Out on the Hawaiian reef we have many different types of sea slugs and nudibranches, and the varicose nudibranch are the most-commonly seen by divers and snorkelers. These three-inch-long slugs stand out on the brown reef because they are yellow, black and blue colors, and look like a little pile of scrambled eggs on the reef.
These small, slow-moving creatures are highly poisonous, and they do not seem to have any predators. Like most nudibranches they feed on sponges that grow on the reef in dark caves and cracks and the sponges are poisonous so the nudibranch recycles the poison to emit from mucus that grows on the outside of the nudibranch’s skin. Often in the sea, marine life that does not produce poisonous skin will eat poisonous animals to obtain the toxin to use for its own defense.
The varicose phyllidia is a type of nudibranch, and most nudibranches worldwide are extremely colorful. There are groups of divers that travel around the world just looking for nudibranches to take pictures of. In Hawai‘i, we do not have a lot of these slugs because they do better in calm water areas because most nudibranches have soft skin and don’t do well in the surf. The varicose phyllidia is different, as it adapted to deal with surf by becoming hard and rigid. The word “nudibranch” means “exposed gills” or “nude branchia,” but the varicose phyllidia evolved to have their gills tucked under their hard body to protect them.
In Hawai‘i, we often have marine-life species that are very different than species in the rest of the Pacific Ocean because Hawai‘i is a young island chain and has not had time to develop barrier coral reefs.
In Tahiti, Palau and the Philippines you will find huge, calm lagoons that were made over millions of years of coral growth, but in Hawai‘i we do not have many calm lagoons and the surf constantly crashes onto the lava reefs, causing the marine life to adapt to a more-rugged environment. In one calm lagoon in the Philippines, I counted over 200 nudibranches and 50 seahorses in a one-hour dive, but here in Hawai‘i, most divers are happy to find a single nudibranch or seahorse on a dive.
Since the varicose nudibranchs are easy to catch, often aquarium owners pick them off of the reef to put in their saltwater tank because they are so colorful to look at. The only problem is when you disturb one of these toxic sea slugs they shoot out toxins into the saltwater and in a tank that kills all of the fish!
You can see the varicose phyllidia in action on my underwater educational web page at underwater2web.com. I am also making a new movie about nudibranches worldwide called “The Life of a Nudibranch.”
Terry Lilley is a marine biologist living in Hanalei and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawai‘i go to www.reefguardianshawaii.org.