Jeff Rotman / Symbiotic emperor shrimp (Periclimenes imperator / Zenopontonia rex) living in the gills of a spanish nudibranch (Hexabranchus sanguineus) Red Sea, Egypt

Nigel Cattlin / Spanish slug (Arion vulgaris) with closed pneumostome on grass, light grey colour variation. Berkshire, England, UK. August.

Nigel Cattlin / Spanish slug (Arion vulgaris) on grass, tan colour variation. Berkshire, England, UK. August.

Magnus Lundgren / Wild Wonders of China / Free swimming juvenile spanish dancer (Hexabranchus sanguineus) in the night, Kenting National Park, Hengchun Peninsula, Taiwan

Magnus Lundgren / Wild Wonders of China / Free swimming juvenile spanish dancer (Hexabranchus sanguineus) in the night, Kenting National Park, Hengchun Peninsula, Taiwan

Robert Thompson / Spanish slug (Arion vulgaris) in garden, Banbridge, Co. Down, Northern Ireland. August

Robert Thompson / Spanish slug (Arion vulgaris) in garden, Banbridge, Co. Down, Northern Ireland. August

David Fleetham / Imperial commensal shrimp (Periclimenes imperator / Zenopontonia rex) on Spanish dancer nudibranch (Hexabranchus sanguineus) at night. Philippines.

Claudio Contreras / Spanish shawl nudibranch (Flabellina iodinea), Salsipuedes Island Protected Area, Gulf of California (Sea of Cortez), Mexico, July

Tony Wu / Spanish dancer nudibranch (Hexabranchus sanguineus) scavenging the reef at night in Ambon, Indonesia

Alex Hyde / Spanish Slug (Arion vulgaris), Derbyshire, UK. August. Invasive species.

Pascal Kobeh / Diver in front of two mating Spanish dancers (Hexabranchus sanguineus) Madagascar. Indian Ocean. September 2012.

Alex Mustard / Spanish dancer nudibranch (Hexabranchus sanguineus) moves across coral reef in daylight. It is unusually to see this large nudibranch during daylight. Nuweiba, Sinai, Egypt. Gulf of Aqaba, Red Sea.

Nick Upton / Iberian slug (Arion vulgaris) an invasive species in northern Europe, crossing forest path after rain, near Stralsund, Mecklenberg, Germany

Fred Bavendam / Spanish Dancer (Hexabranchus sanguineus) nudibranch egg ribbon, Kimbe Bay, Papua New Guinea

Fred Bavendam / Spanish Dancer (Hexabranchus sanguineus) nudibranch pair mating, Hawaii

Chris Newbert / Shrimp (Periclimenes sp) living among the gills of the Spanish Dancer (Hexabranchus sanguineus) nudibranch, 50 feet deep, Papua New Guinea

Chris Newbert / Shrimp (Periclimenes sp) living among the gills of the Spanish Dancer (Hexabranchus sanguineus) nudibranch, 50 feet deep, Papua New Guinea

Norbert Wu / Spanish Dancer (Hexabranchus sanguineus) nudibranch, Red Sea

Constantinos Petrinos / Spanish dancer {Hexabranchus sanguineus} Sulawesi Indonesia

Roberto Rinaldi / Emperor shrimp (Periclimenes imperator / Zenopontonia rex) hiding among the feathery gills of a spanish dancer nudibranch / sea slug (Hexabranchus sanguineus), La Runion, Indian Ocean

Nature Production / Eggs of nudibranch Spanish Dancer {Hexabranchus lacera} laid on coral reef, Red Sea, Egypt

Birgitte Wilms / Emperor Shrimp (Periclimenes imperator / Zenopontonia rex) on Spanish Dancer (Hexabranchus sanguineus) nudibranch, Red Sea, Egypt

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.

Roman Willi / Spanish slug (Arion vulgaris) feeding on a flower, Lucerne, Switzerland. September.

Roman Willi / Spanish slug (Arion vulgaris) feeding on a petal, Lucerne, Switzerland. September.

David Fleetham / Spanish dancer nudibranch (Hexabranchus sanguineus) on coral reef at night, Hawaii, Pacific Ocean.

David Fleetham / Spanish dancer nudibranch (Hexabranchus sanguineus) on a reef at night, portrait, Indonesia, Indo-Pacific Ocean.

David Fleetham / The red eggmass of a Spanish dancer nudibranch, (Hexabranchus sanguineus), Hawaii, Pacific Ocean.

Magnus Lundgren / Wild Wonders of China / Free-swiiming juvenile Spanish dancer, (Hexabranchus sanguineus) Green Island, Taiwan.

Alex Mustard / Portrait of a Spanish shawl nudibranch (Flabellinopsis iodinea). Palos Verdes, Los Angeles, California, United States of America. North East Pacific Ocean.