The virus that causes dengue fever is highly virulent, and is spread by Aedes mosquitoes. But spraying insecticides to control the mosquitoes creates problems of environmental residues, so researchers inside and outside Taiwan have sought to develop biocontrols. Much attention has focused on two fish species that are renowned as "mosquito hunters": the mosquitofish (Gambusia affinis) and the guppy (Poecilia reticulata). According to studies, a male mosquitofish can eat 44 Aedes larvae in one day, and the male guppy comes a close second at 42.

But these figures pale in comparison to the appetite of Taiwan's native paradise fish (Macropodus opercularis). In the wild, paradise fish eat small insects and mosquito larvae, and experiments by the Taiwan Endemic Species Research Institute have shown that a male paradise fish can eat as many as 374 mosquito larvae in one day-eight times as many as the so-called mosquito hunters. Science experiments by children at Tajen Elementary School in Taichung City also showed what big eaters the fish are: one weighing just 0.6 grams can eat around 300 mosquito larvae in a day. Paradise fish really are mosquito hunters worthy of the name. But sadly this is of little help in the fight against the disease-carrying bugs, for paradise fish are now so rare that they are listed as a Grade 2 endangered species.

Natural mosquito lamps

However, 30 to 40 years ago, watercourses and paddy fields in the plains of western Taiwan, and in the Lanyang Plain to the east were full of native paradise fish. The fish were close companions of agricultural life, and because of the striking black-green spots on their gill covers they were known by many names, such as three-spot, cover-spot fighting fish, Taiwan fighting fish and Taiwan goldfish, as well as paradise fish.

Fighting fishes (family Belontiidae) are widely distributed in Europe, Asia and Africa, and paradise fish are scattered throughout mainland China, Taiwan and Southeast Asia. Like the imported fighting fish that are often seen in aquariums, native Taiwanese paradise fish are around four to six centimeters long. Most fighting fish species are brightly colored and eye-catching in appearance. Both males and females of Taiwan's native paradise fish have ten vertical blue bars along their gray-green bodies, intermingled with a pale red hue. In sunlight this coloration creates a shimmering interplay of blue and green velvet sheens. The male's dorsal and pelvic fins have long threadlike rays, and the tips of its red tail fins are extended in a swallowtail shape. The varied effects of their fins rippling in the water are very pleasing to the eye.

In nature, male animals that make courtship displays often have very striking looks, and the purpose of the exaggerated and colorful appearance of male paradise fish is indeed to attract the opposite sex. Unusually, the male paradise fish also plays a crucial role in bringing up the next generation.

Tragicomic courtship

From April to October each year is the breeding season for paradise fish. When they begin breeding, amid aquatic plants such as duckweed one can see the male fish bobbing up and down under the surface of the water, repeatedly swallowing air and blowing out bubbles of air and mucus. Many of the bubbles float up to the surface and burst, so the fathers-to-be have to work continuously for hours at a stretch to build thick nests of bubbles for their future eggs to hatch. Once a male paradise fish has finished his bubble nest, he waits beside it for a female fish to rise to his bait.

When a female approaches, the male fish promptly begins to serenade her by swimming back and forth all around her, spreading his swallowtail fins. These overtures display the rainbow sheen of his coloration to best effect, and his whole body trembles with excitement. However, if the female is unmoved by the male's display of passion, his excitement may turn to anger, and he will show his true fighting nature by driving away the unfeeling female.

But if the female does respond, the male goes into a complex courtship display, repeatedly pecking her back, and then the two chase each other round in circles, head to tail. After they have repeated this ritual many times, the male fish swims onto the female's back and turns her belly up, so that when she releases her eggs they float up and lodge in the bubble nest. In the space of ten to 20 minutes, she releases ten to 20 eggs. The fish continue for several hours, repeating this process again and again, until 300-400 eggs have been fertilized.

King of kids

With mating successfully completed, the male paradise fish gives the female her marching orders, and sets to work to protect the eggs. Many experts believe that mother paradise fish have no maternal instincts, and would not hesitate to eat their own eggs. While the male fish are protecting the eggs they are very intolerant of intruders. They stay on guard and do not rest until the eggs are hatched. The tiny fry are very vulnerable if they leave the bubble nest, so the father fish busily guides them back with his mouth, as if herding ducks. Only when the little fish can feed themselves and finally leave the nest does the father relinquish his task and begin a new stage in his life.

According to fish breeders, a single pair of paradise fish can produce as many as six broods in one breeding season. When the eggs hatch, the fry are only about one millimeter long. They do not begin to eat until they are three days old. In three months they grow to three to four centimeters, and at six months they reach full size. The following year, they are able to breed. The largest only grow to seven or eight centimeters long. The fish can live up to six or seven years.

