The genus Amphiprion includes about 29 species of marine clownfish, also called anemone fish. This genus, along with the maroon clownfish, (Premnas biaculeatus, the only species in its genus), make up the clownfish subfamily Amphiprioninae within the damselfish family Pomacentridae. Found in shallow lagoons and reefs in the Indo-Pacific, all clownfish species form close mutualistic relationships with about 10 poison-producing anemone species, especially those in the anemone genera Heteractis and Stichodactyla, and species Entacmaea quadricolor. Anemonefish have a preferred anemone host species, but most will live with several different anemones. Living their whole lives within an anemone's tentacles, anemonefish provide protection from predators, possibly nutrients through their fecal matter, and help to remove parasites. Their bright coloration may also lure foodfish to the anemone, and their movements increase water circulation around the anemone. Anemonefish secrete protective mucus that allows them to survive anemone poisons, and gain the benefit of a safe home. Omnivores, anemonefish eat zooplankton and algae. Anemonefish live in hierarchical groups and are serial hermaphrodites: all start off life as males, then later reverse sex to become breeding females. As males, they protect the eggs and larvae, which are laid at the base of their host anemone. Many clownfish species are bred in captivity, and are popular fishes for marine aquaria.
(Fautin and Allen 1992; Wikipedia 2012; Zubi 2009)
Water temperature and chemistry ranges based on 143 samples.
Depth range (m): 0.305 - 66.9273
Temperature range (°C): 24.508 - 29.282
Nitrate (umol/L): 0.016 - 3.114
Salinity (PPS): 32.019 - 37.566
Oxygen (ml/l): 4.149 - 4.802
Phosphate (umol/l): 0.071 - 0.546
Silicate (umol/l): 0.506 - 6.058
Depth range (m): 0.305 - 66.9273
Temperature range (°C): 24.508 - 29.282
Nitrate (umol/L): 0.016 - 3.114
Salinity (PPS): 32.019 - 37.566
Oxygen (ml/l): 4.149 - 4.802
Phosphate (umol/l): 0.071 - 0.546
Silicate (umol/l): 0.506 - 6.058
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.
Evolution and Systematics
Clownfish and anemones gain protection from predators thanks to their mutualistic relationship.
"For protection, clownfish seek refuge amongst the tentacles of sea anemones. The tentacles contain harpoon-like stinging capsules called nematocysts that the anemones employ to capture prey and ward off predators.
In a yet-to-be resolved biological mystery, clownfish have mucus on their skin that somehow protects them against the sting of their host anemone. As a result, the clownfish are able to stick near their host which is avoided by most other fish in the sea.
'The clownfish gets protection by hiding sting-free among the tentacles. If you remove the clownfish, large butterfly fishes will eat the anemone,' said John Randall, an ichthyologist at the University of Hawaii at Manoa.
Butterfly fish are predators of the sea anemone. In certain areas of the tropics where clownfish, sea anemone, and butterfly fish exist, clownfish scare off butterflyfish from their host anemone. Research has shown that if the clownfish are removed from the anemone, butterfly fish will move in and devour the anemone. So, the protection of the anemone afforded by the clownfish is part of the mutual relationship.
In addition to scaring off predators, some scientists speculate that clownfish waste may serve as a nutrient for the anemones…
There are more than 1,000 species of sea anemones found throughout the world's oceans. Only ten of these species share their niche with clownfish, which thrive in the tropical waters of the Indian and Pacific oceans.
Each individual host anemone is home to one group of clownfish, which contain a dominant breeding pair and up to four smaller, subordinate fish. There are 28 known species of clownfish, so more than one species of clownfish may take to any given species of anemone." (Roach 2003)
Learn more about this functional adaptation.
The skin of clownfish is protected from sea anemone stings by a coating of mucus.
