Aquatic species, as the name suggests, are those that live primarily in water. Aquatic species can be divided into marine species and freshwater species according to their habitats. The so-called super aquatic species refer to the aquatic species with strong survival ability and their own “super ability”. Most of these species possess abilities that are different from those of other life forms, so studying them is of great significance to our survival and development.

The alligator gar (Atractosteus spatula), known as the “ancient killer” and the “overlord of the water”, is one of the ten most ferocious freshwater fish in the world. They are strangely shaped and extremely fierce, making them the top predators in the water, and always at the top of the food chain. As a predatory freshwater species, their destructive power in water cannot be underestimated. They prey heavily on other aquatic animals, posing a potential threat to aquatic ecosystems.

This is one of the reasons why alligator gar has been able to survive for over 100 million years Previously, alligator gar has been found in many places in China, and the action of “draining the lake water” to capture alligator gar in Ruzhou of Henan Province has attracted widespread attention from the Chinese public (Gao and Guo, 2022). Taking this as an opportunity, this study carried out a comprehensive study and analysis of alligator gars. In this study, the speciological history, morphological structure, feeding behavior, reproduction mode, distribution range and survival status of alligator gars were deeply explored from the biological perspective, and on this basis, the harm of alligator gars and its existence value were dialectically thought, in order to provide a theoretical basis for the scientific management and prevention of alligator gars.

1 The Speciological History of Alligator Gar

1.1 Origin and evolution of alligator gar

The alligator gar (Atractosteus spatula), also known as the Fu’e, Daque’e, Chuiguqueshan etc. in Chinese, and is a giant freshwater carnivorous species (Figure 1). According to the fossil record, the alligator gar first appeared in the early Cretaceous period, known as the “prehistoric monster”. Some studies have found that the alligator gar is evolved from the ancient fish species of Holostei. The species of Holostei are intermediate between Chondrostei and Teleostei in evolutionary stage. They began to appear in the late Permian, flourished in the Jurassic period, and declined significantly from the Cretaceous period. Only a few species, including the alligator gar, remain today.

Figure 1 The alligator gar (picture source: Baidu pictures)

Although alligator gar has survived for more than 100 million years and has been highly evolved, they still retain some morphological features of their earliest ancestors with little or no apparent change, such as shark-like spiral intestines, short skewed tail fins, and swim bladder lungs that allow them to breathe in air and water. For this reason, alligator gar is also called “living fossil” or “primitive fish”.

1.2 Classification and nomenclature of alligator gar

According to biological classification, alligator gar belongs to Atractosteus genus in the family of Lepisosteidae in Lepisosteiformes in Holostei in Actinopterygii in Osteichthyes in Vertebrata in Chordata.

But this was not the original taxonomic position of the alligator gar, at first all species of Lepisosteiformes were classified only in Lepisosteus genus. This is why in 1803 Lacépède named it Lepisosteus spatula when he first described the alligator gar. Later, Rafinesque changed the alligator gar to Atractosteus adamantinus in 1818 and assigned it to Atractosteus genus. Still for a long time Atractosteus was considered an alias for Lepisosteus. Atractosteus was not officially recognized until 1976, when E.O. Wiley renamed the alligator gar as Atractosteus spatula, and to date.

2 Morphological Structure of Alligator Gar

2.1 Physiological characteristics of alligator gar

Alligator gar is the largest species in the Lepisosteidae and one of the largest freshwater fish found in North America. The mature alligator gar typically measures 1.2 to 1.8 m in length, weighs 45 to 72 kg, and is slightly larger in females than in males. The largest recorded alligator gar currently (captured on 14 February 2011), was 2.572 m in length, 148 kg in weight, and 120 cm in circumference.

Alligator gar has a torpedo-shaped body, and the features of the mature alligator gar are more pronounced, including a heavy body, wide head, short and wide nose, protruding snout, short snout like an crocodile, and two rows of teeth like dagger in the upper and lower jaws. The color of alligator gar is usually dark olive green, with a faded grayish white on both sides of the back, and a white belly.

2.2 Unique structure of alligator gar

2.2.1 Alligator gar’s scales

The scales of alligator gar are large and hard rhomboid hinged scales, which usually have serrated edges and are composed of a tough inner bone layer and a hard adenosine outer layer, and are essentially homologous to dental enamel. The scales reach a Mohs hardness of 5, which are even harder than ordinary steel and are highly defensive, almost impenetrable (Sherman et al., 2016).

2.2.2 Alligator gar’s swim bladder

The thick, spongy, and highly vascularized swim bladder of alligator gar (Figure 2) can not only provides buoyancy, but also assists in breathing like a lung (Tyus and Harold, 2011), so it is also known as the swim bladder lung. Alligator gar’s swim bladder lungs enable them to survive well even in hypoxic environments, and can survive for a short time on land. This is one of the reasons why alligator gar has been able to survive for over 100 million years.

Figure 2 Highly vascularized swim bladder of alligator gar (Omar-Ali et al., 2016)

2.2.3 Alligator gar’s mouth

Alligator gar’s mouth is wide and abrupt, and looks like a crocodile, which makes people shiver at the sight. Moreover, their mouths can rapidly expand from front to back, creating a suction force to capture prey (Lemberg et al., 2019).

3 Feeding Behavior of Alligator Gar

3.1 The feeding habits of alligator gar

Alligator gar is an unusually fierce predatory fish. Young fish feed on small fish and insects, while adult fish prey on almost all living things in the water, including various fish, crustaceans, amphibians, reptiles, birds, small and medium-sized mammals, and even carrion.

