Siamese fighting fish

Due to their popularity, Siamese fighting fish are highly abundant in captivity. However, wild specimens are categorised by the IUCN as vulnerable, indicating the species is likely to become endangered without conservation efforts.[18] The primary threats are habitat destruction and pollution, caused by urban and agricultural development across central Thailand.[18]

Some people in Thailand and Malaysia are known to have collected wild bettas at least by the 19th century, observing their aggressive nature and pitting them against each other in gambling matches akin to cockfights. In the wild, betta spar for only a few minutes before one fish retreats; domesticated betta, bred specifically for heightened aggression, can engage for much longer, with winners determined by a willingness to continue fighting; once a fish retreats, the match is over. Fights to the death were rare, so bets were placed on the bravery of the fish rather than its survival.[27]

The popularity of these fights garnered the attention of king Rama III of Thailand, who regulated and taxed the matches, and collected fighting fish of his own. In 1840, he gave some of his prized fish to Danish physician Theodore Edward Cantor, who worked in the Bengal medical service.[26] Nine years later, Cantor published the first recorded article describing these fish, giving them the name Macropodus pugnax. In 1909, British ichthyologist Charles Tate Regan found there was a related species already named Macropodus pugnax, and thus renamed the domesticated Siamese fighting fish, Betta splendens, or "splendid fighter".[28]

Betta splendens first entered the Western aquarium trade in the late 19th century; the earliest known arrival is 1874 in France, when French aquaria expert and ichthyologist Pierre Carbonnier began importing and breeding several specimens. In 1896, German tropical fish expert Paul Matte brought the first specimens into Germany from Moscow, most likely from the strain developed by Carbonnier.[26] This indicates bettas were already somewhat established in France and Russia by the turn of the 20th century. Fighting fish were also present in Australia by 1904, based on an article written by British-born zoologist Edgar Ravenswood Waite and published by the Australian Museum in Sydney.[26] Waite indicates that Australian specimens were brought from Penang, Malaysia, near the border with Thailand.[26] He also makes reference to two articles about "fighting fish" published by Carbonnier in 1874 and 1881. Bettas may have first entered the United States in 1910, via importers in California; there is also evidence they were imported in 1927 from Cambodia.[26]

While it is unclear when bettas became popular in the aquarium trade, the early 20th century marked the first known departure from centuries of breeding bettas for aggression, to instead selecting for color, finnage, and overall beauty for ornamental purposes.[26] In 1927, an article was published in Germany describing the long, flowing fins of the "veiltail" breed, which indicates an emphasis on aesthetic beauty.[29] In the 1950s, an American breeder created a larger and longer-finned veiltail, while around 1960, Indian breeders discovered a genetic mutation that allowed for two caudal fins, producing the "doubletail" variety. Within that decade, a German breeder created the "deltatail" characterized by its broader, triangular fins.[30]

In 1967, a group of betta breeders formed the International Betta Congress (IBC), the first formal interest group dedicated to Siamese fighting fish. The IBC aimed to breed varieties that would be healthier and more symmetrical in fins and body shape, with an emphasis on animal welfare.[30]

In January 2014, a large population of bettas was discovered in the Adelaide River Floodplain in the Northern Territory, Australia.[31] As an invasive species they pose a threat to native fish, frogs and other wetland wildlife.[31] Bettas have also become established in subtropical areas of the United States, namely southern Texas and Florida, although an assessment by the U.S. Fish and Wildlife Service determined they were no threat to natural ecosystems.[32]

As tropical fish, bettas prefer a water temperature of around 75–82 °F (24–28 °C), but have been observed surviving temporarily at extremes of 56 °F (13 °C) to 95 °F (35 °C). When kept in colder climates, aquarium heaters are recommended, as colder water weakens their immune system and makes them susceptible to certain diseases.[33]

Bettas are also affected by the pH of the water: a neutral pH of 7.0 is ideal, but slightly higher levels are tolerable.[34] Due to their labyrinth organ, bettas can endure low oxygen levels, but cannot survive for long in unmaintained aquaria, as poor water quality makes all tropical fish more susceptible to diseases like fin rot.[35] Thus, notwithstanding the betta's well known tolerance of still water, a mechanical filter is considered necessary for their long-term health and longevity.[36][37][38][39] Similarly, live aquatic plants provide a supplemental source of filtration, in addition to crucial enrichment to the betta.[37]