"The mystery behind the clownfish and sea anemone relationship is how the clownfish avoids being stung and killed by its host anemone. Of the numerous theories that have been presented over the years to explain this relationship, the focus is now on a layer of mucus that coats the clownfish. 'The fish are not immune to being stung,' said [Daphne] Fautin. 'But their mucus coat protects them. The debate is the source of the mucus.' One theory holds that the fish produce the mucus themselves and that it contains chemicals that prevent the anemone nematocysts from stinging as they do other fish in the sea. The other theory is that the clownfish rub themselves against the anemone tentacles in elaborate dances, smearing anemone mucus over themselves. This coating tricks the anemone into confusing the fish for itself. 'There is evidence for both,' said Fautin. 'And since there is a wide variety of anemone hosts, and 28 species of fish, I am convinced these views present two ends of a spectrum, and a combination is probably true for many.'" (Roach 2003)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:273
Specimens with Barcodes:230
Species With Barcodes:21
Statistics of barcoding coverage: Amphiprion cf. chrysopterus
Public Records: 0
Specimens with Barcodes: 3
Species With Barcodes: 1
Fishes in the pomacentrid subfamily Amphiprioninae, most of which are in the genus Amphiprion, are known as clownfishes or anemonefishes. As a result of their close associations with anemones clownfishes are indirectly threatened by any factors that cause populations of their anemone associates to decline. Clownfishes often occur with anemones living on coral reefs. Because coral reefs have been seriously and negatively impacted by global climate change, populations of reef dwellers such as clownfishes are expected to suffer as well (IUCN 2009). Globally, coral reefs are seriously declining as a result of mass bleaching, ocean acidification, and other environmental impacts. In 1998, one of the most severe global coral bleaching events in recorded history led to the complete disappearance of several anemone species used by clownfishes in the corals reefs around Sesoko Island, Japan, causing local clownfish population declines (Arvedlund and Takemura 2005 and references therein).
The acidification of the oceans as they absorb more carbon dioxide due to increasing atmospheric carbon dioxide levels poses an interesting and potentially serious threat to clownfishes. Recent studies have indicated that lowering the pH of seawater (i.e., making it more acidic) may disrupt the ability of larval clownfish to detect the chemical signals necessary for recognizing and locating appropriate reef sites and anemone hosts (Munday et al. 2009).
Increasing water temperatures pose additional direct threats to clownfishes. Juvenile clownfish have been shown to develop faster as water temperatures increase (assuming sufficient food is available). This may yield immediate benefits to individuals, such as earlier reproduction, but more rapid growth would likely also mean that individuals would disperse shorter distances from their parents’ anemone before their developmental stage triggers the instinct to find their own anemone. The resulting decrease in dispersal distance could mean greater competition for local dwelling places, greater likelihood of predation, and increased inbreeding. A further threat to clownfishes associated with warming ocean temperatures is that they are known to reproduce only within a very narrow temperature range and an increase in temperature could therefore disrupt breeding. High temperatures have also been shown to reduce egg survival (IUCN 2009).
It is possible that clownfishes might be able to adapt physiologically and/or behaviorally to cope with the new conditions produced by global climate change. For example, one species of clownfish has recently been shown to use soft corals as an alternative habitat, something previously observed only in captivity (Arvedlund and Takemura 2005). It remains unknown, however, to what degree clownfishes can adapt to their changing environment and, in particular, whether they will be able to adapt sufficiently quickly to match the rapidly changing environmental conditions that lie ahead.
Both clownfishes and their sea-anemone hosts face other threats, unrelated to climate change, as well. Both are highly sought after for the aquarium trade and have become increasingly popular targets for collection. However, reef destruction and degradation due to human activities, including global climate modification, remain the greatest threat at present (IUCN 2009).
Clownfish or Anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species are recognized, one in the genus Premnas, while the remaining are in the genus Amphiprion. In the wild, they all form symbiotic mutualisms with sea anemones. Depending on species, Clownfish are overall yellow, orange, or a reddish or blackish color, and many show white bars or patches. The largest can reach a length of 18 centimetres (7.1 in), while the smallest barely can reach 10 centimetres (3.9 in).
Distribution and habitat
Clownfish are native to warmer waters of the Indian and Pacific oceans, including the Great Barrier Reef and the Red Sea. While most species have restricted distributions, others are distributed elsewhere. Clownfish live at the bottom of shallow seas in shallow reefs or lagoons. There are no clownfish in the Atlantic.
