefecture, Japan, Pacific Ocean.

Tony Wu / Dragon moray eel (Enchelycore pardalis) female releasing eggs. Male had already spawned and returned to the safety of coral, Kagoshima Prefecture, Japan, Pacific Ocean.

Tui De Roy / Green turtle (Chelonia mydas) sleeping on sea floor at cleaning station amongst Garden eels (Heteroconger sp.) as Scalloped hammerhead sharks (Sphyrna lewini) swim above, Darwin Island, Galapagos, Ecu...

Linda Pitkin / Yellowfin goatfish (Mulloidichthys vanicolensis) shoal schooling over coral reef, Marsa Alam, Egypt, Red Sea, July.

Doug Perrine / Shortfin mako shark (Isurus oxyrinchus) swimming close to water surface, Baja California Sur, Mexico, Pacific Ocean. Endangered.

Doug Perrine / Blue shark (Prionace glauca) with mouth open, underwater portrait, Baja California Sur, Mexico, Pacific Ocean.

Doug Perrine / Blue shark (Prionace glauca) dorsal view, Baja California Sur, Mexico, Pacific Ocean.

Doug Perrine / Blue shark (Prionace glauca) with small parasites attached on side of head and under mouth, accompanied by Pilot fish (Naucrates ductor), Baja California Sur, Mexico, Pacific Ocean.

Doug Perrine / Blue shark (Prionace glauca) with small parasites attached on side of head and under mouth, accompanied by Pilot fish (Naucrates ductor), Baja California Sur, Mexico, Pacific Ocean.

Doug Perrine / Reef manta ray (Mobula alfredi) with Saddle wrasses (Thalassoma duperrey) and Hawaiian cleaner wrasses (Labroides phthirophagus) at cleaning station on coral reef, Keahole Point, North Kona, Hawaii Is...

Doug Perrine / Day octopus (Octopus cyanea) hunting on coral reef, exhibiting white colouration while expanding webbing between arms to trap fleeing prey with Manybar goatfish (Parupeneus multifasciatus) waiting clo...

Doug Perrine / Day octopus (Octopus cyanea) hunting on coral reef with Manybar goatfish (Parupeneus multifasciatus) waiting to catch any small fish that escape, Makako Bay, North Kona, Hawaii Island, Hawaii, Pacific...

Doug Perrine / Hawaiian spinner dolphin (Stenella longirostris longirostris) pod, with male performing genital buzz courtship behaviour on female. A shoal of Black durgon triggerfish (Melichthys niger) swimming behi...

D. Parer & E. Parer-Cook / Humpback Whale (Megaptera novaeangliae) males chasing female during heat run near snorkeler, Tonga

D. Parer & E. Parer-Cook / Humpback Whale (Megaptera novaeangliae) blowing bubbles, Tonga

Franco Banfi / RF - Atlantic walrus (Odobenus rosmarus) swimming in shallow water, Spitsbergen, Svalbard, Norway, Arctic Ocean. (This image may be licensed either as rights managed or royalty free.)

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater portrait, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater portrait, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater view, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater view, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Franco Banfi / Atlantic walrus (Odobenus rosmarus) underwater portrait, Spitsbergen, Svalbard, Norway, Arctic Ocean.

Shane Gross / Plainfin midshipman (Porichthys notatus) fry, newly hatched and attached to yolk sacs in nest hidden under a rock in the intertidal zone, Vancouver Island, British Columbia, Canada, Pacific Ocean. Und...

Doug Allan / Arctic murres (Uria aalge) diving underwater for food beside iceberg, Baffin Island, Lancaster Sound, Nunavut, Canada, Arctic Ocean.

Doug Allan / Southern fur seals (Arctocephalus gazella) swimming beside Giant kelp (Macrocystis pyrifera), South Georgia, Atlantic Ocean.

Doug Allan / Atlantic cod (Gadus morhua) swimming in fish farm net, Newfoundland, Canada, Atlantic Ocean.

Doug Allan / Beluga (Delphinapterus leucas) swimming underwater, Lancaster Sound, Nunavut, Baffin Bay, Canada.

Doug Allan / Antarcturus isopod (Antarcturus sp.) standing on seaweed with other isopods, Signy Island, South Orkneys, Antarctica, Southern Ocean, June 1980.

Doug Allan / Giant antarctic sea spider (Decolopoda australis), Signy Island, South Orkneys, Antarctic, Southern Ocean, June 1980.

Doug Allan / Scientist measuring dimensions of a Sea anemone (Actiniaria sp.) with False clownfish (Amphiprion ocellaris) amongst its tentacles as another diver swims behind, Wakatobi National Marine Park, Sulawes...

Doug Allan / Banded sea snake (Laticauda colubrina) swimming down from water's surface, Raja Ampat Marine Reserve, Indonesia, Ceram Sea.

Doug Allan / Branching sponges (Porifera sp.) growing amongst coral with fish shoaling behind, Raja Ampat Marine Reserve, Indonesia, Ceram Sea.

Doug Allan / Sponge (Porifera sp.) with Red banded cleaner shrimps (Stenopus hispidus) inside, Raja Amat Marine Reserve, Indonesia, Ceram Sea.

Doug Allan / Aerial view of iceberg, Antarctic Peninsula, Antarctica, Southern Ocean, February 2008.

Doug Allan / Emperor penguins (Aptenodytes forsteri) diving underwater, Cape Washington, Antarctica, Southern Ocean.

Doug Allan / Common guillemots (Uria aalge) returning to the surface after diving for food, Lancaster Sound, Nunavut, Arctic Canada, Arctic Ocean.

David Shale / Glowing sucker octopus (Stauroteuthis syrtensis) at a depth of 830m, Gulf of Maine, Atlantic Ocean.

David Fleetham / Purple fairy basslets (Pseudanthias tuka) schooling around hard coral reef with scuba diver in background, Philippines, Pacific Ocean. Model released.

David Fleetham / A dive instructor hand signalling to student diver over a reef, Molokini Islet. off Maui, Hawaii, Pacific Ocean. Model released.

David Fleetham / Tiger shark (Galeocerdo cuvier) swimming over seabed, Tiger Beach, Bahamas, Atlantic Ocean.

David Fleetham / Great white shark (Carcharodon carcharias) swimming close to ocean surface, Guadalupe Island, Baja California, Mexico, Pacific Ocean.