We found this on a beach at the Perhentian Islands, Malaysia in March, 2016 and cannot identify it. It's some sort of worm-type creature with many legs and bluish back about 15cm long. The legs undulated along the body as it swam.


sea millipede

belly side:

sea millipede underneath


It's hard to identify from the photos provided, but I think it is Chloeia flava (a species of polycaete worm, within the phylum Annelida), also known in English as the "Golden Fireworm". The size is roughly similar to what you describe (they are typically about 7-10 cm long). The individual you observed looks like it lives in sandy bottom environments (not a typical environment for a nudibranch), and this worm does also. It is commonly found all across the warmer Indo-Pacific as well, and looks like the individual observed in your photo.

If it is not this species, I think it is another species of the same family, Amphinomidae.

Chloeia flava

  • $\begingroup$ Thanks! We're happy to have seen a nudibranch in real life. We just learned about their existence a few months ago when a blue dragon washed up in Australia. We're lucky we weren't stung by this one since we were swimming very close to it to study it. $\endgroup$ – Kim Apr 1 '16 at 2:01
  • $\begingroup$ Sorry my answer was edited. It is not a nudibranch or mollusk but a polycaete worm. If you want to find a nudibranch I recommend checking tidal pools or near coral. If you aren't a diver yet, the best way to observe them is to take up scuba diving. $\endgroup$ – Sudachi Apr 1 '16 at 3:47
  • $\begingroup$ :( So not a nudibranch but still very interesting. $\endgroup$ – Kim Apr 1 '16 at 5:09
  • $\begingroup$ Sorry for my mistaken edit. My mind was so set on a nudibranch that I didn't even spot the mistake even though I visited the wikipedia page for C. flava. $\endgroup$ – fileunderwater Apr 1 '16 at 7:43
  • 1
    $\begingroup$ @fileunderwater No problem! Thank you for the edit. It was my first post and I learned how to format posts now. $\endgroup$ – Sudachi Apr 1 '16 at 13:44

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