Clownfish are omnivorous and can feed on undigested food from their host anemones, and the fecal matter from the clownfish provides nutrients to the sea anemone. Clownfish primarily feed on small zooplankton from the water column, such as copepods and tunicate larvae, with a small portion of their diet coming from algae, with the exception of Amphiprion perideraion, which primarily feeds on algae. They may also consume the tentacles of their host anemone.
Symbiosis and mutualism
Clownfish and sea anemones have a symbiotic, mutualistic relationship, each providing a number of benefits to the other. The individual species are generally highly host specific, and especially the genera Heteractis and Stichodactyla, and the species Entacmaea quadricolor are frequent clownfish partners. The sea anemone protects the clownfish from predators, as well as providing food through the scraps left from the anemone's meals and occasional dead anemone tentacles. In return, the clownfish defends the anemone from its predators, and parasites. The anemone also picks up nutrients from the clownfish's excrement, and functions as a safe nest site. The nitrogen excreted from clownfish increases the amount of algae incorporated into the tissue of their hosts, which aids the anemone in tissue growth and regeneration. It has been theorized that the clownfish use their bright coloring to lure small fish to the anemone, and that the activity of the clownfish results in greater water circulation around the sea anemone. Studies on anemonefish have found that clownfish alter the flow of water around sea anemone tentacles by certain behaviours and movements such as "wedging" and "switching." Aeration of the host anemone tentacles allows for benefits to the metabolism of both partners, mainly by increasing anemone body size and both clownfish and anemone respiration.
There are several theories about how they can survive the sea anemone poison:
- The mucus coating of the fish may be based on sugars rather than proteins. This would mean that anemones fail to recognize the fish as a potential food source and do not fire their nematocysts, or sting organelles.
- The coevolution of certain species of clownfish with specific anemone host species and may have acquired an immunity to the nematocysts and toxins of their host anemone. Experimentation has shown that Amphiprion percula may develop resistance to the toxin from Heteractis magnifica, but it is not totally protected, since it was shown experimentally to die when its skin, devoid of mucus, was exposed to the nematocysts of its host.
Clownfish are the best known example of a fish that are able to live among the venomous sea anemone tentacles, but there are several others, including juvenile threespot dascyllus, certain cardinalfish (such as Banggai cardinalfish), Bucchich's (or anemone) goby and juvenile painted greenling.
In a group of clownfish, there is a strict dominance hierarchy. The largest and most aggressive female is found at the top. Only two clownfish, a male and a female, in a group reproduce through external fertilization. Clownfish are sequential hermaphrodites, meaning that they develop into males first, and when they mature, they become females. If the female clownfish is removed from the group, such as by death, one of the largest and most dominant males will become a female. The remaining males will move up a rank in the hierarchy.
Clownfish lay eggs on any flat surface close to their host anemones. In the wild, clownfish spawn around the time of the full moon. Depending on the species, clownfish can lay hundreds or thousands of eggs. The male parent guards the eggs until they hatch about six to ten days later, typically two hours after dusk.
Most clownfish are protandrous hermaphrodites, meaning they alternate between the male and female sexes at some point in their lives. Anemonefish colonies usually consist of the reproductive male and female and a few juveniles, who help tend the colony. Although multiple males co-habit an environment with a single female, polygamy does not occur and only the adult pair exhibit reproductive behavior. However, if the largest female dies, the social hierarchy shifts with the breeding male exhibiting protandrous sex reversal to become the breeding female. The largest juvenile will then become the new breeding male after a period of rapid growth. The existence of protandry in clownfish may rest on the case that non-breeders modulate their phenotype in a way that causes breeders to tolerate them. This strategy prevents conflict by reducing competition between the males for one female. For example, by purposefully modifying their growth rate to remain small and submissive, the juveniles in a colony present no threat to the fitness of the adult male, thereby protecting themselves from being evicted by the dominant fish.