Despite frequently being displayed and sold in small containers in the pet trade, bettas do best in larger environments; while they can survive in cups, bowls, and other confined spaces, they will be happier, healthier, and longer-lived in a larger surface area.[36][37][38] Although some betta enthusiasts claim there is a minimum tank size, determining a strict baseline is somewhat arbitrary and subject to debate.[40] The general consensus is that the ideal tank should be no less than 9–19 litres (3–5 US gallons), though a tank of just 4 litres (1 gallon) can also suffice if it is cleaned regularly—at least every other day for small, unfiltered tanks—and maintained at an acceptable temperature of 24–26°C (75–78°F).[41]

Although males bettas are solitary and aggressive towards one another, they can generally cohabit with many types of fish if there is adequate space and hiding places. However, compatibility varies based on the temperament of the individual betta, and it is advised to carefully supervise the betta's interaction with other fish. Tankmates must be tropical, communal, and nonterritorial; coldwater fish like goldfish have incompatible temperature requirements, while aggressive and predatory fish are likely to nip at the betta's fins or erode their slime coat.[42] Species that shoal, such as tetras and danios, are considered most ideal, since they usually keep to themselves and can endure the territorial nature of bettas with their numbers.[42] Brightly coloured fish with large fins, such as male guppies, should be avoided, as they may invite fin nipping by the male betta. Potential tankmates should usually be added before the male betta so they can establish their respective territories beforehand, rather than compete with the betta.

Female bettas are less aggressive and territorial than males, and thus can live with a greater variety of fish; for example, brightly coloured or large-finned fish will not usually disturb a female. Generally, female fighting fish can also tolerate larger or more numerous tankmates than males.[42][43] However, like male bettas, a female's tolerance of other fish will vary by individual temperament.

It is not recommended to keep male and female bettas together, except temporarily for breeding purposes, which should always be undertaken with caution and supervision.

Bettas are fairly intelligent and inquisitive, and thus require stimulation; otherwise they can become bored and depressed, leading to lethargy and a weaker immune system.[44][45] Decorations such as plastic or live plants, rocks, caves, driftwood, and other ornaments provide crucial enrichment—provided they do not have rough textures or jagged edges, which can damage the delicate fins. In the wild, Siamese fighting fish spend most of their time concealing themselves under floating debris or overhanging plants to avoid potential predators.[46] Floating plants and leaves can help bettas feel more secure, while also giving males an anchor from which to build their bubble nests.[46] Abundant vegetation of any kind is generally recommended to provide maximum security and to cater to the betta's instinct to claim protective territory.

Indian almond leaves are increasingly popular for providing something closer to the natural foliage under which bettas would hide in the wild. The leaves allegedly confer several health benefits through their tannins,[47] and are used to treat certain ailments, such as swim bladder disease, and stabilize the pH of the water.[48]

When properly kept and fed a correct diet, Siamese fighting fish generally live between three and five years in captivity, though in rare cases may live as long as seven to ten years.[49] One study found that bettas kept in tanks of several gallons and provided with proper nutrition and "exercise"—in the form of being chased around by a stick for a short period—lived over nine years; by contrast, a control group of bettas confined to small jars lived far fewer years.[50] A larger tank with proper filtration, regular maintenance, and an abundance of decor and hiding spaces, along with a rich, protein-based diet, increases the likelihood of a long lifepsan.

Like all tropical fish in captivity, bettas are susceptible to several kinds of diseases, usually caused by bacterial, fungal, or parasitic infections. Most illnesses result from poor water quality and cold water, both of which weaken the immune system.[51] The four most common illnesses are white spot, velvet, fin rot, and dropsy; with the exception of the latter, which is incurable, these ailments can be treated with a combination of over-the-counter fish medication, increased water temperature, and/or regular water changes.[52]

Over a century of intensive selective breeding has produced a wide variety of colors and fin types, and breeders around the world continue to develop new varieties.[53] Often, the males of the species are sold preferentially in stores because of their beauty relative to the females, which almost never develop fins or vibrant colours as showy as their male counterparts; however, some breeders claim to have produced females with fairly long fins and bright colours.

