Reproduction in Nereidid Polychaetes: Physiological and Biochemical Aspects

In spite of the large diversity of lifestyles displayed by polychaetes, the families Nereididae together with Spionidae and Syllidae, are covered most in the literature. With regards to reproduction, most physiological and biochemical studies have been carried out on nereidids. This is mostly due to their semelparous reproductive strategy, in which a phase of somatic growth is followed by a shift of metabolic resources towards gamete production. In many species, epitokous metamorphosis, a reorganization of the body structure, accompanies the sexual maturation of the animal. This involves remodeling of the body musculature, increasing circulatory and respiratory capacity and enlarging of the eyes. These adaptations allow the animal to become active swimmers in preparation for a single spawning event leading to the death of the animal. At the same time, the reproductive effort is high with up to 80% of the total energy allocated to gamete biomass. These dramatic processes have attracted researchers since the end of the 19th century. This chapter will focus on the reproductive biochemistry and physiology of nereidid polychaetes including past and more recent studies not covered in past reviews. Biochemical and physiological aspects of polychaete reproduction have been investigated only a limited number of species and most findings reviewed here will cover the genus Nereis (now renamed to Alitta), Perinereis, and Platynereis. 34Nereidids display a simple reproductive organization with gametes and coelomic cells developing freely floating in the coelomic fluid (CF) which make them accessible for physiological and biochemical studies. In recent years, Platynereis dumerilii has emerged as a model species for development, evolution, and aspects of chronobiology. The chapter will present an overview on past and more recent findings not covered in previous reviews. Aspects included are the environmental control of gametogenesis, the origin of germ cells, oocyte, and male gamete development. A focus is set on the role of eleocytes which play a key role during gametogenesis. They are known suppliers of vitellogenin (VTG), the major storage protein incorporated by the oocytes. However, eleocytes have additional functions as stores for amino acids and for purine compounds providing precursors for gamete nucleic acid synthesis. Another aspect covers the role of the discoidal lipoprotein as a lipid carrier which is incorporated by both male and female germ cells but is also involved in the transport of hemoglobin degradation products to the eleocytes. Finally, an overview of the hormonal control of gametogenesis will be presented highlighting the recent discovery of methyl farnesoate (MF) as the long-sought juvenile hormone, which has been postulated more than 60 years ago, and the identification of corazonin as one of the gonadotropins, which have also remained unknown for a long time.