The reproductive cycle of clownfish is often correlated with the lunar cycle. Rates of spawning for clownfish peak at approximately the first and third quarters of the moon. The timing of this spawn means that the eggs will hatch around the full moon or new moon periods. One explanation for this lunar clock is that spring tides produce the highest tides during full or new moons. Nocturnal hatching during high tide may reduce predation by allowing for a greater capacity for escape. Namely, the stronger currents and greater water volume during high tide protects the hatchlings by effectively sweeping them to safety. Before spawning, clownfish exhibit increased rates of anemone and substrate biting, which help prepare and clean the nest for the spawn.
In terms of parental care, male clownfish are often the caretakers of eggs. Before making the clutch, the parents often clear an oval sized clutch varying in diameter for the spawn. Fecundity, or reproductive rate, of the females usually ranges from 600 to 1500 eggs depending on the size of the female. In contrast to most animal species, the female only occasionally takes responsibility for the eggs, with males expending most of the time and effort. Male clownfish care for their eggs by fanning and guarding them for 6 to 10 days until they hatch. Studies have shown that, in general, eggs develop more rapidly in a clutch when males fanned properly and that fanning represents a crucial mechanism of successfully developing eggs. This suggests that males have the ability to control the success of hatching an egg clutch by investing different amounts of time and energy towards the eggs. For example, a male could choose to fan less in times of scarcity or fan more in times of abundance. Furthermore, males display increased alertness when guarding more valuable broods, or eggs in which paternity was guaranteed. Females, on the other hand, display generally less preference for parental behavior than males. All these suggest that males have increased parental investment towards the eggs compared to females.
In the aquarium
Clownfish make up 43% of the global marine ornamental trade, and 25% of the global trade comes from fish bred in captivity, while the majority are captured from the wild, accounting for decreased densities in exploited areas. Public aquaria and captive breeding programs are essential to sustain their trade as marine ornamentals, and has recently become economically feasible. It is one of a handful of marine ornamentals whose complete life cycle has been closed in captivity. Members of some clownfish species, such as the maroon clownfish, become aggressive in captivity; others, like the false percula clownfish, can be kept successfully with other individuals of the same species.
When a sea anemone is not available in an aquarium, the clownfish may settle in some varieties of soft corals, or large polyp stony corals. Once an anemone or coral has been adopted, the clownfish will defend it. As there is less pressure to forage for food in an aquarium, it is common for clownfish to remain within 2-4 inches of their host for their entire lifetime. Clownfish, however, are not obligately tied to hosts, and can survive alone in captivity.
- Genus Amphiprion:
- Amphiprion akallopisos – Skunk clownfish
- Amphiprion akindynos – Barrier reef anemonefish
- Amphiprion allardi – Twobar anemonefish
- Amphiprion barberi
- Amphiprion bicinctus – Twoband anemonefish
- Amphiprion chagosensis – Chagos anemonefish
- Amphiprion chrysogaster – Mauritian anemonefish
- Amphiprion chrysopterus – Orange-fin anemonefish
- Amphiprion clarkii – Yellowtail clownfish
- Amphiprion ephippium – Saddle anemonefish
- Amphiprion frenatus – Tomato clownfish
- Amphiprion fuscocaudatus – Seychelles anemonefish
- Amphiprion latezonatus – Wide-band Anemonefish
- Amphiprion latifasciatus – Madagascar anemonefish
- Amphiprion leucokranos – Whitebonnet anemonefish
- Amphiprion mccullochi – Whitesnout anemonefish
- Amphiprion melanopus – Fire clownfish
- Amphiprion nigripes – Maldive anemonefish
- Amphiprion ocellaris – Clown anemonefish
- Amphiprion omanensis – Oman anemonefish
- Amphiprion pacificus – Pacific anemonefish
- Amphiprion percula – Orange clownfish
- Amphiprion perideraion – Pink skunk clownfish
- Amphiprion polymnus – Saddleback clownfish
- Amphiprion rubrocinctus – Red Anemonefish
- Amphiprion sandaracinos – Yellow clownfish
- Amphiprion sebae – Sebae anemonefish
- Amphiprion thiellei – Thielle's anemonefish
- Amphiprion tricinctus – Three-band anemonefish
- Genus Premnas:
- Premnas biaculeatus – Maroon clownfish
In popular culture
Allard's clownfish (Amphiprion allardi).
