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 ...
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 specializ ...
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. 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 ...
.
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
An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set of ...
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 (th ...
rather than
homologous
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
*Sequence homology, biological homology between DNA, RNA, or protein sequences
* Homologous chrom ...
; the gonozooid phases of say, the Tunicata did not evolve from anything like say, a Bryozoan.
Examples of species with gonozooids
''
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 freshwat ...
''
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.
''
Doliolum nationalis
''Doliolum'' is a genus of tunicates, the members of which move via jet propulsion. A detailed description can be found at .
Species
The World Register of Marine Species lists the following species: 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 ...
, 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 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 kno ...
, new gonozooids are produced and the life cycle closes.
''
Celleporella hyalina
''Celleporella hyalina'' is a species of bryozoans
Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about l ...
''
Another colonial gonozooid-forming animal is the Bryozoan ''Celleporella hyalina''''.'' Male and female gonozooids are budded from a layer of
sterile
Sterile or sterility may refer to:
*Asepsis, a state of being free from biological contaminants
* Sterile (archaeology), a sediment deposit which contains no evidence of human activity
*Sterilization (microbiology), any process that eliminates or ...
gastrozooids. The gonozooids then produce larva, which will undergo metamorphosis 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 prop ...
appearing in the population. This beneficial phenotype may in turn spread throughout the population and increase its resistance against the stressing factor.