For you parents who scan my articles to make sure they are appropriate for young children to read (they're usually not!), you may want to exercise caution since this week's topic is... nudies! No, not the lovely mermaids I hope to encounter "in the deep" (probably only when I'm seriously narc'ed). I'm talking about one of my favorite groups of critters... the shell-less snails known as nudibranchs. Now human nudies can also be quite appealing if photographed tastefully, but most of these nudies are appealing regardless of the talent (or lack thereof) of the photographer. I think my images of this week's subject, the Spanish shawl (Flabellina iodinea), prove that point!
Catalina is generally not a location to dive if you are looking for these beautiful snails. They are more common on the mainland and in the northern Channel Islands where the water is colder and nutrients are more abundant. Why? Because these nutrients increase plankton populations, and many of the critters nudibranchs eat feed on those small invertebrates and other drifters. Where there is ample food, there can be ample carnivores... like myself. Of course the breakdown in the refrigeration systems at Vons recently limited all our abilities to munch... and it is a rare day I find my favorite pan dulce there (hint, hint). Maybe I'll lose some of my flab-ellina.
When I do find nudibranchs off our island, they tend to be in the deeper, colder water where nutrients are more abundant. However, diving to depths between 100 and 200 feet does not give me a lot of "bottom time" (no relation to nudies, folks) to allow me to film them. Of course there are often exceptions to any rule. The Spanish shawl nudibranch is one. It is perhaps the most commonly seen nudibranch here, and may be found within easy diving depths allowing divers to really observe... and film them.
Like the lovely Latinas of our island and south of the border, this is one attractive nudibranch. It's brilliant coloration is responsible for its common name. This "shawl" often is seen fluttering in the currents as it tries to maintain a stable grip on the substrate. It seems to prefer locations bathed by moving waters since the water also brings food to the hydroids, bryozoa and other invertebrates nudibranchs usually feed on.
This stunning snail has a deep purple body with bright neon orange gills known as cerata. The gills extend out from the body like a shag carpet. Nudibranchs with gills that cover their dorsal surface like this are often members of a group known as aeolids. Another common group, the dorids, has their gills or cerata bunched together in a circlet near the rear of the body. On the head region there are two bright red sensory structures known as rhinophores. This species is very fussy about what it eats, and prefers an orange colored hydroid known as Eudendrium. An orange pigment in this food source is used to create the purple, orange and red colors of the nudibranch's body. I'm sure glad I'm not the color of the food I eat!
The flashy Flabellina has been recorded from Vancouver Island, Canada, to Baja California and on to the Galapagos Islands. It is rare north of San Francisco, and in the southern parts of its range it is usually found in deeper, colder water. Known as submergence, this is the same phenomenon seen off our island when looking for nudibranchs from colder waters further north.
Now these nudibranchs follow the Wrigley Doublemint philosophy of "double your pleasure, double your fun" because each individual has both male and female sex organs. If you look closely at the lower left image, you can actually see these structures (yes, I know, shameless... I should be censored). When they mate, each fertilizes the other. The Spanish shawl then lays bright pinkish orange egg ribbons, usually right on their prey. However, the young will not be able to feed on the hydroids. After about a week the eggs hatch into tiny larvae which drift and feed in the plankton. This offers a dispersal mechanism for them to colonize new areas or mix up the genes a bit between populations.
The bright colors of nudibranchs are said to be warning coloration to possible predators. Some are known to possess chemical defenses which give them a nasty taste, causing predators to spit them out. Others incorporate stinging cells from tiny relatives of the jellyfish (or sea jellies if you prefer) as part of their defense "budget." If disturbed by a predator, the Spanish shawl may go "airborne" (er, waterborne) and swim away through rapid side-to-side motions of the body. Kind of reminds me of myself when I dive... which is one of the reasons I dive solo, so no one can see!
© 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 DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!
The Spanish shawl in all its colorful glory and its egg mass (lower right).
This document maintained by
Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia