The maturation and breeding biology of Harmothoë imbricata (Polychaeta: Polynoidae)

The embryos of Harmothoë imbricata (L.) are protected during their early development as a mass under the scales of the female parent. Eggs are shed into the space under the scales through nephridial papillae which elongate and turn dorsally as the female matures. The mass is held together by an adhesive layer surrounding each egg. Maturation begins in September and breeding occurs during March and April, when each female spawns twice in succession. Males contain mature sperm throughout the breeding season and shed only a small proportion at each spawning. As the polychaetes mature, pairs are formed with the male resting on the dorsal surface of the female. When the female spawns, the male sheds sperm over the egg mass and the two animals then separate. The sperm has a long acrosome to penetrate the adhesive layer around the egg. Embryo-protecting species in other groups of polychaeces show parallel specialization in sperm structure and in breeding behaviour. A chronology has been established for the development of embryos up to their release from the parent, and this can be used to estimate the spawning date of females with embryo masses collected on the shore. Individuals grow at widely varying rates after settlement on the shore, and no year classes could be detected in the population. All individuals spawn in the first spring after settlement, and at least some survive to spawn again in one or more subsequent springs.