Pink skunk clownfish (Amphiprion perideraion).
Yellow clownfish (Amphiprion sandaracinos) and sea anemone off Sulawesi, Indonesia.
Yellowtail clownfish (Amphiprion clarkii) with sea anemone.
Orange-fin anemonefish (Amphiprion chrysopterus) is one of the few anemonefish with a white tail.
- Fautin, Daphne; Gerald Allen (1997). Field Guide to Anemone Fishes and Their Host Sea Anemones (2 ed.). Perth, Australia: Western Australian Museum. ISBN 978-0-7309-8365-1.
- Porat, D.; Chadwick-Furman, N.E. (2005). "Effects of anemonefish on giant sea anemones: Ammonium uptake, zooxanthella content and tissue regeneration". Marine and Freshwater Behaviour and Physiology 29 (1): 43–51. doi:10.1080/10236240500057929.
- Fautin, D.G.; Guo, C.; Hwang, J.S. (1995). "Costs and benefits of the symbiosis between the anemoneshrimp Periclimenes brevicarpalis and its host Entacmaea quadricolor.". Marine Ecology Progress Series 129: 77–84. doi:10.3354/meps129077.
- "Clown Anemonefish". Nat Geo Wild : Animals. National Geographic Society. Retrieved 2011-12-19.
- Amphiprioninae at the Encyclopedia of Life
- Holbrook, S. J. and Schmitt,R. J. Growth, reproduction and survival of a tropical sea anemone (Actiniaria): benefits of hosting anemonefish, 2005, cited in 
- "Clown Anemonefishes, Amphiprion ocellaris". Marinebio. The MarineBio Conservation Society. Retrieved 2011-12-19.
- Szczebak, Joseph T.; Raymond P. Henry; Fuad A. Al-Horani; Nanette E. Chadwick (2013). "Anemonefish oxygenate their anemone hosts at night". Journal of Experimental Biology 216 (9): 970–976. doi:10.1242/jeb.075648.
- Joseph T. Szczebak, Raymond P. Henry, Fuad A. Al-Horani, Nanette E. Chadwick (2012-11-03). "Anemonefish oxygenate their anemone hosts at night". The Journal of Experimental Biology. Retrieved 2013-09-15.
- Mebs, D. 1994. "Anemonefish symbiosis: Vulnerability and Resistance of Fish to the Toxin of the Sea Anemone." Toxicon. Vol. 32(9):1059–1068.
- Lieske, E.; and R. Myers (1999). Coral Reef Fishes. ISBN 0-691-00481-1
- Debelius, H. (1997). Mediterranean and Atlantic Fish Guide. ISBN 978-3925919541
- Fretwell, K.; and B. Starzomski (2014). Painted greenling. Biodiversity of the Central Coast. Retrieved 29 January 2015.
- Stephanie Boyer. "Clown Anemofish". Florida Museum of Natural History. Retrieved 2013-09-15.
- Robert M. Ross (1978-02-10). "Reproductive Behavior of the Anemonefish Amphiprion melanopus on Guam". Copeia. Retrieved 2013-09-15.
- Peter Buston (2004-08-18). "Does the presence of non-breeders enhance the fitness of breeders? An experimental analysis in the clown anemonefish Amphiprion percula". Springer-Verlag. Retrieved 2013-09-15.
- Swagat Ghosh, T. T. Ajith Kumar, T. Balasubramanian (2011-08-04). "Determining the level of parental care relating fanning behavior of five species of clownfishes in captivity". Indian Journal of Geo-Marine Sciences. Retrieved 2013-09-15.
- Dhaneesh, K.V.; R. Vinoth; Swagat Gosh; M. Gopi; T.T. Ajith Kumar; T. Balasubramanian (2013). Sundaresan, J., ed. "Hatchery Production of Marine Ornamental Fishes: An Alternate Livelihood Option for the Island Community at Lakshadweep". Climate Change and Island and Coastal Vulnerability (Capital Publishing Company) 17: 253–265. doi:10.1007/978-94-007-6016-5_17.
