Nudibranchs, or sea slugs, are in the phylum Mollusca and the class Gastropoda, which means that they’re snails. But they’ve lost their shell during the course of evolution, though a vestigial shell is retained in the early larval stage. They’re also often toxic or venomous, and have therefore adopted aposematic (warning) coloration (note: because many reef fish are also colorful but not aposematic, there may be some camouflage or species-recognition involved here too.) Nudibranchs are in fact some of the world’s most gorgeous animals. If you do a Google Image search for “nudibranch”, these are the first four rows of pictures that come up. It’s like an LSD vision of animals:
Most nudibranch species are benthic: that is, they crawl around on the substrate looking for food, and that food is other invertebrates, including sponges, coelenterates, or other nudibranchs. This video shows some lovely nudibranchs and their habit of crawling around on the substrate (often reefs). Some, however, can swim for a short while if disturbed.
A few species, however, are completely pelagic, hunting in the open ocean. Deep Sea News reports one of these species, in the genus Philliroe (there are only two species, and the one shown is not identified), has undergone convergent evolution to a fishlike form. Part of its body has a split “tail fin,” another a “dorsal fin”, and it even has a pair of horns that act as stabilizing pectoral fins:
They get up to 5 cm long: about two inches. Here’s another view:
Here’s another shot of one nomming its preferred prey: jellyfish. Another remarkable aspect of the slug is that it’s bioluminescent—it glows in the dark.
The author of the piece raises a few hypotheses for its fishlike shape, including the silly idea that females simply prefer fishlike males (if that were the case, there would probably be sexual dimorphism in shape, with males looking fishier than females). But then the author of the piece (“R. R. Helm”) suggests what’s probably the correct reason: evolution has molded the animal to be fishlike because such an appearance enables it to swim faster. After all, that form of convergent evolution also molded the ancestors of icthyosaurs and porpoises, land-dwelling reptiles and mammals, respectively, forming one of the most famous cases of convergent evolution in diverse taxa.
There are a few other bits of information in the piece, but I’ll let you read it for yourself. I’ll just add one other video of another example of convergence in the group: Cephalopyge trematoides, another pelagic nudibranch that’s evolved a shape like an eel, though it’s not very adept at swimming like an eel.
Remember, convergent evolution doesn’t mean that one species evolves by natural selection acting directly to make it resemble another (that’s mimicry); rather, it is the independent evolution of unrelated taxa to similar forms to deal with similar environmental challenges. (See WEIT for a figure showing the remarkable convergence of many Australian marsupials and non-Australian placental mammals.)
h/t: Ant
Wow, pelagic nudibranchs! Now that is new to me. And beautiful creatures too.
When I kept reef tanks, I had a couple nudibranchs, but they were difficult to keep because of their diet. I learned of many reef species in the hobby that should stay in the ocean as captivity spells their doom.
Your documentation of convergent evolution of the Australian marsupials and non-Australian placental mammals was probably my favorite part of WEIT, and an iron-clad argument for WEIT to boot.
“…should stay in the ocean as captivity spells their doom.”
Ain’t that the truth. 🙁 One of my worst reef-keeping memories is of ordering a Chelidonura varians and watching it waste away over the next week or so. Now of course they have the Aiptasia-eating nudi that apparently not only thrives but breeds in reef tanks…
I actually ordered some Aiptasia-eating nudi and they all died too… It was my second attempt of keeping them and after that I didn’t get any more. The ultra-beautiful Mandarin Gobi was another that wasted away because I didn’t have enough zooplankton. It’s a fun hobby, but my conscious got the better of me after a few years and I stopped. I did have some great successes too though.
I have had the privilege of seeing a few different species of nudibranchs when spelunking around tide pools in San Diego and down in Mexico.
The example of convergent evolution to a fish is amazing. I am very impressed, even though on principle it should not be surprising.
There were many fav. sections for the WEIT book for me, but the part that really impressed me was on biogeography. That was b/c my knowledge of that subject was pretty poor. It influenced my teaching on the evidence for evolution.
As a diver, I’m always on the lookout for these beauties, but they are annoyingly difficult to find. You need to learn to ignore the big, scaly, fellas swimming by and keep your nose down in the weeds. Every find is exciting.
Interesting that two groups (vertebrates and molluscs), with such different body plans, should both converge on such similar shapes for pelagic swimming.
Agree. That’s interesting.
Yes, that’s what I found most interesting about this. One of the common examples of convergent evolution is ichthyosaur-fish*-cetacean, which are all vertebrates, so it is nice to see an invertebrate with the same “blueprint”.
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* See #12.
Beautiful fractalicious species 🙂 Nudibranch is a much better name than sea slug.
Fascinating, and quite cool!
More flamboyant than a Nudie Suit, to boot!
Well, that was a disappointment.
Concur. Definitely an anticlimax.
cr
I’ve only seen one nudibranch, in the lagoon in Rarotonga, decades ago. It was bright red with orange feathery gills, several inches long and swimming above the coral. Maybe a Spanish Dancer?
Just before that, c. 1980, the BBC made the epoch-making David Attenborough series Life on Earth, with magnificent colour photography. I remember a segment, several minutes long, just showing various nudibranchs. I had no idea such colourful creatures existed.
cr
What a fascinating animal and evolutionary convergence! I wonder if this nudi commandeers the jellyfishes’ nematocysts, like other cnidarian-eating nudis do.
This part of the Deep Sea News article was interesting as well:
Amazing – if you like nudibranchs look at Peter Godfrey-Smith’s website –
http://metazoan.net/
Totally cool. So glad to know this creature exists.
There is no such thing as a fish.
These are really beautiful creatures. They remind me of the wonderful invented creatures of cartoonist Jim Woodring. That in turn reminds me that I once sent him a link to a photo of a brightly colored starfish which also reminded me of his work. He replied “Don’t tell ME there’s no God!”, which I hadn’t thought to do at all but afterwards figured maybe I should have…
I hate getting told things like that–the tellers always seem so sure that they’ll meet with nothing but agreement.