External Fertilization in Chordates

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The phylum chordata represents the whole diverse class of vertebrates, animals with a vertebral column, as well as lancelets and tunicates. Two fertilization strategies are employed by members of chordata: internal fertilization, where the gametes, or sperm and egg, meet inside the body of one parent, and external fertilization, where sperm and egg meet outside the body. External fertilization is necessarily limited to aquatic environments, since sperm need a liquid medium to swim through in order to reach an egg.

Subphylum Cephalochordata

Cephalochordata is a very small subphylum made up of species of lancelets. Lancelets are small, fish-like animals that represent some of the most primitive characteristics of the phylum, including a dorsal nerve chord supported by a notochord rather than a spine. Females and males produce eggs and sperm, respectively, from sets of paired gonads, and release them simultaneously for fertilization during the spawning season. During spawning, the gonads of the lancelets rupture and the gametes are flushed out into the water. The fertilized gametes form fish-like larvae.

Subphylum Urochordata

The subphylum urochordata, also known as tunicates, may not seem to belong in the phylum chordata at first glance. They generally only have notochords in the larval stages and the adults are frequently completely immobile, looking as much like plants as animals. Tunicate reproduction is a complicated affair. Some tunicates reproduce asexually, and of those that reproduce sexually, most are hermaphrodites, producing male and female gametes. Some colonial species hold eggs and take in sperm through their siphon, or mouth, but solitary species release both eggs and sperm for external fertilization. Fertilized eggs form a free swimming tadpole that finds a new home and becomes an immobile adult.

Subphylum Vertebrata: Fish

Some fish are capable of live birth, but most use external fertilization. Spawning behavior varies among species, but commonly one or both parents prepare a nest for the eggs to rest on. There is a courtship ritual to ensure that the eggs and sperm meet for fertilization and then both individuals release large numbers of gametes. Some species continue to provide some parental care, such as mouth brooding, where one parent (in some species it's the female; in others the male) allows the young to shelter from predators in his or her mouth; but most young fish, or fry, are on their own.

Subphylum Vertebrata: Amphibians

Amphibians live part of their lives in the water and part on the land, but almost all frogs and most other amphibians use external fertilization in aquatic environments to breed. Whether a pond or the well of a leaf, the female and male meet on a breeding ground, where the female deposits a mass of eggs and the male deposits sperm on top of the mass. The eggs of most amphibians develop into an aquatic larval stage that undergoes metamorphosis to an amphibious adult .


About the Author

Based in Wenatchee, Wash., Andrea Becker specializes in biology, ecology and environmental sciences. She has written peer-reviewed articles in the "Journal of Wildlife Management," policy documents,and educational materials. She holds a Master of Science in wildlife management from Iowa State University. She was once charged by a grizzly bear while on the job.

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