Color
Coordinated Sponge and Slug OutfitsBright is beautiful. Bright is bold. And bright is normally thought of in terms of things that are supposed to stand out from the crowd.
This is often the case in the underwater world of sea slugs or nudibranchs. Along the California coast, there are so many types that are bright, bold and beautiful. This not only seems to make them stand out against the drabber hues found around the kelp beds, but also serve as a warning to potential predators that these gastropods are not ones they want to mess with!
Yet, there is a bunch of beautiful bright and bold colored sea slugs that are not out to advertise their presence. In fact, their intense colors serve the opposite function. It helps them blend in with their chosen habitat - sponges that are equally bright tones.
Such
is the case with the Crimson Nudibranch (Rostanga pulchra). Under
normal circumstances, its flame red color should stick out like a
sore thumb. But instead, it carries out the daily affairs of a
nudibranch life - eating, mating, laying egg masses - practically
unnoticed because it sticks to living on the equally bright Velvet
Red Sponge (Ophlitaspongia pennata). In fact, practically everywhere
such a sponge is found, so will this matching sea slug. That could
make identification of both animals easy, except this nudibranch also
can be found on another red sponge species, and another red colored
nudibranch, the Blood Spot (Aldisa sanguinea), will live on the
Velvet Red Sponge.
This red on red would seem to be a classic protective camouflage scenario, and there is no question that the mollusks are well disguised and can crawl about in the open with impunity on their spongey abode. Yet if a person were to remove a Crimson Nudibranch from its normal residence, no predator would instantly snap it up. It has no known enemies (aside, of course, from people who move it away from its sponge to see if anything will eat it). Even that infamous "nudibranchivour" sea slug, Navanax (Navanax inermis), which will swallow whole other species like Hermissenda, is repelled by the Crimson Nudibranch. Thus, this red sea slug's color is not considered to be protective coloration.
Then why does the Crimson Nudibranch so perfectly match those red sponges? No, it is not so underwater photographers can take some dazzling pictures - that is just another side benefit. The reason seems to be that they eat the red sponges and then incorporate the pigments into their own bodies. The old "you are what you eat" idea is carried to the extreme!
Of
course, not all nudibranchs, which feed on sponges, are so "into
their meals," (or their meals into them), but they can be just as
discriminating in their tastes. The big six-inch long yellow Lemon
Nudibranch (Anisodoris nobilis) is also unaffected by predators and
it is often seen seemingly strutting about the reef, flaunting that
fact. When hungry, it will go for a sponge and chomp away with its
rasping teeth. This species is known to feed on a half dozen or more
types of sponges, yet an individual Lemon Nudibranch tends to stick
to a certain kind. Move it to another area and it will search out its
favorite again.
When it comes to the determination to eat a particularly desirable sponge, the Salted Nudibranch (Doriopsilla albopunctata) likewise don't let some little inconvenience stand in its way - the little inconvenience of having no teeth. It secretes a fluid out of its mouth over the surface of the red sponge Acarnus erithacus, the yellowish Boring Sponge (Cliona celata), and other types. The surfaces are thus softened and the sea slug can slurp up the residue.
Yes, bright can be bold and beautiful, even when it is red on red, yellow on yellow, white on white - although perhaps not when decorating a house, but when it comes to sea slugs that decorate the exterior of sponges. And the divers who take the time to pursue the otherwise uninteresting, non-photogenic sponges, and successfully find these colorful opistobranchs - they are certainly bright ones too!
Nancy Vander Velde is a frequent contributor to California Diving News and has illustrated several books.
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