Constantinos Petrinos / Chromodorid nudibranch (Chromodoris annae) on Sea Squirt (Polycarpa aurata) Lembeh Strait, North Sulawesi, Indonesia.

Franco Banfi / Nudibranch (Cratena peregrina) on a sponge, Ist Island, Croatia, Adriatic Sea, Mediterranean.

Franco Banfi / Nudibranch (Janolus cristatus) Vela Luka, Korcula Island, Croatia, Adriatic Sea, Mediterranean

Wild Wonders of Europe / Lundgren / Gibraltar nudibranch (Tambja ceutae) Pico, Azores, Portugal, July 2009

Magnus Lundgren / Wild Wonders of China / Dorid nudibranch (Chromodoris strigata) feedings on sponges. Kenting National Park, Hengchun Peninsula, Taiwan

Doug Perrine / Pelagic nudibranch / Blue sea slug (Glaucus atlanticus) along with a much smaller relative, (Glaucilla sp) feeding on a Portuguese man o' war, (Physalia utriculus) Kona, Hawaii, Hawaiian Islands,...

Franco Banfi / Naked sea butterfly sea slug (Clione limacina) swimming in front of iceberg. Tasiilaq, East Greenland. April.

Magnus Lundgren / Wild Wonders of China / Dorid nudibranch (Hypselodoris festiva) Sai Kung, Hong Kong, China.

Georgette Douwma / Swollen phyllidia (Phyllidia varicosa) Puerto Galera, Philippines.

Linda Pitkin / Nudibranch (Chromodoris willani), Triton Bay, near Kaimana, West Papua, Indonesia

Doug Perrine / Blue dragon / Blue seaslug (Glaucus atlanticus) feeding on a Portuguese man o' war (Physalia utriculus) along with a much smaller relative, Glaucilla sp. (upside down); Glaucus and Glaucilla store th...

Tony Wu / Blue sea slug (Glaucus atlanticus) approaching the venom-filled tentacles of a Indo-Pacific Portuguese man-of-war (Physalia utriculus). Both were washed ashore as part of a mass, multi-day stranding o...

Tony Wu / Blue sea slug (Glaucus atlanticus) that was washed ashore with a mass, multi-day stranding of thousands of Indo-Pacific Portuguese man-of-war (Physalia utriculus). This species predates Portuguese man...

Georgette Douwma / Nudibranch (Phyllidia marindica), Lembeh Strait, North Sulawesi, Indonesia. December.

Georgette Douwma / Two Nudibranchs (Chromodoris willani), West Papua, Indonesia. March.

Alex Mustard / Nudibranchs (Nembrotha chamberlaini) feeding on Tunicates (sea squirt) on a coral reef. Anilao, Batangas, Luzon, Philippines. Verde Island Passages, Tropical West Pacific Ocean..

Georgette Douwma / Nudibranch (Hypselodoris bullockii) with a ribbon of eggs. Mabul, Malaysia.

Georgette Douwma / Nudibranch (Chromodoris willani) Mabul, Malaysia.

Pete Oxford / Sky blue Phyllidia Dorid nudibranch (Phyllidia coelestis) Coral reef, Fiji, South Pacific.

Alex Mustard / Portrait of Nudibranch (Hypselodoris infucata) moving over the seabed. Seraya, Bali, Indonesia. Java Sea.

Alex Mustard / Hooded nudibranchs (Melibe leonina) filter feeding beneath bull kelp. Hussar Point, Browning Pass, Port Hardy, Vancouver Island, British Columbia, Canada.

Alex Mustard / A colourful nudibranch (Tambja verconis) crawls over the seabed. Blairgowrie, Mornington Penisular, Victoria, Australia. Port Philip Bay, Bass Strait.

Franco Banfi / Sap sucking seaslug (Plakobranchus ocellatus) Blue water mangrove, Raja Ampat, Irian Jaya, West Papua, Indonesia, Pacific Ocean

Duncan Usher / Turbinate Monodont (Monodonta turbinata) shells collected from North Sea seen in ultraviolet light, Spain. Sequence 2 of 2

Jeff Rotman / Blue Candy Nudibranch (Phyllidia arabica varicosa) Red Sea, Egypt

Sue Daly / Nudibranch (Phyllidiopsis shireenae) Lembeh Straits, Sulawesi, Indonesia

Sue Daly / Nudibranch (Chromodoris willani) Lembeh Straits, Sulawesi, Indonesia

Wild Wonders of Europe / Lundgren / Nudibranch (Tambja ceutae) with a Skeleton shrimp (Caprellidae) riding on its back, Pico & Faial, Azores, Portugal, July 2009

Wild Wonders of Europe / Lundgren / Nudibranch (Dendronotus frondosus) on Sea-mat / Lacy crust bryozoan (Membranipora membranacea)Saltstraumen, Bodö, Norway, October 2008

Wild Wonders of Europe / Pitkin / A violet Nudibranch / Sea slug (Flabellina affinis) feeding on hydroids, 'Turtle Rock', Passage du Cavallo, Lavezzi Archipelago, Corsica, France, September 2008

