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By the end of this section, you will be able to:
  • Discuss internal and external methods of fertilization
  • Describe the methods used by animals for development of offspring during gestation
  • Describe the anatomical adaptions that occurred in animals to facilitate reproduction

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

External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). [link] shows salmon spawning in a shallow stream. Frogs, like those shown in [link] , corals, molluscs, and sea cucumbers also spawn.

Photo shows many salmon swimming up a shallow creek.
Salmon reproduce through spawning. (credit: Dan Bennett)
Photo shows mating toads.
During sexual reproduction in toads, the male grasps the female from behind and externally fertilizes the eggs as they are deposited. (credit: "OakleyOriginals"/Flickr)

Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.

External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms like sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs and developing young in the water provides opportunities for predation resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals, and the offspring produced through this method must mature rapidly. The survival rate of eggs produced through broadcast spawning is low.

Internal fertilization

Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization. In oviparity    , fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. Chicken eggs are an example of this second type.

Questions & Answers

what are structure of the cell
wana Reply
what did Darwin say about evolution
Hope Reply
effect of planning beans of using fertilizer
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what do we mean by transgenic organisms?
FADILAT Reply
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They are the regions of the pancreas that contains the endocrine cell
Iyadi
is the studly of life
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what is biology
Asunta Reply
is the study of living organism and their interection with one another and their environment.
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what is soil
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the top layer of the earth in which plant's, tree's
Ahmad
type of soil
Asunta
function of cell wall
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function of cell wall
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To protect the cell from bursting
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to protect the cell from bursting
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to protect all other internal components of the cell
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sex cells
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hlo
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procryotic cell and some eucaryotic cell
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applications of biology
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hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
Namawejje
eukaryotic cells have DNA in their nucleus while prokaryotic cells have their DNA present freely in their cytoplasm.
FADILAT
deviation from mendelian
Ogali Reply
what is lethal allele
Ogali
a lethal allele is an allele that can cause poor development or even death of an organism
Olaoye
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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