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Illustration shows a hydra, which has a stalk-like body with tentacles growing out the top. A smaller hydra is budding from the side of the stalk.
Hydra reproduce asexually through budding.

Watch a video of a hydra budding.

Fragmentation

Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.

For example, in many sea stars, asexual reproduction is accomplished by fragmentation. [link] illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

 Illustration shows a sea star with one long arm and four very short arms.
Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual.

Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximate size are formed.

Parthenogenesis

Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water flees, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced.

Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males.

Sexual reproduction

Sexual reproduction is the combination of (usually haploid) reproductive cells from two individuals to form a third (usually diploid) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes—male and female—determined at conception. However, in the animal kingdom, there are many variations on this theme.

Hermaphroditism

Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates such as earthworms, slugs, tapeworms and snails, shown in [link] , are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.

Photo shows a land snail.
Many snails are hermaphrodites. When two individuals mate, they can produce up to one hundred eggs each. (credit: Assaf Shtilman)

Sex determination

Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.

Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system.

The sex of some species is not determined by genetics but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development. This is referred to as environmental sex determination, or more specifically as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-dependent.

Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first male.” Oysters, for example, are born male, grow, and become female and lay eggs; some oyster species change sex multiple times.

Section summary

Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring. Asexual reproduction occurs through fission, budding, and fragmentation. Sexual reproduction may mean the joining of sperm and eggs within animals’ bodies or it may mean the release of sperm and eggs into the environment. An individual may be one sex, or both; it may start out as one sex and switch during its life, or it may stay male or female.

Questions & Answers

what is photosynthesis
Conast Reply
photosynthesis is the process by which green plants manufacture their own food from water and carbon dioxide using sunlight as a source of their energy releasing oxygen as a by-product.
Everine
what are runners
Godfrey Reply
what is an environment
Bwambale Reply
diagrams are not indicated, why?
Simon Reply
diagrams for what?
Everine
A vaccination is a treatment which makes the body stronger against an infection. The body fights infections using the immune system, which is made up of millions upon millions of cells including T cells and B cells.
Nworie
define the term immunity
Everine
can simply defined as an exemption from an attack
Nworie
is the ability of an organism to resist a particular infection or toxin by a specific antibodies or sensitized white blood cells
Adeb
difference between cell and cell membrane
Sunday Reply
cell is a basic unit of living body however cell membrane is a protective layer of cell.
Suhail
very good
Everine
which topic would you like to discuss today?
Everine
vaccine
Nworie
what do you understand by the term vaccine?
Everine
guy I need the answer of this question what occurs during photosynthesis
REAL Reply
Photosynthesis is the process that plants use to convert light into food. During this process, plants create carbohydrates starting with only carbon dioxide and water. Sunlight provides the energy that makes photosynthesis possible.
Stanley
@Stanley,that's how I could as well put it
Evans
what are different between genotype and phenotype
Martha Reply
what is light microscope and electronic microscope
Martha
what occurs during photosynthesis
Martha
what are importance of biology
Martha
what is the meaning of computer science
Martha
what is ecology
Adeyemo Reply
what is zoology
Adeyemo
zoology branch of biology that study animals
Moses
a branch of biology which deals with the study of animals
Anita
A plant is a living thing that grows in the earth and has a stem, leaves, and roots
Bello Reply
what is a plant
dalitso Reply
A plant is a living thing that grows in the earth and has a stem, leaves, and roots
Bello
a plant is an is a living thing that is classified under autothrops because it makes its own food through the process of photosynthesis .
Joanne
a plant is also known as autotropic category and a plant have root & stem
Anita
what is plant
Odion Reply
Plants are autotrophic
Baramox
Then animal are heterotrophic
Iyiola
what are organisms
Godfred Reply
Living structures
Aryavarta
living and non living which charectrised by different properties
Anita
organisms living and nonliving components of the environment with distinct xtics
Tony
the diagram below show how the blood of a human embryo flows close to the mothers blood in the placenta . which substances are represent at x in higher concentrations than at y
Joyce Reply
what is ecology
Odion Reply
what is cell
Odion
what is cell
Odion
Is the basic unit of life
Agatha
what's biology
Okebe
biology is the study of life
Ali
I want to know more about biology
Okebe
What are enzymes?
Zanga
if you want to know more, please start studying biology from the beginning systematically
Best
enzymes are biological catalyst that speed up d rate biochemical reactions
Ning
What is a cell
Awal Reply
What are tissues
Awal
A cell is the smallest unit of living organisms.
hamidat
Tissues are group of similar cells performing a particular function.
hamidat

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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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