The Japanese green syllid Megasyllis nipponica, is a bristle worm native from the coast of Japan, but now is know to live in coastal parts of California where is stablished. This little dance seen in the GIF have a reproductive reason, and researchers now explaing better what is happening, and you may not be prepared to this.
The brown and dancing body segment is called the stolon, which detaches from the worm in a unique reproductive process called stolonisation or schizogamy. When gonads are matured, the stolon developed a head, which even posses eyes, antennae and bristles, then the stolon can break off and swim away, to break apart to release eggs or sperm. This bizarre process continues before the stolon detaches when it develops nerves and a ‘brain’, allowing it to swim independently from the original body.
Video and figure by Nakamura et al 2023
Reference (Open Access): Nakamura et al. 2023 Morphological, histological and gene-expression analyses on stolonization in the Japanese Green Syllid, Megasyllis nipponica (Annelida, Syllidae). Sci Rep