3.2 Predation pattern of alligator gar

Although the alligator gar moves slowly, it is a fierce ambush predator. It is possible to hide under floating wood, debris, or hanging branches in the water day and night to stay still, quietly waiting for unsuspecting prey to swim within reach, then suddenly pouncing on the prey, and finally successfully capturing it.

Due to bone structure issues, the alligator gar does not have the strong bite force of sharks and crocodiles. Although the teeth of an alligator gar can pierce prey, they have no chewing function. They can only eat by swallowing, and can swallow prey up to a quarter the size of their bodies.

4 Reproduction Mode of Alligator Gar

Alligator gars usually lay eggs through male and female mating from April to June every year. When male alligator gars surround female alligator gars and perform a series of activities that trigger egg release, female alligator gars will lay eggs in the water, laying 150 000 to 280 000 eggs at a time, and then male alligator gars will release a large number of reproductive glands to fertilize the eggs. However, this open fertilization method makes the fertilization rate of alligator gars not very high. The fertilized eggs of alligator gars are incubated under relatively harsh conditions, which are influenced by water quality, temperature and water flow. The incubation period of alligator gars is usually 6 to 8 days, and the young will leave the water plants or gravels after 10 days of birth. Male alligator gars reach sexual maturity around 6 years old, while females are up to 11 years old. Therefore, the reproductive ability of alligator gars is not as impressive as their spawning capacity, and the reproduction rate is not as fast.

Eggs of the alligator gar are usually light yellow to olive in color, sticky and easily attached to water plants or gravels. It is worth mentioning that the eggs of the alligator gar contain “ichthyotoxin”, a highly toxic toxin that can defend against crustacean predators and kill other fish if swallowed, and is also effective against humans because it is a protein toxin that can cause disease if ingested.

5 Distribution Range of Alligator Gar

5.1 Natural range

Alligator gars inhabit freshwater environments such as rivers, lakes and marshes in the tropics and subtropics year-round, and occasionally in brackish water areas such as estuaries or seashores. They are mainly distributed along the southeastern coast of the United States, west to Texas and Oklahoma, east to Florida, north to the Mississippi River basin, lower Ohio River and Missouri River basin, and south to Mexico (Figure 3).

Figure 3 Distribution status of alligator gars in North American watersheds (Smith, N. G. et al., 2020) Note: Light gray area: never present; dark gray area: distribution unknown; yellow area: artificially released; green area: breeding area; orange area: present but no breeding record; red area: extinct

5.2 Beyond the natural range

In addition to North America, some notable occurrences of alligator gars have been reported in other parts of the world, for example, in the Caspian Sea north of Turkmenistan (November 2008), in canals in Singapore (January 2011), in wells in Dadar, India (August 2015), and on the banks of reservoirs in Cyprus (June 2020), among others. Previously, alligator gars were also found in the provinces of Henan, Shandong, Yunnan, Hunan, Jiangsu, Beijing, Guangxi, Fujian, Sichuan, and Guangdong in China (Han, 2022). Most of these alligator gars outside their natural range do not naturally form populations, which is related to the unsuitable climate and water quality of the invaded areas for their reproduction. Almost all of the alligator gar invasions reported today are cases of human abandonment or release.

6 Survival Status of Alligator Gar

6.1 Natural enemies of alligator gar

In the natural distribution area, the natural enemies of alligator gars are American crocodiles, large raptors, otters, mink and alligators, etc. Among them, otters and mink are the natural enemies of young alligator gars; in the unnatural distribution area, alligator gars have almost no natural enemies for the time being.

6.2 Population status of alligator gar

In the natural distribution area, the alligator gar has been treated as a target for elimination for nearly half a century because of its strong destructive ability, which has brought fatal harm to all kinds of organisms in the water, and has been hunted on a large scale and continuously, and even reached the endangered level. Thankfully, it has now been realized that alligator gars are not only of great significance to bioscience research, but are also very important to the maintenance of the biological chain. Experts in the natural range of the alligator gar have also been actively protecting it by means of habitat protection and research on enrichment breeding.

In the non-natural distribution area, the lack of natural enemies of the alligator gar poses a potential threat to the ecological balance and biodiversity in the invasive waters, so most of these alligator gars are caught and then treated in a non-pollution manner.

7 Conclusion

As a “living fossil” that has survived for more than 100 million years on earth, the research value of alligator gars is self-evident, and as an important part of the ecosystem, alligator gars are also very important to the maintenance of the biological chain, and in addition, in the natural distribution area, alligator gars have natural enemies and the population can be maintained at a suitable level. In this study, we believe that the alligator gar should not be hunted in its natural range, but should be sustainably maintained. For the population size of alligator gars in natural distribution areas, monitoring and supervision can be strengthened to regulate and protect them in a timely manner. For the ornamental fish trade of alligator gars, it should be strictly controlled and investigated, and even if it is difficult to prohibit it, the hazards of alligator gars should be clarified in the trade and informed that they should not be released or abandoned.

For the alligator gars that have already appeared in unnatural distribution areas, we can lure and catch them for pollution-free treatment to prevent them from threatening the ecosystem of invasive waters. At present, the invasion of alligator gars is not that bad, because although the invasive alligator gars are powerful individually, their reproductive capacity and reproduction speed are limited, and the climate and water quality of the invaded areas are mostly unsuitable for their reproduction. However, we should not relax our vigilance, but should strengthen the prevention and control of the introduction of the link, strengthen the popularization of science, avoid the artificial abandonment and release of alligator gars, and block their “entry into the water” from the source, so as to achieve the purpose of prevention.

Acknowledgments

This research was funded by the Scientific Research Fund of Cuixi Academy of Biotechnology. Thanks Dr. Xuanjun Fang, Director of Hainan Institute of Tropical Agricultural Resources, for providing ideas for this research and for his guidance and assistance in the writing and revision of this paper.

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