In biology, Gonozooids are any of the reproductive individuals of

Tunicates A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ca ...

, Bryozoan, or

Hydrozoan Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialize ...

colonies that produce

gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

. Gonozooids may play a role in labour division or in

alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...

. A gonozooid typically has hardly any other function than reproduction, amounting to little more than a

motile Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

. The production of gonozooids amounts to one aspect of certain classes of alternation of generations. In biological terms the various forms are examples of evolutionary strategies and are largely

analogous Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ...

rather than homologous; the gonozooid phases of say, the Tunicata did not evolve from anything like say, a Bryozoan.

Examples of species with gonozooids

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Hydractinia symbiolongicarpus ''Hydractinia symbiolongicarpus'' is one of the 30 + ''Hydractinia'' species known worldwide. This saltwater animal, from here on referred to as ''Hydractinia'', is at the base of the Cnidaria phylum and in many ways, is similar to the freshwate ...
''

Polyps from the Cnidaria ''Hydractinia symbiolongicarpus'' form colonies where labour is divided between reproduction (gonozooids), feeding (gastrozooids) and defense (dactylozooid). Even though they are genetically identical, there is a distinct morphological difference between those polyps. The gonozooids have gametes-producing organs called gonophores on their body column. They lack the mouth and tentacles present on gastrozooids, and the stinging cells present on the dactylozooid. They can only survive as part of the colony.

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Doliolum nationalis ''Doliolum'' is a genus of tunicates, the members of which move via jet propulsion Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the mov ...
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The Tunicate ''Doliolum nationalis'' does not form colonies, but is a free swimming-organism. It exhibits alternation of generation in its life cycle. In this organism, the gonozooids are hermaphrodites. They both produce and fertilize

ova , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and series made specially for release in home video formats without prior showings on television or in theaters, though the first part of an OVA s ...

, which in turn develop into oozooids, the asexual stage. This oozoid develops further and takes on a quite different form than the gonozooid. Through

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

called

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...

, new gonozooids are produced and the life cycle closes.

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Celleporella hyalina ''Celleporella hyalina'' is a species of bryozoans belonging to the family Hippothoidae. It has almost cosmopolitan distribution In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most ...
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Another colonial gonozooid-forming animal is the Bryozoan ''Celleporella hyalina''''.'' Male and female gonozooids are budded from a layer of sterile gastrozooids. The gonozooids then produce larva, which will undergo

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

and become the founder of another colony.

Roles of gonozooid individuals in the life cycle

Division of labour

In animals such as ''Hydractinia symbiolongicarpus'', gonozooids are part of a colony. Each individual has a role that benefits the colony as a whole. The gonozooids that produce the gametes can sexually reproduce. As the whole colony is genetically identical, it is not necessary for each polyp to individually reproduce, allowing more energy and resources to be used for other tasks such as feeding and defense.

Sexual/Asexual reproduction frequencies

In certain animals, stressful environments have been known to lead to an increase in gonozooid frequencies. Higher gonozooid frequencies lead to more sexual reproduction and thus more offspring being genetically different from their parents. The resulting higher

genetic variance Genetic variance is a concept outlined by the English biologist and statistician Ronald Fisher in his fundamental theorem of natural selection. In his 1930 book ''The Genetical Theory of Natural Selection'', Fisher postulates that the rate of ch ...

increases the chances of beneficial

phenotypes In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

appearing in the population. This beneficial phenotype may in turn spread throughout the population and increase its resistance against the stressing factor.

References

{{Reflist Animal reproductive system