Sea Slugs

Nudibranchs, sea hares, headshield slugs, and their kin are among the ocean's most visually striking invertebrates. These soft-bodied molluscs range from cryptic camouflage to brilliant warning coloration, and many species are known from only a handful of records.

21 photos · These species on iNaturalist

Wavy Bubble ShellMicromelo scriptusHaleʻiwa, Oʻahu, Hawaiʻi, USA · 2018Coral reef Hawaiʻi
Tiger Butterfly Sea SlugCyerce nigraNorth Minahasa, Sulawesi Utara, Indonesia · 2024Coral reef Coral Triangle
Scott Johnson's NudibranchArdeadoris scottjohnsoniPūpūkea, Oʻahu, Hawaiʻi, USA · 2016Coral reef Hawaiʻi
Orangemargin NudibranchArdeadoris sp. 2Haleʻiwa, Oʻahu, Hawaiʻi, USA · 2017Coral reef Hawaiʻi
Diana's ChromodorisChromodoris dianaeCagayancillo, Palawan, Philippines · 2023Coral reef Coral Triangle
Willan's ChromodorisChromodoris willaniCagayancillo, Palawan, Philippines · 2023Coral reef Coral Triangle
Glossodoris cf. cinctaLembeh Strait, Sulawesi Utara, Indonesia · 2013Sand and muck slopes Coral Triangle
White-spotted NudibranchGoniobranchus albopunctatusPūpūkea, Oʻahu, Hawaiʻi, USA · 2019Coral reef Hawaiʻi
Gem NudibranchGoniobranchus geminusKuda Giri wreck, Maldives · 2021Coral reef Indian Ocean
Goniobranchus kunieiUchelbeluu Reef, Koror, Palau · 2017Coral reef Pacific Islands
Midway NudibranchGoniobranchus sp. #5Pūpūkea, Oʻahu, Hawaiʻi, USA · 2016Coral reef Hawaiʻi
Bertsch's NudibranchHypselodoris bertschiWaikiki, Honolulu, Hawaiʻi, USA · 2022Coral reef Hawaiʻi
Jolly Green GiantMiamira sinuataOʻahu, Hawaiʻi, USA · 2018Coral reef Hawaiʻi
Variable NoumeaVerconia variansPūpūkea, Oʻahu, Hawaiʻi, USA · 2015Coral reef Hawaiʻi
Mushroom Coral ReticulidaReticulidia fungiaNgercheu Island, Peleliu, Palau · 2017Coral reef Pacific Islands
Dusky NembrothaNembrotha kubaryanaOlango Island, Cebu, Philippines · 2023Coral reef Coral Triangle
Gloomy NudibranchTambja morosaWaikiki, Honolulu, Hawaiʻi, USA · 2022Coral reef Hawaiʻi
Indian CaloriaCaloria indicaWaikiki, Honolulu, Hawaiʻi, USA · 2022Coral reef Hawaiʻi
Phyllodesmium sp.Raja Ampat, Papua Barat, Indonesia · 2019Coral reef Coral Triangle
Porites-eating NudibranchPhestilla sibogaeRa, Fiji · 2025Coral reef Pacific Islands
Melibe colemaniNorth Minahasa, Sulawesi Utara, Indonesia · 2024Coral reef Coral Triangle

Field marks

Flatworm or nudibranch?

Polyclad flatworms share the same reefs and often wear the same bright colors as nudibranchs. They are not related. Look at the head first, then the back.

Feature	Nudibranch	Flatworm
Head	Two rhinophores: separate stalks, often ringed or folded, that many species can pull into a sheath.	Two pseudotentacles: pinched folds of the front edge, not separate stalks. Clusters of tiny eyespots sit between them.
Back	A gill plume near the rear on most dorids. Aeolids carry finger-like cerata instead.	No gills and no cerata. The animal breathes through its whole body surface.
Body	Fleshy and thick, with a muscular foot underneath.	Paper-thin, often only a few millimetres deep. The edge is usually ruffled.
Movement	A slow crawl on the foot.	A fast, smooth glide. Many species also swim by rippling the body edge.
Group	Mollusca: related to snails and octopus.	Platyhelminthes: no close relation to any slug.

Quick test in the water: if you see stalks on the head or a plume on the back, it is a nudibranch. If the animal is flat as a leaf with a wavy edge, it is a flatworm.

Some flatworms copy toxic nudibranchs on purpose. Pseudoceros imitatus is named for its resemblance to the warty phyllidiid slugs. The head still gives it away.