Betta splendens can be hybridized with B. imbellis, B. mahachaiensis, and B. smaragdina, though with the latter, the fry tend to have low survival rates. In addition to these hybrids within the genus Betta, intergeneric hybridization of Betta splendens and Macropodus opercularis, the paradise fish, has been reported.[54]

A male betta

Wild bettas exhibit strong colours only when agitated. Over the centuries, breeders have been able to make this coloration permanent, and a wide variety of hues breed true. Colours among captive bettas include red, orange, yellow, blue, steel blue, turquoise/green, black, pastel, opaque white, and multi-coloured.

The betta's diverse colours are due to different layers of pigmentation in their skin. The layers, from deepest within to the outermost,, consists of red, yellow, black, iridescent (blue and green), and metallic (not a colour itself, but reacts with the other colours). Any combination of these layers can be present, leading to a wide variety of colours within and among bettas.[55]

The shades of blue, turquoise, and green are slightly iridescent, and can appear to change colour with different lighting conditions or viewing angles; this is because these colours (unlike black or red) are not due to pigments, but created through refraction within a layer of translucent guanine crystals. Breeders have also developed different colour patterns such as marble and butterfly, as well as metallic shades like copper, gold, or platinum, which were obtained by crossing B. splendens to other Betta species).[56]

Some bettas will change colours throughout their lifetime, a process known as marbling, which is attributed to a transposon, in which a DNA sequence can change its position within a genome, thereby altering a cell.[57] Koi bettas have mutated over time and in some case change colours or patterns throughout their lifetime (known as true Koi), due to the defective gene that causes marbling not being repaired in the color layers after some time.[58]

Halfmoon betta male

Common colours:[59]

• Super Red
• Super Blue
• Super Yellow
• Opaque
• Super Black
• Super White
• Orange
• Marble
• Candy
• Nemo
• Galaxy Nemo
• Koi
• Alien
• Copper
• Cellophane
• Gold
• Galaxy Koi

Dalmatian orange male

Rarer colours:[59]

• Super Orange
• Metallic
• Turquoise
• Lavender
• Mustard Gas
• Grizzle
• Green

Metallic Doubletail male

Colour patterns:[59]

Crowntail male

Breeders have developed several different finnage and scale variations:[59]

• Veiltail – Extended finnage length and non-symmetrical tail; caudal fin rays usually only split once; the most common tail type seen in pet stores.
• Crowntail – Fin rays are extended well beyond the membrane and consequently the tail can take on the appearance of a crown; also called fringetail
• Combtail – Less extended version of the crown tail, derived from breeding crown and another finnage type
• Halfmoon – "D" shaped caudal fin that forms a 180° angle, the edges of the tail are crisp and straight
• Over-Halfmoon or Super Delta Tail – Caudal fin exceeds 180° angle (a byproduct of trying to breed half-moons), which can sometimes cause problems because the fins are too big for the fish to swim properly[60]
• Rosetail – Variation with so much finnage that it overlaps and looks like a rose
• Feather tail – Similar to the Rosetail, with a rougher appearance
• Plakat – Short fins that resemble the fins seen in wild-type bettas
• Halfmoon plakat – Short-finned Halfmoon; plakat and halfmoon cross
• Double tail or Full-moon – Tail fin is duplicated into two lobes and the dorsal fin is significantly elongated, the two tails can show different levels of bifurcation depending on the individual
• Delta tail – Tail spread less than that of a Halfmoon (less than 180°)
• Super Delta (aka SD or SDT) – Enhanced version of the Delta; one step closer to the Halfmoon variety in that their tails have a span between 130 -170 degrees
• Half-Sun – Combtail with caudal fin going 180°, like a half-moon
• Elephant Ear – Pectoral fins are much larger than normal, often white, resembling the ears of an elephant
• Spade Tail – Caudal fin has a wide base that narrows to a small point

• 
  Purple and blue female.
• 
  Super Yellow Plakat Halfmoon male and female.
• 
  Cellophane Big Ear male
• 
  Super Orange female and male.
• 
  Yellow Halfmoon
• 
  White Rosetail male

In general, studies have shown that females exhibit similar aggressive behaviours to males, albeit less frequently and intensely.[70] An observational study examined a group of female Siamese fighting fish over a period of two weeks, during which time they were recorded attacking, flaring, and biting food. This indicated that when females are housed in small groups, they form a stable dominance order, or "pecking order". For example, the fish ranked at the top showed higher levels of mutual displays, in comparison to the fish who were of lower ranks. The researchers also found that the duration of the displays differed depending on whether an attack occurred.[71] The results of this research suggest that female Siamese fighting fish warrant as much scientific study as males, as they seem to have variations in their behaviours as well.