Apart from their shouldering the responsibilities of parenthood alone, another characteristic that makes male paradise fish interesting to researchers is their combative nature. Chiu Kuo-hsun, a postgraduate student at National Sun Yat-sen University's Institute of Marine Biology who researches the "agonistic behavior" of paradise fish, says that many fish species display such aggressive behavior, but it occurs most commonly, and is easiest to observe, in fighting fish.

In fact, compared with the artificially selected fighting fish of various commercial species, which show agonistic behavior in both sexes regardless of the season, "Taiwanese paradise fish are rather docile, and only become more aggressive during their breeding season. They only become real 'warriors' when competing with other males for the attention of a female, or when guarding their offspring in the bubble nest," says Lin Ming-liang, a technical development officer at Taipei County Bureau of Agriculture's fish breeding station.

Ten attacking moves

"Paradise fish are territorial, and male fish only display agonistic behavior when they need to compete for resources. A male that loses has to look for another territory," says Chiu Kuo-hsun. He describes how the fighting rituals of paradise fish follow clear rules: when two males face off, they first try to scare each other by spreading their broad tail fins. Unless one of them backs down at this point, they then go into what researchers term a "mouthlock" in which they grip each other's mouths in a firm bite as they pull and push each other and shake their bodies.

This head-to-head encounter usually decides the contest, but if it does not the combatants give their red and swollen mouths a rest and begin following each other round in circles. "They never do anything cowardly like biting their opponent's tail from behind-they always fight with honor, 'man to man,'" says Chiu Kuo-hsun. Chiu says that the fish also understand that discretion is the better part of valor, and will first fight in ways that have a relatively small cost. If they can determine victory by ritual displays, and thereby achieve their goal of proclaiming their territory, they will not embark on a life-and-death battle.

A Hungarian researcher has listed ten modes of combative behavior in paradise fish. They include displaying at a distance; pushing their heads against each other like fighting bulls; swimming in parallel in the same direction, body to body; then displaying head-to-tail, pecking each other, and going into a mouthlock for over two minutes. Researchers have also discovered that there are differences between individual paradise fish, which they divide into aggressive hawks and more docile doves. Chiu Kuo-hsun is investigating what relationships there may be between paradise fishes' agonistic behavior and their biological structure.

Second gills-labyrinths

Although paradise fish are small in size, their diverse natural history makes them a favorite subject of academic research. Many people who keep paradise fish know that they can adapt to poor water quality, and the aquarium does not need an air pump because the fish can poke their mouths out of the water and directly breathe air. This is why in the past they could be seen everywhere in the Taiwanese countryside, in ditches, ponds and paddy fields with only a small trickle of water.

The reason the paradise fish can breathe air is because it has a specialized auxiliary respiratory organ-the labyrinth-located above the primary gills. Air that the paradise fish sucks into its mouth enters the labyrinth chamber, which connects its throat to its gill chambers. The labyrinth chamber is full of capillary blood vessels, and oxygen in the air is absorbed into these capillaries to pass into the fish's body.

Associate Professor Lee Tsung-han of National Chung Hsing University's life sciences department explains that the paradise fish's labyrinth organ is the result of environmental adaptation. Unlike ocean fish, freshwater fish live in restricted bodies of water, and may often be faced with the situation of water in pools or streams drying up. As the amount of water is reduced, its quality is also likely to worsen as it becomes turbid or full of rotting vegetable matter. Water temperatures may fluctuate dramatically too. In response to such variable conditions, many freshwater fishes have developed secondary respiratory organs. They use labyrinths, their mouth cavity or their skin as additional breathing surfaces, to overcome the dangers of drought. The labyrinth fishes that live in the rivers of tropical Asia and Africa rely on this remarkable adaptation for their survival. Those among them that are kept as ornamental fish include gouramis, paradise fish and Siamese fighting fish (bettas), all of which are members of the family Belontiidae.

Too heavy a mission

In Taiwan, aquarium dealers often keep a paradise fish in a glass of water to show how resilient and easy to keep they are. Li Ming-liang also confirms that paradise fish can resist cold and heat-even at summer temperatures of 36 to 37 oC, paradise fish at the Taipei County fish breeding station are full of energy. Researchers have also demonstrated the fishes' remarkable appetite for mosquito larvae. These factors have caused the paradise fish's reputation as an "environmental" fish to grow ever stronger. Public health and environmental protection agencies also show a continuing interest in them, and are eager to breed and release them in large numbers. But if paradise fish really have such tremendous vitality, why have they all but disappeared from the wild in Taiwan?

