2000-03 Solar-Powered Nudibranch

By Denise Nielsen Tackett and Larry Tackett

Nudibranchs are among the most beautiful animals in the sea. Like mobile rainbows, they bring unexpected bits of color to the reef, delighting both novice and experienced divers. Their bright colors generally warn of their toxicity but many, like the Solar-Powered Nudibranch (Phyllodesmium longicirra), are harmless. They are colored to blend with their surroundings so they are less visible to would-be predators. This species is an aeolid nudibranch, meaning it has many vertical projections, called cerata, on its body. Because their cerata wave like streamers in the wind, aeolids take their name from the Greek god of the winds, Aeolus.

Many aeolids feed on hydroids and corals, and store the undischarged stinging cells from their prey in special sacs in their cerata. When confronted with a predator, they can release the stored stinging cells for defense. The Solar-Powered Nudibranch goes a step further. It feeds on leather corals (genus Sarcophyton) and stores, instead, the symbiotic zooxanthellae (algae) from the coral in its cerata. The zooxanthellae continue to live and produce nutrients that benefit the nudibranch, just as they did for the coral. If you look closely at the cerata, you can see the zooxanthellae. They are small gold specks arranged like mini solar panels poised to collect sunlight and convert it into energy via photosynthesis. This process led to the nudibranch’s common name, Solar-Powered Nudibranch.

The cerata are also useful for respiration (aeolids have no gills) and defense. Although it is incredibly well-camouflaged, the Solar-Powered Nudibranch is occasionally confronted with a predator. When alarmed it casts off several cerata, which begin to wiggle and distract the predator while the nudibranch slips safely away. The nudibranch will regrow the discarded cerata, just as sea stars regrow missing arms.

The Solar-Powered Nudibranch can reach six inches in length and its cerata can be two inches tall. Look closely for it in reef and rubble areas of the Indo-Pacific, where it is often seen feeding on leather corals.