Types of reproduction

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Fuse: to join two or more parts together into one.

Gamete: specialized cells found in your reproductive organs that have half the amount of DNA of somatic cells. These cells combine to make a fertilized egg... more

Invertebrate: the major division of animals that lack a backbone (spinal column)....more

Vertebrate: the major division of animals that have a backbone (spinal column) which is made up of several different pieces... more

External Fertilization

Palmyra atoll

Corals aren't exactly swift movers, so how do they find a mate? Image by Jim Maragos/U.S. Fish and Wildlife Service.

You dive into the ocean along a coral reef. You see beautiful corals, full of color and life. But you notice that these animals don’t actually move. How do animals that don’t move find a mate?

They do it by releasing eggs or sperm into the water, in the hopes that the cells will find each other and the eggs will be fertilized. Most aquatic organisms that reproduce sexually use this method of fertilization. Because it happens outside of the body, we call this “external fertilization”.

External fertilization in animals usually occurs in water or in damp areas in a process called spawning. Moisture in the environment keeps the gametes from drying out. Moisture is also important so the eggs and sperm can move around freely, which lets them meet and fuse.

Agalychnis callidryas mating

Frogs mate in a position called "amplexus," where the male hugs the female's back as they both release gametes. Image by Andrew DuBois on Flickr.

In aquatic vertebrates like fish, salamanders, or frogs, some males and females will release eggs near each other so the eggs have a better chance of meeting. Grunions are fish that swim on to shore and mate in the sand. The female buries her tail in the sand and lays her eggs, then a male wraps around her and fertilizes the eggs she laid. Male and female salmon fish leave the ocean at the same time, to swim for miles and miles up river together. They gather in a special spot called a spawning ground. There, females make nests in river beds and fill them with eggs, and males release sperm over these nests. In most frogs, the male will hug the female’s back and fertilize the eggs as she lays them.

Purple sea urchin

Many aquatic animals release eggs and sperm into the water. Because it may be difficult for these gametes to find one another, they are often released by individuals of the same species around the same time. Image by Steve Lonhart/NOAA.

In aquatic plants and invertebrates like coral and sea anemones, the motion of the water moves eggs and sperm around so they meet by chance. Sounds a bit unlikely, right? To help make sure these eggs and sperm will find one another, thousands or even millions of eggs or sperm are released. The release of these eggs and sperm are often timed together, to give them a higher chance to find one another.

 

 


Additional images via Flickr. Mantis shrimp via Wikimedia Commons by Rickard Zerpe.

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Bibliographic details:

  • Article: External Fertilization
  • Author(s): Sarala Pradhan, Ioulia Bespalova
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: July 16, 2019
  • Date accessed: December 13, 2019
  • Link: https://askabiologist.asu.edu/external-fertilization

APA Style

Sarala Pradhan, Ioulia Bespalova. (2019, July 16). External Fertilization. ASU - Ask A Biologist. Retrieved December 13, 2019 from https://askabiologist.asu.edu/external-fertilization

American Psychological Association. For more info, see http://owl.english.purdue.edu/owl/resource/560/10/

Chicago Manual of Style

Sarala Pradhan, Ioulia Bespalova. "External Fertilization". ASU - Ask A Biologist. 16 July, 2019. https://askabiologist.asu.edu/external-fertilization

MLA 2017 Style

Sarala Pradhan, Ioulia Bespalova. "External Fertilization". ASU - Ask A Biologist. 16 Jul 2019. ASU - Ask A Biologist, Web. 13 Dec 2019. https://askabiologist.asu.edu/external-fertilization

Modern Language Association, 7th Ed. For more info, see http://owl.english.purdue.edu/owl/resource/747/08/
Peacock mantis shrimp
Most fish, bivalves (like clams), and crustaceans, like this peacock mantis shrimp, use external fertilization.

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