"Paradise fish do have some adaptive mechanisms that other fish lack, but their strong ability to adapt does not mean that they like a dirty environment," stresses Lee Tsung-han. Although paradise fish are not too fussy about water quality, in fact they are very vulnerable in the face of all kinds of disturbance by humans. The first deadly assault on them was the heavy pollution of Taiwan's rivers with chemicals. Then, ubiquitous concreting of watercourses robbed them of their habitat. They were also unable to withstand the rapacious competition of introduced tilapia for space and food. Finally, in August 1990, after an islandwide population survey by Associate Professor Tseng Ching-hsien of National Tsing Hua University's Institute of Bioinformatics and Structural Biology, the Council of Agriculture had no choice but to list the paradise fish as a protected species.

Community favorite

Wild paradise fish have gradually disappeared from our natural environment. But they are the brightest and most eye-catching of all Taiwan's river fish, so for more than 20 years people in the aquarium trade have been selectively breeding local paradise fish. Dealers say that the pet fish known as "colored rabbits" in aquarium shops are the descendents of wild paradise fish.

As well as the fish that survive in aquarium shops, in 1997 staff from the Taipei County fish breeding station discovered six individual paradise fish at locations in the Sanchih and Kungliao areas of Taipei County, and in Miaoli County. They were delighted, and in view of a growing epidemic of dengue fever at the time, embarked on a program for the large-scale breeding of these "mosquito hunters." Two years ago, they overcame the difficulties of "volume production," and bred tens of thousands of paradise fish, which they began to supply on request to campuses and communities. With sponsorship from private enterprise, the county government also held a big ceremony to mark the release of the fish into the section of the Tanshui River near the Erhchung flood relief channel. This event started a trend for high schools and elementary schools to raise paradise fish. Paradise fish, which in the past only a small number of organizations such as the Tachia River Ecological Conservation Society had been concerned about, came to be used in nativist education as a species that children can breed and observe in practice.

Lin Ming-liang states that any ponds, ditches or agricultural irrigation cisterns that contain some freshwater plankton and aquatic plants that attract mosquitoes are all suitable for populating with paradise fish. To date, communities, schools and parks have come from as far away as Pingtung and Tainan in the south to "adopt" paradise fish. In recent years, with the continuing growth of the community regeneration movement, the paradise fish has become a favorite with communities seeking to rebuild local vitality.

Not just a mosquito catcher

In fact, the Council of Agriculture normally forbids the artificial breeding and sale of protected species, for fear that inbreeding will damage the wild gene pool. But with the resurgence of dengue fever, and with people's enthusiasm for repopulating Taiwan's rivers with the indigenous paradise fish, conservation agencies merely suggest that breeders should enlarge their sources of genetic material, to avoid narrowing the fishes' gene pool. "Will breeding from a small number of individuals reduce genetic diversity? Will widespread releases endanger the extremely rare remaining wild populations? These are questions that will need to be followed up," suggests Liao Yun-chih, a doctoral student at the Academia Sinica's Institute of Zoology who has photographed and raised paradise fish.

Will the newly released paradise fish be able to adapt to life in the wild, and achieve the desired the effect of controlling mosquitoes? For many people, this is a focus of even greater concern. In fact, it is not only paradise fish that have disappeared from our lives-most of Taiwan's native freshwater fish are now gone. Only if the overall environment can also be gradually improved will the paradise fish be able to spread out from the small number of places where they are released and survive in the long term, and only then will they have any chance of exercising their "talent" for hunting mosquitoes. Let's hope that one day paradise fish can swim freely, and eat mosquito larvae, in every corner of Taiwan where there is water, and not just in a few isolated spots.

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From building a bubble nest, to pecking at the female's back to show his affection, to helping her turn onto her back to lay her eggs, so that the fertilized eggs can float up into the bubble nest-during the long and difficult process of rearing the next generation, the male paradise fish shoulders much of the work.

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Week-old fish fry hide among river vegetation. This rare and remarkable shot of baby paradise fish in the wild was captured by Chan Chien-ping, principal of Chungshan Elementary School in Tungshih Township, Taichung County, who for many years has been involved in conservation work to rehabilitate the ecology of the Tachia River. (photo by Chan Chien-ping)

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Where ponds are alive with healthy paradise fish, there is no need to worry about dengue fever. (photo by Chan Chien-ping)

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Returning the waters of country ponds to a pristine state is something that many people hope can be achieved by conservation of the paradise fish. (photo by Chan Chien-ping)