- Taylor, M., Green, E. and Razak, T. (2003). From ocean to aquarium: A global trade in marine ornamental species. UNEP world conservation and monitoring centre (WCMC). pp. 1–64. Retrieved 18 April 2013.
- Shuman, Craig; Gregor Hodgson; Richard F. Ambrose (2005). "Population impacts of collecting sea anemones and anemoneﬁsh for the marine aquarium trade in the Philippines". Coral Reefs 24: 564–573. doi:10.1007/s00338-005-0027-z.
- Watson, Craig; Jeffery Hill (2006). "Design criteria for recirculating, marine ornamental production systems". Aquacultural Engineering 34 (3): 157–162. doi:10.1016/j.aquaeng.2005.07.002.
- Hall, Heather; Douglas Warmolts (2003). "23". In James C. Cato, Christopher L. Brown. Marine Ornamental Species: Collection, Culture and Conservation. Wiley-Blackwell. pp. 303–326. ISBN 978-0-8138-2987-6.
- Daphne Gail Fautin (1991). "The anemonefish symbiosis: what is known and what is not" (PDF). Symbiosis 10: 23–46.
- Ronald L. Shimek (2004). Marine Invertebrates. Neptune City, NJ: T.F.H. Publications. p. 83. ISBN 978-1-890087-66-1.
- Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Amphiprion in FishBase. December 2011 version.
- Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Premnas in FishBase. December 2011 version.
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Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods and plants. Here, the individual is born one sex and changes sex at some point in their life. They can change from a male to female (protandry), or from female to male (protogyny). Despite which sex the organism changes to, those that change gonodal sex can have both female and male germ cells in the gonads or can change from one complete gonodal type to the other during their last life stage.
Protandry refers to organisms that are born male and at some point in their lifespan change sex to female. Protandrous animals include clownfish. Clownfish have a very structured society. In the Amphiprion percula species, there are zero to four individuals excluded from breeding and a breeding pair living in a sea anemone. Dominance is based on size, the female being the largest and the male being the second largest. The rest of the group is made up of progressively smaller non-breeders, which have no functioning gonads. If the female dies, the male gains weight and becomes the female for that group. The largest non-breeding fish then sexually matures and becomes the male of the group.
Other examples of protandrous animals include:
- The ctenophore Coeloplana gonoctena. In this organism the females are bigger than the males and are only found during the summer. In contrast males are found year round.
- The flatworms Hymanella retenuova and Paravortex cardii.
- Laevapex fuscus, a gastropod, is described as being functionally protandric. The sperm matures in late winter and early spring, and the eggs mature in early summer, and copulation occurs only in June. This shows that males cannot reproduce until the females appear, thus why they are considered to be functionally protandric.
Protogyny refers to organisms that are born female and at some point in their lifespan change sex to males. Common model organisms for this type of sequential hermaphroditism are wrasses. They are one of the largest families of coral reef fish and belong to the Labridae family. Wrasses are found around the world in all marine habitats and tend to bury themselves in sand at night or when they feel threatened. In wrasses, the larger of the two fish is the male, while the smaller is the female. In most cases, females and immature have a uniform color while the male has the terminal bicolored phase. Large males hold territories and try to pair spawn while small to mid-size initial-phase males live with females and group spawn. In other words, both the initial and terminal phase males can breed; they differ however in the way they do it.
In the California Sheephead (Pimelometopon pulchrum), a type of wrasse, when the female changes to male, the ovaries degenerate and spermatogenic crypts appear in the gonads. The general structure of the gonads remains ovarian after the transformation and the sperm is transported through a series of ducts on the periphery of the gonad and oviduct. Here sex change is age dependant. For example, the California sheephead stays a female for four years before changing sex.
Other examples of protogynous organisms include:
- The isopods Cyathura polita and C. carinata
- The tanaidacean Heterotanais oerstedi.
- The echinoderms, Asterina pancerii and A. gibbosa are also protogynous and they brood their young.
- Protogyny sometimes occurs in the frog Rana temporaria, where old females sometimes change to males.
