The Blue Dragon, a Solar-Powered Nudibranch

by B. N. Sullivan

Pteraeolidia ianthinaThis is the third article in our series on 'Exotic Underwater Nudies', i.e., nudibranchs. We began the series last month with the Spanish Dancer, exceptional for its large size and unique swimming behavior. Next we presented the lovely Gold Lace nudibranch, a species endemic to Hawaii. Today we'll take a look at a very different kind of nudibranch -- different both in form, and in how it makes its living.

Meet the Blue Dragon (Pteraeolidia ianthina). This is one of a handful of nudibranch species that is, in effect, solar powered. I know what you're thinking: A solar-powered sea slug? How is that possible?

It happens in a round-about way. The Blue Dragon is carnivorous. It eats hydroids -- stinging organisms that are related to corals. The hydroids are colonized by zooxanthellae -- microscopic plants. As plants large and small are known to do, the zooxanthellae convert energy from sunlight into sugars through the process of photosynthesis.

So, the Blue Dragon eats hydroids, which contain zooxanthellae in their tissues. The Blue Dragon is one of several nudibranch species that has evolved a method of extracting the zooxanthellae from the hydroids it eats and storing them in its own body. Once in place in the nudie's tissues, the zooxanthellae continue to produce sugars via photosynthesis, thus nourishing the creature that has now become their new host. In this way, the Blue Dragon qualifies as a solar powered nudibranch.

Zooxanthellae are found in the tissues of many marine animals, including corals. In fact, it is the presence of zooxanthellae that give corals their colors. The same is true for the Blue Dragon nudibranch. The coloration of this species varies, partly due to the concentration of zooxanthellae that a given individual carries in its tissues. Juveniles of this species are a nearly transparent white. Only as they mature and accumulate zooxanthellae in their tissues do they take on their characteristic color.

Pteraeolidia ianthinaThe Blue Dragon stores zooxanthellae in its cerata -- those fluffy looking bits that cover its body. In fact, one hypothesis about the Blue Dragon's form posits that the cerata may have evolved in order to provide more surface area for the zooxanthellae to inhabit. More surface area means more space for zooxanthellae, as well as a greater likelihood that they will be exposed to the light they need for photosynthesis.

Speaking of sunlight, as you might expect, the Blue Dragon nudibranch works the day shift on the reef. It needs to be exposed sunlight so that its zooxanthellae can produce nutrients. Not only is this creature seen most frequently during daylight hours, it also tends to dwell at shallower depths than some other nudibranchs. Naturally, there is more sunlight available at shallower depths than in deeper water.

About the photos on this page: In the photo at the top of the page, you can see how the cerrata are arrayed along the entire length of the Blue Dragon. Zooxanthellae are stored in the cerata. The bottom photo is a 1:1 macro of the head of a Pteraeolidia ianthina. The feathery looking bits protruding upward from its head are its rhinophores, the organs that sense 'smells' in the water. The animal uses its rhinophores to locate food, much like a mammal on land would use its nose. The striped appendages facing forward are oral tentacles, which act like 'feelers' so that the nudie can pinpoint where the food is, by touch.

Each photo shows a different individual. The photo at the top was taken at Siapadan Island, off the coast of Borneo. The bottom photo was taken near Manado, Sulawesi Island, Indonesia.

14 comments:

  1. It's awesome!!!! And that beautiful, blue colour!!!!!!!

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  2. A couple of months back I saw a Nat Geo article about these guys. Such an amazing variety of vibrant colors and shapes!

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  3. Your blog always keeps me in awe!! My usual day to check you out is Wednesday, but I've been missing WW lately.

    Anyway, I greatly enjoy your blog. I have an award for you!! You can pick it up on my blog.

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  4. Wow! I love visiting your blog and learning new things from under the sea. What a great nudibranch! When I first saw the photo I thought you were going to say this was a lei someone must have tossed overboard. How unique and pretty this is!

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  5. Thank you all.

    @ Antigoni - I like the color, too.

    @ ScienceGuy - They are amazing little animals.

    @ ArmyFamily - Thank you for the nice compliments, and also for the award.

    @ 2Sweet - Thanks. Now that you mention it, I can see why you might have thought that first photo was of a lei tossed in the water.

    Bobbie

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  6. Fantastic Bobbie! What an amazing looking animal, and I love your commentary. There's a couple of other critters out there that co-opt photosynthetic organisms for their own benefit.

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  7. This one is so different from the others - and what a color is! Fascinating creatures!

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  8. Kind creepy looking. Reminds me of a stinging caterpillar. Looks like it would hurt if you accidentally brushed against it. Cool color though.

    And again with Indonesia, you are making me homesick!

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  9. You take wonderful underwater pictures. Beautiful.

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  10. @ Chris (a free man) - Thank you. Yes, you are right, there are more 'solar powered' critters out there than most people realize.

    @ Kathy (kml) - They really are fascinating.

    @ Shannon - They're not dangerous to humans - 'cept maybe if you took a bite of one. You should learn to dive whether you get a chance to return to Indonesia or not!

    @ Connie - Thank you very much.

    Bobbie

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  11. Amazing and beautiful, as usual! Question, though: which of those bits are its nudie bronchi, you know, its exposed lungs?

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  12. Hi Lavender - I think you mean gills (not lungs)? This nudibranch belongs to a suborder called the Aeolids. One characteristic of this suborder is that they do not have gills in the form of a branchial plume like, say, the Spanish Dancer. Instead they "breathe" (sort of) through their cerata. My understanding is that blood flows very close to the surface of the cerata, where gas exchange is accomplished. Hope this is clear. If not, let me know and I'll try again, or look for a reference to point you to.

    Bobbie

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  13. Umm... yes, I meant gills! And yes, your reply makes sense to me. I have enough grasp of the concept of gas exchange to picture what you mean. Thanks!

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  14. Incredible creatures and images Bobbie, thanks for sharing your info and photographic abilities with us !!

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Bobbie & Jerry

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