Maybe "Parasol Feather Duster" (old names: Demonax medius or Sabella media).
Another one, darker (orange on one side, balck or dark brown on the other).
About 1/2" long, saw several of them in various red/yellow/orange colors speeding along just below the surface of the water.
Face shot! See other photos for rest of worm. About 3" long but very thin, with long bristles.
I found these thin pinkish worms under two rocks in the sandy and rocky middle intertidal zone. They are thinner than a toothpick, and about that long. In both cases they were living with dozens of other worms in the muddy sand under a rock, building little tunnels and piling up the sand in tall clumps with holes at the top, as visible in the photo of the single worm.
I think this olive green and reddish polychaete worm is Nereis vexillosa or Banner Sea-nymph. It was found under a rock in the middle intertidal zone at low tide. It was about 2" long when contracted, and 4" long when spread out.
Christmas Tree worms in Guadeloupe, Cousteau Marine park.
Trial for using iNaturalist in the field for this project.
The annelids (also called "ringed worms"), formally called Annelida (from Latin anellus "little ring"), are a large phylum of segmented worms, with over 2,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments.They are bilaterally symmetrical, triploblastic, coelomate organisms.They have parapodia for locomotion. Although most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include...