Blue Dragon Nudibranch

As I was diving the Sea Tiger Shipwreck off the South shore of Oahu, I noticed an exotic blue-colored-sea-slug-looking organism that I have never come across before. I was immediately mesmerized and stared at it for a long time. It had bright blue rings around its tubular body and, due to its extremely fluorescent color, really stuck out on the side of the ship. Later when I went home I looked it up and found that it was a Blue Dragon Nudibranch with the scientific name Pteraeolidia ianthina.


Pteraeolidia ianthina are normally found in waters anywhere from 10 to 20 meters deep on rocky areas with drop-offs. There are two things that make this nudibranch very unique. The first is that P. ianthina houses zooxanthellae in their tissues. Zooxanthellae are microscopic algae that conduct photosynthesis. Photosynthesis is the process by which sunlight is converted into energy, such as sugar, that the body can use. Zooxanthellae share some of this energy with P. ianthina and in return P. ianthina provide a safe habitat for which the zooxanthellae can live. This is called a symbiotic relationship because both organisms are benefitting from one another. Depending on the number of zooxanthellae present on each P. ianthina, P. ianthina body color can range from dark blue, to lavender, to golden brown.

The second unique characteristic of P. ianthina is that it can sting. Pteraeolidia ianthina tend to prey on hydroids, which are very small organisms that are composed of stinging cells called nematocysts. When P. ianthina digest these hydroids, they retain some of their nematocysts in those bright colored rings around its body. In this way, P. ianthina avoid getting eaten. Watch out if you ever come across a P. ianthina because if you touch it you will get stung!

Below is a short YouTube video I found that describes this organism. The narrator sounds kind of funny, but he has good things to say! Enjoy!

-by Rebecca Weible



Hoegh-Guldberg, O. & Hinde, R. (1986). Studies on a Nudibranch that Contains Zooxanthellae I. Photosynthesis, Respiration and the Translocation of Newly Fixed Carbon by Zooxanthellae in Pteraeolidia ianthina. Proceedings of the Royal Society of London. Series B, Biological Sciences, 228(1253), 493-509.

Pittman, C. & Fiene, P. Pteraeolidia ianthina (Angas, 1864). Sea Slugs of Hawai’i. Retrieved from



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