Wild Wonders of Europe / Pitkin / A violet Nudibranch / Sea slug (Flabellina affinis) feeding on hydroids, 'Turtle Rock', Passage du Cavallo, Lavezzi Archipelago, Corsica, France, September 2008

Wild Wonders of Europe / Pitkin / A violet Nudibranch / Sea slug (Flabellina affinis) feeding on hydroids, 'Turtle Rock', Passage du Cavallo, Lavezzi Archipelago, Corsica, France, September 2008

Juergen Freund / A pair of nudibranchs {Cromodoris annae} Indo-pacific

Hiroya Minakuchi / Magnificent Chromodorid Nudibranch (Chromodoris magnifica), Celebes Sea

Hiroya Minakuchi / Ann's Chromodorid Nudibranch (Chromodoris annae), Celebes Sea

Hiroya Minakuchi / Nudibranch (Chromodoris sp), Celebes Sea

GEORGETTE DOUWMA / Nudibranch (Hypselodoris infucata) Bali, Indonesia.

GEORGETTE DOUWMA / Nudibranch {Tambja morosa} Komodo, Indonesia.

Kerstin Hinze / Sea Slug (Clione limacina) Hokkaido, Japan February 2007

Doug Perrine / Pair of Blue striped nudibranch {Tambja mullineri} Galapagos

Doug Perrine / Lettuce backed nudibranch {Tridachia crispata}, Dominica, Caribbean

Constantinos Petrinos / Nudibranch {Chromodoris lochi} Sulawesi, Indonesia

Constantinos Petrinos / Nudibranch (Phyllidiopsis annae) Sulawesi, Indonesia

Doug Perrine / Blue dragon nudibranch {Pteraeolidia ianthina} Gato Island, Cebu, Philippines

GEORGETTE DOUWMA / Nudibranch {Hypselodoris bullockii} previously known as {Chromodoris bullockii} Mabul, Malaysia, Indo-Pacific. Very variable in colour.

Fred Bavendam / Limpet (Thyca crystallina) living on coral surface and the Blue Sea Star (Linckia laevigata), Manado, North Sulawesi, Indonesia

Constantinos Petrinos / Dorid nudibranch {Tambja morosa} Secret Bay, Bali Indonesia

Roberto Rinaldi / A nudibranch / sea slug (Chromodoris vibrata) on the coral reef of Sipadan, Borneo

Luis Quinta / Blue sea slug (Glaucus atlanticus) juvenile, portrait, Portugal, Atlantic Ocean.

Luis Quinta / Blue sea slug (Glaucus atlanticus) juvenile, feeding on Blue button hydroid colony (Porpita porpita), Portugal, Atlantic Ocean.

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.

Joel Sartore / Photo Ark / Regal goddess nudibranch (Felimare picta) portrait, Pure Aquariums, Nebraska. Captive, occurs in Atlantic Ocean.

Joel Sartore / Photo Ark / Regal goddess nudibranch (Felimare picta) portrait, Pure Aquariums, Nebraska. Captive, occurs in Atlantic Ocean.

Tony Wu / Purple-lined nembrotha nudibranch (Nembrotha purpureolineata) portrait. East China Sea, Kagoshima Prefecture, Japan.

Lucas Bustamante / Pacific spotted scorpionfish / Stone scorpionfish (Scorpaena mystes) with a Galapagos sea slug (Tambja mullineri) resting on its head, North Seymour Island, Galapagos National Park, Pacific Ocean.

Lucas Bustamante / Panamic cushion star / Cortez starfish (Pentaceraster cumingi) two arms reaching over rock, with two Galapagos sea slugs (Tambja mullineri) close by, North Seymour Island, Galapagos National Park, Pac...

Georgette Douwma / Composite image of marine invertebrates on black background. Top row from left: Tube anemone (Ceriantharia), Yellow sea cucumber (Colochirus robustus), Nudibranch (Nembrotha lineolata), Squat shri...

Alex Mustard / Nudibranch (Phyllidia varicosa) portrait on white background, Bitung, North Sulawesi, Indonesia.

Magnus Lundgren / Loch's chromodoris (Chromodoris lochi) nudibranch, close up, Raja Ampat, West Papua, Indonesia, Pacific Ocean.

Alex Mustard / Portrait of a Nudibranch (Nembrotha milleri) on a coral reef. Dauin, Dauin Marine Protected Area, Dumaguete, Negros, Philippines. Bohol Sea, tropical west Pacific Ocean.

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) feeding on blue button (Porpita porpita), Tenerife, Canary Islands.

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) feeding on Blue button hydroid colony (Porpita porpita) Tenerife, Canary Islands. October

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) swimming,Tenerife, Canary Islands. September

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) on white background,Tenerife, Canary Islands. September

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) with Blue button hydroid colony (Porpita porpita) on which it feeds. Tenerife, Canary Islands. October

Sergio Hanquet / Blue dragon seaslug (Glaucus atlanticus) with blue button hydroid colony (Porpita porpita) on which it feeds. Tenerife, Canary Islands. October

David Fleetham / Blue dragon nudibranch (Pteraeolidia ianthina) which has evolved a method of capturing and farming microscopic plants (zooxanthellae) in its body. Hawaii, USA