Intracellular calcium signaling in the fertilized eggs of Annelida

• We reviewed about the historical studies on fertilization with Annelida.
• Fertilization of Annelida illustrates drastic interactions between egg and sperm.
• Fertilization of Annelida is distinguished in several independent steps.
• We showed 2 steps Ca2+ increases corresponds to the sperm entry and egg activation.
• Annelida contributes to the further study of Ca2+ function at fertilization by sperm.

Fertilization is such a universal and indispensable step in sexual reproduction, but a high degree of variability exists in the way it takes place in the animal kingdom. As discussed in other reviews in this issue, recent works on this subject clarified many points. However, important results on the mechanisms of fertilization are obtained mainly from a few restricted model organisms. In this sense, it is utterly important to collect more information from various phyla. In this review, we have re-introduced Annelida as one of the most suitable models for the analysis of fertilization process. We have briefly reviewed the historical works on the fertilization of Annelida. Then, we have described recent findings on the two independent Ca2+ increases in the fertilized eggs of Annelida, which arise from two different mechanisms and may have distinct physiological roles toward sperm entry and egg activation. We propose that the Ca2+ increase in the fertilized eggs reflect the specific needs of the zygote in a given species.