Predators include the Pacific staghorn sculpin Leptocottus armatus
. This species swarms as epitokes during spawning, and swarms have been observed during spring and summer in CA. A close relative, Nereis virens, swarms about midnight in the dark of the moon during summer. During swarming, males can be distinguished by the white sperm showing through the body wall, while the posterior segments of females are red due to the eggs inside. In another relative, the Atlantic species Platynereis megalops, the females in a swarm bite off the posterior, egg-bearing segments of the swarming males. After being swallowed and broken out of the male segments, the sperm pass through the wall of the female pharynx into her coelomic space, where they fertilize the eggs. The fertilized eggs are shed through ruptures in the body wall. The females do not shed their eggs until they are fertilized, and the eggs cannot be fertilized while free in the seawater.
Members of Family Nereidae seem to be very inefficient swimmers. While swimming as while crawling, they undulate their bodies back and forth in metachronous waves. While swimming these waves are of large amplitude and proceed from the back to the front of the animal (retrograde waves). These waves produce a current which tends to move the animal backward. At the same time, the frantic waving of the parapodia produces a current to drive the animal forward. The result is that the animal doesn't go much of anywhere and mostly thrashes around. The similar appearing Nephtys worms (Family Nephtyidae) use a much smaller metachronous body wave while swimming and can swim forward much more efficiently.
Nereids may be tubedwellers, mud borers, commensal, or free-crawling and free-swimming. Nereis brandti is the largest polychaete worm on our coast.