Alex Mustard / Sacoglossan sea slug (Costasiella sp.) tiny <5mm along, on algae. Anilao, Batangas marine protected area, Luzon, Philippines. Verde Island Passages, Tropical West Pacific Ocean.

Magnus Lundgren / Wild Wonders of China / Sacoglossan sea slug (Cyerce nigricans) Xiaoliuqiu Island, Taiwan

Magnus Lundgren / Wild Wonders of China / Sacoglossan sea slug (Cyerce nigricans) Xiaoliuqiu Island, Taiwan

Pascal Kobeh / Lined Thuridilla / Orange-edged sapsucking nudibranch / sea slug (Thuridilla lineolata), Sulu sea, Philippines

Linda Pitkin / Nudibranch (Thuridilla lineolata) Sangeang, off Sumbawa, Indonesia.

Georgette Douwma / Sea slug (Plakobranchus ocellatus) on sand. Raja Ampat, West Papua, Indonesia.

Alex Mustard / Tiny (7mm) herbivorous sap-sucking seaslug (Costasiella kuroshimae) crawling over piece of algae next to its circular egg ribbon. Seraya, Bali, Indonesia. Java Sea

Alex Mustard / Details of ruffled mantle extensions on the Sacoglossan sea slug (Tridachia crispata) These also contain the chloroplasts of their algae food, that continue to photosynthesise inside the mollusc and p...

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

Juergen Freund / Seaslugs (Plakobranchus ocellatus) Raja Ampat, West Papua, Indonesia

Christian Ziegler / Lettuce Sea Slug (Elysia crispata), Bastimentos Marine National Park, Bocas del Toro, Panama

Pete Oxford / Lettuce Sea Slug (Elysia crispata), Bonaire, Netherlands Antilles, Caribbean

Juergen Freund / Sacoglossan sea slug / Sap sucker (Elysia sp) a herbivore that feeds, mates and lays eggs on one type of host algae, Batangas, Philippines

Chris Newbert / Sacoglossan (Cyerce nigricans) 70 feet deep, Papua New Guinea

Ingo Arndt / Sea Slug (Echinospira sp) larva swimming, Weddell Sea, Antarctica

Ingo Arndt / Sea Slug (Echinospira sp) larva swimming, Weddell Sea, Antarctica

Chris Newbert / Sea Slug (Elysiidae) portrait, 50 feet deep, Solomon Islands

Alex Mustard / Seaslug (Elysia sp.) portrait, Raja Ampat, West Papua, Pacific Ocean.

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

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.

David Fleetham / Ornate sapsucking slug / Ornate elysia (Elysia ornata) on a reef, Yap, Federated States of Micronesia, Pacific Ocean.

Franco Banfi / Nudibranch (Thuridilla hopei), Punta Campanella Marine Protected Area, Amalfi Coast, Italy, Tyrrhenian Sea, Mediterranean.

Franco Banfi / Nudibranch, (Thuridilla hopei), Ponza island, Italy, Tyrrhenian Sea, Mediterranean

Francisco Marquez / Mexican dancer nudibranch (Elysia diomedea) on white background, Islas Marias Archipelago, Marias Biosphere Reserve, Mexico.

Francisco Marquez / Mexican dancer nudibranch (Elysia diomedea) on white background, Islas Marias Archipelago, Mexico.

Francisco Marquez / Mexican dancer nudibranch (Elysia diomedea) on white background, Islas Marias Archipelago, Marias Biosphere Reserve, Mexico.

Francisco Marquez / Mexican dancer nudibranch (Elysia diomedea) on white background, Islas Marias Archipelago, Marias Biosphere Reserve, Mexico.

Pascal Kobeh / Lettuce seaslug (Elysia crispata). Bahamas.

Alex Mustard / Lettuce leaf sea slug (Elysia crispata) on a coral reef. Jardines de la Reina, Gardens of the Queen National Park, Cuba. Caribbean Sea.

Sergio Hanquet / Sea snail (Oxynoe olivacea) on seaweed (Caulerpa prolifera) Tenerife, Canary Islands.

Sergio Hanquet / Sea snail (Oxynoe benchijigua) on seaweed (Caulerpa prolifera) Tenerife, Canary Islands.

Frederique Olivier / Nudibranch (Chromodoris annae), with a Common reef octopus (Octopus cyanea) camouflaged amongst the coral, Uepi Island, Solomon Islands.

Shane Gross / Cose up of pattern detail on the back of a Lettuce Sea Slug (Elysia crispata) The Bahamas.