Nudis: The Naked Truth

Sea Clown Nudibranch (Triopha catalinae)
Sea Clown Nudibranch (Triopha catalinae), photo curtesy of Todd Huspeni during our field course to the Pacific Northwest in 2013

Dawn slowly emerges over the coastal horizon, spilling rays of light over the rocks and tidepools dotting the ocean shoreline.  The world slowly wakes up with the occasional calls of sandpipers and gulls growing more frequent.  The rolling crash of the low tide waves against the rocky shore creates a rain of mist forming rainbows in the beams of dawn.  Even the occasional sound of sea lions arguing for space from the large rock down the shoreline drifts through the morning.  In a tidepool, a ray of dawn spills over a soft vibrant body, slowly twisting and moving rhythmically in the brisk water.  The body of a nudibranch!  Wait, what kind of body did you think I was talking about?

Nanaimo Dorid Nudibranch (Acanthodoris nanaimoensis)
Nanaimo Dorid Nudibranch (Acanthodoris nanaimoensis), photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013
Opalescent Nudibranch (Hermissenda opalescens)
Opalescent Nudibranch (Hermissenda opalescens) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

What is a nudibranch?

White lined Nudibranch (Dirona albolineata)
White lined Nudibranch (Dirona albolineata) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

Yes, some people do refer to multiple nudibranchs as nudis, I didn’t make that up for the title.  Unless you are a beachcomber, diver, or marine biologist, there is a good chance you never heard of these gorgeous creatures.  Nudibranchs are mollusks, this is a phylum of animals that also includes snails/slugs, clams, oysters, abalone, chitons, limpets, and octopi.  Quite the group, but they do all have something in common: a soft body surrounding their organs (1).  

Many mollusks, including larval but not adult nudibranchs, have a shell around part of their body.  Even though larval nudibranchs can have a shell, their name derives from the lack of a shell as an adult.  Nudibranch literally means “naked gill” (nudi = naked and branch = gill) (1,2).  What nudibranchs lack from a shell, they make up with other defense mechanisms to protect them.  Some nudibranchs have stinging cells, some are toxic, some are camouflaged, and some are bright vibrant colors that warn predators to back off (1,2,3). 

Colorful Dirona Nudibranch (Dirona picta)
Colorful Dirona Nudibranch (Dirona picta) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013
Yellow Margin Dorid Nudibranch (Cadlina luteomarginata)
Yellow Margin Dorid Nudibranch (Cadlina luteomarginata) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

With their soft bodies and lack of a shell, nudibranchs are also commonly called “sea slugs”.  Although, a lot of people will call anything living in the ocean with a slimy soft body a sea slug, so not all sea slugs are nudibranchs.  

There are over 3,000 known species of nudibranchs in all of the world’s oceans (2).  They are so diverse in color and size so it’s hard to describe what exactly a nudibranch looks like.  Most nudibranchs do not grow longer than 4 cm (1.5 inches) although the largest known species (the Spanish Dancer: Hexabranchus sanguineus) can get up to 60 cm (almost 2 feet) long (2,4)!  I have never personally seen a nudibranch that big.  Most nudibranchs, despite their size, will only live for about a year though.

Stubby Frond-Aeolis Nudibranch (Dendronotus subramosus)
Stubby Frond-Aeolis Nudibranch (Dendronotus subramosus) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

Sweet Nudi Facts

I mentioned above how some nudibranchs have stinging cells as a form of defense.  What’s even cooler is that some nudibranchs have this power because they steal it from jellyfish or anemones after the nudibranch eats them.  Kind of like adding insult to injury, not only does the nudibranch eat them but it also steals their superpower for itself (3).

White-and-Orange-Tipped Nudibranch (Janolus fuscus)
White-and-Orange-Tipped Nudibranch (Janolus fuscus) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

Nudibranchs are hermaphrodites.  This means that they have both female and male genitals so if they come across any other nudibranch that is the same species as them, they can reproduce with each other.  They can not reproduce with themselves though, it still takes two to tango for these fellas (4).

Sea Lemon Nudibranch (Dorididae)
Sea Lemon Nudibranch (Dorididae) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013
Diamondback Tritonia Nudibranch (Tritonia festiva)
Diamondback Tritonia Nudibranch (Tritonia festiva) photo curtesy of T. Huspeni during our field course to the Pacific Northwest in 2013

Citations

  1. Sept, J.D. (2009). Molluscs and Brachiopods: Phyla Mollusca & Brachiopoda. The Beachcomber’s Guide to Seashore Like in the Pacific Northwest: revised edition, pp 53-102.
  2. “A Collage of Nudibranch Colors” (On-line) Smithsonian: Ocean: Find Your Blue. Accessed 2021 at https://ocean.si.edu/ocean-life/invertebrates/collage-nudibranch-colors
  3. Osterloff, E. “Nudibranchs: psychedelic thieves of the sea” (On-line) Natural History Museum. Accessed 2021 at https://www.nhm.ac.uk/discover/nudibranchs-psychedelic-thieves-of-the-sea.html
  4. “Cephalopods, Crustaceans, & Other Shellfish: Spanish Dancer Hexabranchus sanguineus” (On-line) Oceana: Protecting the World’s Oceans. Accessed 2021 at https://oceana.org/marine-life/cephalopods-crustaceans-other-shellfish/spanish-dancer
  5. For more information check out the blog on this website which is working to promote nudibranch knowledge for everyone: https://ilovenudis.com/

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