There has been much research in the courtship behaviour between male and female Siamese fighting fish. Studies generally focus on the aggressive behaviours of males during the courtship process. For example, one study found that when male fish are in the bubble nest phase, their aggression toward females is quite low. This is due to the males attempting to attract potential mates to their nest, so eggs can successfully be laid.[72] It has also been found that in determining a suitable mate, females often "eavesdrop" on pairs of males that are fighting. When a female witnesses aggressive behaviour between males, she is more likely to be attracted to the male who won. In contrast, if a female did not "eavesdrop" on a fight between males, she will show no preference in mate choice. In regards to the males, the "loser" is more likely to attempt to court the fish who did not "eavesdrop", while the "winner" showed no preference between females who “eavesdropped” and those who did not.[73]

One study considered the ways in which male Siamese fighting fish alter their behaviours during courtship when another male is present. During this experiment, a dummy female was placed in the tank. The researchers expected that males would conceal their courtship from intruders; instead, when another male fish was present, the male was more likely to engage in courtship behaviours with the dummy female fish. When no barriers were present, the males were more likely to engage in gill flaring at an intruder male fish. The researchers concluded that the male was attempting to court the female and communicate with its rival at the same time.[74] These results indicate the importance of considering courtship behaviour, as the literature has suggested there are many factors that can dramatically affect the ways in which both male and females can act in courtship settings.

Studies have found that Siamese fighting fish often begin with behaviours that require high cost, and gradually decrease their behaviours as the encounter proceeds.[72] This indicates that Siamese fighting fish will first begin an encounter using much metabolic energy, but will gradually decrease, as to not use too much energy, thus making the encounter a waste if the fish is not successful. Similarly, researchers have found that when pairs of male Siamese fighting fish were kept together in the same tank for a three-day period, aggressive behaviour was most prevalent during the mornings of the first two days of their cohabitation. However the researchers observed that the fighting between the two males decreased as the day progressed. The male in the dominant position initially had metabolic advantage; although as the experiment progressed, both fish became equal in regards to metabolic advantages.[75] In regards to oxygen consumption, one study found that when two male bettas fought, the metabolic rates of both fish did not differ before or during the fight. However, the fish who won showed higher oxygen consumption during the evening following the fight. This indicates that aggressive behaviour in the form of fighting has long-lasting effects on metabolism.[76]

Siamese fighting fish are popular models for studying the neurological and physiological impact of certain chemicals, such as hormones, since their aggression is the result of cell signaling and possibly genes.[77]

One study investigated the effect of testosterone on female Siamese fighting fish. Females were given testosterone, which resulted in changes to fin length, body coloration and gonads that resembled typical male fish. Their aggressive behaviour was found to be elevated when interacting with other females, but reduced when interacting with males. The researchers then allowed the females to interact with a control group of unaltered females; when the female fish stopped receiving testosterone, those who were exposed to the normal females still exhibited male-typical behaviours. In contrast, the female fish who were kept isolated did not continue to exhibit the male typical behaviours after testosterone was discontinued.[78]

Another study exposed male Siamese fighting fish to endocrine-disrupting chemicals. The researchers were curious if exposure to these chemicals would affect the ways in which females respond to the exposed males. It was found that when shown videos of the exposed males, the females favoured those who were not exposed to the endocrine-disrupting chemicals, and avoided those males that were exposed. The researchers concluded that exposure to these chemicals can negatively affect the mating success of male Siamese fighting fish.[79]

A psychology study used male Siamese fighting fish to investigate the effects of fluoxetine, an SSRI used primarily as an antidepressant in humans. Siamese fighting fish were selected as prime models due to having comparable serotonin transporter pathways, which accounts for their aggression. It was found that when exposed to fluoxetine, male Siamese fighting fish exhibited less aggressive behaviour than is characteristic of their species.[80] Similarly, research has found that bettas are responsive to serotonin,[81] dopamine,[82] and GABA.[83]