The diversity of protostomes is demonstrated by the classification of these animals into more than 20 phyla and 1,000,000 species. Many species of aquatic protostomes are considered valuable for basic studies of reproduction as well as for fishery resources. The aim of this chapter is to provide a brief overview of the process and mechanism of fertilization in three protostome groups, the mollusks, annelids, and arthropods, in which various modes of sexual reproduction have evolved. Regarding the series of interactions between oocytes and sperm at fertilization, we have described the structural changes in gametes and the regulatory mechanisms of polyspermy block and increases in intracellular Ca2+, which enable the successful fertilization of oocytes. Prior to fertilization, oocytes and sperm are produced and matured in gonads and released at the optimal time. The second half of the article focuses on gonial cell multiplication, oocyte growth, and spawning (oocyte release and sperm release) in bivalve mollusks, in which these processes are precisely regulated by endocrine systems. Although bivalves share many endocrine regulatory molecules with vertebrates, they also employ unique mechanisms such as the use of the neurohormone serotonin (5-hydroxytryptamine, 5-HT), which acts directly on oocytes and sperm to induce oocyte maturation, sperm activation, and spawning.
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