English for

7.5 Reading #3:
 Fertilization 
Sexual reproduction starts with the combination of a sperm and an egg in a process called 
fertilization. This can occur either inside (
internal fertilization
) or outside (
external fertilization
) 
the body of the female. Humans provide an example of the former whereas seahorse reproduction is 
an example of the latter. 
External Fertilization 
In aquatic environments where both eggs and sperm are dispersed into the water, external 
fertilization typically takes place. Fertilization occurs after the sperm contacts the egg. The majority 
of external fertilization occurs during the spawning process, when one or more females release their 
eggs and one or more males release sperm simultaneously in the same location. The temperature of 
the water or the amount of daylight may cause the release of the reproductive material. Almost all 
fish spawn, along with mollusks (like oysters), squid, echinoderms, and crustaceans (like crabs and 
shrimp) (such as sea urchins and sea cucumbers). Salmon are seen spawning in a small stream in 
Figure 157. Corals, mollusks, sea cucumbers, and frogs like those in Figure 158 all spawn. 
Figure 157.
Salmon reproduces through spawning
120
.
 
119
Clark M.,Choi, J. Douglas M. (2020) 
Biology 2e
. Chapter 43, -Pp. 1228-1229. OpenStax, 
https://openstax.org/details/books/biology-2e
)
120
(credit: Dan Bennett)

---

224 
 
Figure 158.
The male toad grabs the female from behind and externally fertilizes the eggs as they are laid 
during sexual reproduction
121
. 
 
Fish pairs that don't broadcast spawn occasionally engage in courtship behavior. This enables 
the female to pick out a specific man. The act of spawning, which initiates the release of eggs and 
sperm, concentrates them in a restricted space, increasing the likelihood of fertilization. 
The eggs are shielded from drying out in an aquatic environment by external fertilization. 
Broadcast spawning can produce a more diverse mix of genes within a population, increasing genetic 
diversity and the likelihood that a species will survive in unfavorable conditions. Broadcast spawning 
is the only method for fertilization and colonization of new environments for sessile aquatic 
organisms like sponges. The fertilized eggs and growing young present in the water create 
opportunities for predation, which leads to the loss of offspring. As a result, each individual must 
produce millions of eggs, and the offspring created using this process must mature quickly. Eggs 
produced through broadcast spawning have a low rate of survival.