The Blue Dragon, a Solar-Powered Nudibranch

by B. N. Sullivan

This 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.

The 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.