Ghiselin proposed three models for hermaphroditism in 1969 in his paper titled “The evolution of hermaphroditism among animals”. The ‘’low-density model’’ states that individuals have characteristics that reduce the opportunity for mating; this model cannot be applied to sequential hermaphroditism. The ‘’gene dispersal model’’ is based on the idea that limitations on dispersal may influence population structure or genetical environment and it can be separated into two versions: the inbreeding version and the sampling-error version. This theory of gene dispersal can be applied to sequential hermaphrodites, especially the inbreeding version. The inbreeding version is based upon the fact that both protandry and protogyny help prevent inbreeding in plants and thus one can make the same assumption that in animals it works by reducing the probability of this occurring among siblings. The sampling-error version is based on the reality that the genetical environment is influenced by genetic drift and similar phenomena in small populations. The two aspects of these hypotheses influenced by hermaphroditism, that is inbreeding and sampling-error, result in the same thing, reduction of genetic variability. In other words being a hermaphrodite would increase genetic variability and thus be considered advantageous to the organism. This theory of gene dispersal can be applied to sequential hermaphrodites, especially the inbreeding version. Lastly, the ‘’size-advantage model’’ states that reproductive functions are carried out better if the individual is a certain size/age. Assuming that the reproductive functions of one sex are better performed at a certain size, then an organism would assume the sex that its size allows to perform the best. This would increase its reproductive potential and fitness. For example, eggs are larger than sperm, thus if you are a big you are able to make more eggs so being female when big is advantageous, however the size advantage relationship is really not as simple as the example just mentioned, but it allows for a better understanding of it.
In most ectotherms body size and female fecundity are positively correlated. This supports Ghiselin’s size-advantage model, which is still widely accepted today. Kazancioglu and Alonzo (2010) performed the first comparative analysis of sex change in Labridae. Their analysis supports the size-advantage model by Ghiselin and suggest that sequential hermaphroditism is correlated to the size-advantage. They determined that dioecy was less likely to occur when the size advantage is stronger than other advantages
Warner suggests that selection for protandry may occur in populations where female fecundity is augmented with age and individuals mate randomly. Selection for protogyny may occur where there are traits in the population that depress male fecundity at early ages (territoriality, mate selection or inexperience) and when female fecundity is decreased with age, the latter seems to be rare in the field. An example of territoriality favoring protogyny occurs when there is a need to protect their habitat and being a large male is advantageous for this purpose. In the mating aspect, a large male has a higher chance of mating, while this has no effect on the female mating fitness. Thus, he suggests that female fecundity has more impact on sequential hermaphroditism that the age structures of the population.
The size-advantage model predicts that sex change would only be absent if the relationship between size/age with reproductive potential is identical in both sexes. With this prediction one would assume that hermaphroditism is very common, but this is not the case. Sequential hermaphroditism is very rare and according to scientists this is due to some cost that decreases fitness in sex changers as opposed to those who don’t change sex. Kazanciglu and Alonzo confirmed this in 2009. They found that the costs of changing sex only favored dioecy when the cost was very large but that some groups favored hermaphroditism. This indicates that the cost of sex change does not explain the rarity of sequential hermaphroditism by itself.
Many studies have focused on the proximate causes of sequential hermaphroditism. The role of aromatase has been widely studied in this area. Aromatase is an enzyme that controls the androgen/estrogen ratio in animals by catalyzing the conversion of testosterone into oestradiol, which is irreversible. It has been discovered that the aromatase pathway mediates sex change in both directions. Many studies also involve understanding the effect of aromatase inhibitors on sex change. One such study was performed by Kobayashi et al. In their study they tested the role of estrogens in male three-spot wrasses (‘’Halichoeres trimaculatus’’). They discovered that fish treated with aromatase inhibitors showed decreased gonodal weight, plasma estrogen level and spermatogonial proliferation in the testis as well as increased androgen levels. Their results suggest that estrogens are important in the regulation of spermatogenesis in this protogynous hermaphrodite.
In the context of the plant sexuality of flowering plants (angiosperms), there are two forms of dichogamy: protogyny—female function precedes male function—and protandry—male function precedes female function.
