Giant pile worms or sea nymphs rise from their burrow when the moon and tides are right. They swim vigorously with the males following the females, eggs or sperm pouring out of each, sometimes through ruptures in the body wall. Their active swimming is made possible by legs that develop shortly before, specifically for the purpose of their nuptial dance.
This is one of the several videos made by Jeff Adams, Marine Water Quality Specialist with Washington Sea Grant and founder of the Beach Naturalist program for Kitsap County.
Other videos on Jeff's YouTube channel Salish Sea Life include diverse marine related topics from jellyfish to orcas.