Dive Dry with Dr. Bill

#290: The Rainbow Dendronotus

I've often written that the reason I continue to dive after nearly 50 years off-and-on underwater is that there's always something new to see... and film. I was out on SCUBA Luv's King Neptune for a few Father's Day dives, and had a spectacular example of this. It reminded me of a dive last summer when I saw my very first thornback ray here in Catalina waters. Although they are reasonably common on the mainland, I had never seen one off the island before.

This weekend my sighting was yet another "first" in Catalina waters, at least for me. For years I had read accounts, and seen pictures taken by friends who dive mainland waters showing the beautiful rainbow Dendronotus nudibranch feeding on tube dwelling anemones known as cerianthids. In all my years I had never seen one here in island waters, and simply assumed there was some barrier to their dispersal to Catalina. Perhaps they did not have planktonic larvae that could drift over with the currents. Then why didn't they hitch a ride on drifting kelp like so many other nudibranchs, or shell-less snails, do? There are plenty of the colorful cerianthid tube anemones in our waters to feed on. Perhaps they couldn't survive a kelp raft trip because their food would be unavailable on the raft during their transit.

We were at Twin Rocks near Goat Harbor for our first dive. I usually go deep there, but stopped at a "shallow" (80 ft) rocky pinnacle to film a few other critters before descending to a mere 120 ft. As I worked my way back towards the shallows, I saw something at 112 ft that caught my eye. It was the tube of one of the cerianthid anemones, but there were brightly colored "hairs" projecting from its sides. As I moved closer and around the tube, I realized I was looking at the first Dendronotus iris or rainbow Dendronotus I had ever seen in our waters!

It was climbing up the anemone's tube, but the wise anemone had already withdrawn deep into it. Often these beautiful snails will surprise the anemone and begin biting pieces off their extended tentacles. Perhaps this nudibranch was a bit too noisy, because it would have to crawl to the opening at the top of the tube and then down into it to munch. This is not unusual feeding behavior for these snails as they will frequently climb down, or get sucked into the tubes to feed at their heart's content. Fortunately their appetites are not as big as mine, and they rarely kill the anemone so it survives to feed (and reproduce) another day.

The coloration on these nudibranchs is spectacular, even for a color-blind diver like myself. I've heard it referred to as iris, but in my mind that's my housemate... or the thingie at the center of my eye. My trusty Funk & Wagnalls didn't define the word "iris" in terms of a color. One major field guide refers to their coloration as highly variable ranging from white to gray to orange and red. The one I observed was definitely at the more colorful end of that spectrum! The only thing that seems consistent is a white line along the margin of the foot. Their average size is reported as 2-4" although they may reach 8" in length. One source even suggested a maximum length of an astounding foot (as in 12")! The one I saw was about 4" but several of my dive friends on ScubaBoard report seeing them near their maximum size.

Earlier field guides had their distribution listed as Unalaska Island, Alaska, to the Coronados Islands off northern Baja. This would suggest that they are a cold water species. However, the latest edition of that same guide states that they are now known all the way south to Cabo San Lucas. If I had to guess, I'd say that their extended range was the result of surveying deeper in the water column. I am told that they are found in shallow water up in the Pacific Northwest, and at the 40-50 ft range off San Diego which tends to have colder water than Catalina. The water temperature was a "balmy" 52 F at the depth I found this one, so in the warmer southern part of their range they may submerge to deeper, colder depths. They prefer soft bottom habitats such as sand or mud, which would be the preferred habitat of their prey as well. However, they are also excellent mid-water swimmers... although I doubt they could match Capt. Bob's Olympic class butterfly speed (neither could I)!

The jaws of this nudibranch are reportedly much more substantial than those of other closely related nudibranchs. Of course its preferred prey is much larger than the usual bryozoan or hydroid fare of most sea slugs. These sea slugs will also eat hydroids and jellyfish. If you look at the image below of an approaching rainbow Dendronotus, taken by Phil Garner, you can see that they must appear very frightening to their prey... at least if their prey could actually see them coming! Given my vivid imagination, they look like something directly out of a Star Wars bar scene. Since their sight is almost as bad as mine, they locate their prey uses chemical senses. In turn, they are eaten by starfish such as the sunflower star.

These nudies lay their eggs directly on the tubes of the cerianthid anemones. Each egg capsule contains about 50 eggs. I find it curious that they would lay their eggs on their prey, when the eggs hatch into veliger larvae 10-20 days later. The larvae would receive no benefit from the close proximity of a meal unless they hang around as they develop, rather than drifting off in the currents as temporary plankton. Perhaps this is the reason they don't disperse across or long (and deep) San Pedro Channel. So how did the one I filmed get here? Another "mystery of the deep."

© 2008 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass," "Common Fish and Invertebrates of the Sea of Cortez" or the new "Sharks and Rays of Southern California" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

The nudibranch's favorite prey, the cerianthid tube anemone, Dendronotus iris flying through the water column;
Phil Garner's excellent shot of one approaching and one climbing up the anemone's tube.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia