Tubeworms are either male or female. One recent discovery indicates that the spawning of female "Lamellibrachia" appears to have produced a unique association with the large bivalve "Acesta bullisi", which lives permanently attached to the anterior tube opening of the tubeworm, and feeds on the periodic egg release (Järnegren et al., 2005). This close association between the bivalves and tubeworms was discovered in 1984 (Boland, 1986) but not fully explained. Virtually all mature "Acesta" individuals are found on female rather than male tubeworms. This evidence and other experiments by Järnegren et al. (2005) seem to have solved this mystery.