E External Fertilization Megha Das, Nitesh Kumar Mishra and Sanjeev Kumar Yadav Department of Zoology, Institute of Science, Bananas Hindu University, Varanasi, UP, India Definition External fertilization or “the ancestor of internal modes of reproduction” is generally a widespread reproductive approach of aquatic animals includ- ing both invertebrates (coelenterates, annelids, mollusks, echinoderms) and vertebrates (fishes and amphibians) in which male and female gam- etes are released into the aquatic environments and fertilization occurs outside of the body (Giese and Kanatani 1987; Jigersten 1972; Parker 1984; Wray 1995; Rouse and Fitzhugh 1994). Introduction Fertilization is the combination of haploid male and female gametes forming diploid zygote to begin a new life. Fertilization may occur inside the body of a female (internal fertilization) or outside the internal milieu (external fertiliza- tion). Our focused area is external fertilization which usually ensues in water or a humid area to ease the travels of sperms to the egg. Females and males spawn freely into the environment which might be triggered by various factors such as water temperature or the length of daylight. The trigger for spawning causes the egg and sperm to be in a small area, enhancing the likelihood of fertilization. This is not only the environmental triggers that signals and controls the spawning process; other male and female physiological components and phenomena also regulate the entire process. Due to the presence of different types of species in the same environment with same breeding season and having same kind of environmental triggers, they face various chal- lenges such as to address the right chance of contact of the gametes of the same species as well as on prevention of getting fertilized by sperms of another species. Thus, to overcome these encounters, these organisms have evolved several mechanisms which we are discussing below. (a) Environment: Gamete concentration is the primary factor for either a male or a female spawner which regulates the probability of external fertilization. Several field experi- ments have documented that in an aquatic milieu gamete can diffuse and rapidly convert diluted resulting in declined fertilized egg ratio. This depends on various aquatic envi- ronmental situations such as high-wave- intensity shores, increasing water flow, or increasing water velocity. These environmen- tal factors are responsible for the very fast © Springer Nature Switzerland AG 2020 J. Vonk, T. K. Shackelford (eds.), Encyclopedia of Animal Cognition and Behavior, https://doi.org/10.1007/978-3-319-47829-6_330-1