The breeding of Arenicola marina from two beaches in the Isle of Man, one exposed to and the other sheltered from wave action, was studied. Both populations spawned only once during the year, in autumn, after an extended period of sexual development. The population living in exposed conditions had a higher proportion of juvenile non-breeding worms than the sheltered populations, due to loss of adult worms (by mortality and/or migration) resulting from unstable environmental conditions. Lugworms from the Isle of Man are capable of becoming sexually mature for the first time in their second year of life and at a size of between 0.5 c.c. and 0.9 c.c. (about 2 to 3 cms. chaetigerous length).

Male spawning in 1950 and 1951, detected by the occurrence of sperm puddles on the surface of the sand, occurred mainly during three days at the later November neap tides, but some slight spawning also took place before and after. Female spawning could only be detected by examination of the coelomic contents and it appeared that during both years the male and female spawning was not simultaneous, there being a few days difference in the start and finish of the process.

In 1951 spawning occurred in nine populations of British lugworms during the neap tides of October, November and December. In all nine populations the male spawning coincided with one of these neap tides, but at St Andrews worms from below low water neaps spawned during spring tides. In four populations the female worms apparently began or finished spawning slightly earlier than the males. In five populations, spawning was epidemic but in two it extended over a period of several weeks.

Possible factors inducing spawning are discussed.