Historically, dichogamy has been regarded as a mechanism for reducing inbreeding (e.g., Darwin, 1862). However, a survey of the angiosperms found that self-incompatible (SI) plants, which are incapable of inbreeding, were as likely to be dichogamous as were self-compatible (SC) plants (Bertin, 1993). This finding led to a reinterpretation of dichogamy as a more general mechanism for reducing the impact of pollen-pistil interference on pollen import and export (reviewed in Lloyd & Webb, 1986; Barrett, 2002). Unlike the inbreeding-avoidance hypothesis, which focused on female function, this interference-avoidance hypothesis considers both gender functions.
In many hermaphroditic species, the close physical proximity of anthers and stigma makes interference unavoidable, either within a flower or between flowers on an inflorescence. Within-flower interference, which occurs when either the pistil interrupts pollen removal or the anthers prevent pollen deposition, can result in autonomous or facilitated self-pollination (Lloyd & Webb, 1986; Lloyd & Schoen, 1992). Between-flower interference results from similar mechanisms, except that the interfering structures occur on different flowers within the same inflorescence and it requires pollinator activity. This results in geitonogamous pollination, the transfer of pollen between flowers of the same individual (Lloyd & Schoen, 1992; de Jong et al., 1993). In contrast to within-flower interference, geitonogamy necessarily involves the same processes as outcrossing: pollinator attraction, reward provisioning, and pollen removal. Therefore, between-flower interference not only carries the cost of self-fertilization (inbreeding depression; Charlesworth & Charlesworth, 1987; Husband & Schemske, 1996), but also reduces the amount of pollen available for export (so-called "pollen discounting"; Harder & Wilson, 1998]). Because pollen discounting diminishes outcross siring success, interference avoidance may be an important evolutionary force in floral biology (Harder & Barrett, 1995, 1996; Harder & Wilson, 1998; Barrett, 2002).
Dichogamy may reduce between-flower interference by minimizing the temporal overlap between stigma and anthers within an inflorescence. Large inflorescences attract more pollinators, potentially enhancing reproductive success by increasing pollen import and export (Schemske, 1980; Queller, 1983; Bell, 1985; Geber, 1985; Schmid-Hempel & Speiser, 1988; Klinkhamer & de Jong, 1990). However, large inflorescences also increase the opportunities for both geitonogamy and pollen discounting, so that the opportunity for between-flower interference increases with inflorescence size (Harder & Barrett, 1996). Consequently, the evolution of floral display size may represent a compromise between maximizing pollinator visitation and minimizing geitonogamy and pollen discounting (Klinkhamer & de Jong, 1993; Barrett et al., 1994; Holsinger, 1996; Snow et al., 1996).
Protandry may be particularly relevant to this compromise, because it often results in an inflorescence structure with female phase flowers positioned below male phase flowers (Bertin & Newman, 1993). Given the tendency of many insect pollinators to forage upwards through inflorescences (Galen & Plowright, 1988), protandry may enhance pollen export by reducing between-flower interference (Darwin, 1862; Harder et al., 2000). Furthermore, this enhanced pollen export should increase as floral display size increases, because between-flower interference should increase with floral display size. These effects of protandry on between-flower interference may decouple the benefits of large inflorescences from the consequences of geitonogamy and pollen discounting. Such a decoupling would provide a significant reproductive advantage through increased pollinator visitation and siring success.
Harder et al. (2000) demonstrated experimentally that dichogamy both reduced rates of self-fertilization and enhanced outcross siring success through reductions in geitonogamy and pollen discounting, respectively. Routley & Husband (2003) examined the influence of inflorescence size on this siring advantage and found a bimodal distribution with increased siring success with both small and large display sizes.
The length of stigmatic receptivity plays a key role in regulating the isolation of the male and female stages in dichogamous plants, and stigmatic receptivity can be influenced by both temperature and humidity. Another study by Jersakova and Johnson, studied the effects of protandry on the pollination process of the moth pollinated orchid, ‘’Satyrium longicauda’’. They discovered that protandry tended to reduce the absolute levels of self-pollination and suggest that the evolution of protandry could be driven by the consequences of the pollination process for male mating success. Another study that indicated that dichogamy might increase male pollination success was the study performed by Dai and Galloway.
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- Plant sexuality
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