Arapaima gigas, also known simply as Arapaima, pirarucu, or paiche,Miranda-Chumacero, G., Wallace, R., Calderón, H., Calderón, G., Willink, P., Guerrero, M., ... & Chuqui, D. (2012). Distribution of arapaima (Arapaima gigas)(Pisces: Arapaimatidae) in Bolivia: implications in the control and management of a non-native population. BioInvasions Record, 1(2).Marková, J., Jerikho, R., Wardiatno, Y., Kamal, M. M., Magalhães, A. L. B., Bohatá, L., ... & Patoka, J. (2020). Conservation paradox of giant arapaima Arapaima gigas (Schinz, 1822)(Pisces: Arapaimidae): endangered in its native range in Brazil and invasive in Indonesia. Knowledge & Management of Aquatic Ecosystems, (421), 47. is a species of arapaima native to the basin of the Amazon River. Once believed to be the sole species in the genus, it is among the largest freshwater fish. It is the best-known species of Arapaimas.
Taxonomy
Arapaima gigas was originally regarded as the only species in the genus Arapaima (monotypic), but the subsequent identification of further species, together with the rarity of specimens and the loss of several type specimens, has led to some uncertainty regarding classification within the genus and the identity of described individuals. Recently, some studies have added extra species to the genus due to common morphological features, including Arapaima leptosoma and Arapaima agassizii.Vitorino, C. A., Oliveira, R. C. C., Margarido, V. P., & Venere, P. C.. (2015). Genetic diversity of Arapaima gigas (Schinz, 1822) (Osteoglossiformes: Arapaimidae) in the Araguaia-Tocantins basin estimated by ISSR marker. Neotropical Ichthyology, 13(3), 557–568. https://doi.org/10.1590/1982-0224-20150037 These speciation events may have occurred due to evidence of "long-distance marine or geodispersal" from similar morphological traits in both fossilized and living specimen within the genus.Perez, M. F., Toma, G. A., Souza, F. H. S., Ferreira, P. N., Ráb, P., & Cioffi, M. B. (2023). Integrating genomic and cytogenetic data to study the evolutionary history of Arapaimas and Arowanas in the Neotropics. In P. M. Galetti Jr. (Ed.), Conservation genetics in the Neotropics. Springer, Cham. Overall, it is unclear if Arapaima gigas truly is the only species in its genus.
Description The species is among the largest known freshwater fish, commonly measuring and reportedly exceptionally reaching lengths of up to . Adults may weigh up to . Of the total body weight, head weight is typically 10.3% to 12.8%.Luxinger, A. O., Cavali, J., Porto, M. O., Sales-Neto, H. M., Lago, A. A., & Freitas, R. T. F. (2018). Morphometric measurements applied in the evaluation of Arapaima gigas body components. Aquaculture, 489, 80–84.
It is an ancient fish, belonging to a group of primitive carnivorous bony-tongued fishes. A. gigas has a streamlined body with dorsal and anal fins set well back towards the tail. While the body is mainly gray to gray-green, its Brazilian local name pirarucu derives from an indigenous word for "red fish", thought to refer to either the red flecks on the scales towards the tail, or the reddish-orange color of its meat.Godinho, H.P., Santos, J.E., Formagio, P.S. and Guimarães-Cruz, R.J. (2005), Gonadal morphology and reproductive traits of the Amazonian fish Arapaima gigas (Schinz, 1822). Acta Zoologica, 86: 289–294. https://doi.org/10.1111/j.1463-6395.2005.00213.x Red coloration typically shows up in males after sexual maturation, with up to 70% of the total body plan in males having red coloration.
The fish have "flexible, armor-like scales" made up of "a hard, mineralized outer layer" and "a tough-but-flexible inner layer" that help protect it from attacks by piranhas. The scales, which are typically 5–7cm but can reach 10cm in larger individuals, are built from collagen fibers in a layered structure. On the body, they overlap each other to form the armor-like characteristic.Lin, Y. S., Wei, C. T., Olevsky, E. A., & Meyers, M. A. (2011). Mechanical properties and the laminate structure of Arapaima gigas scales. Journal of the mechanical behavior of biomedical materials, 4(7), 1145–1156.
Distribution and history A. gigas is native to freshwater in the basin of the Amazon River and Tocantins–Araguaia (Brazil) river basins; it is known to occur in Brazil and Peru. In Bolivia and Peru it is known as paiche, and is considered an invasive species in most parts, affecting local native species and the ecosystem. Other small populations have been found in the Iquitos region of Peru, and four distinct rivers in Ecuador, Colombia and Guyana.
Its populations are increasing and becoming more common in its non-native range, yet is rapidly declining in its native range. It was first found in 1976,La aventura del Paiche en la Amazonía de Bolivia , Laregion.bo. Retrieved 2020-02-27. introduced into the Madre de Dios region in Peru, for aquaculture. In about a decade, arapaimas were found in Bolivian waters due to the connection of the Madre de Dios basin to the Beni River in Bolivia. Rising water of aquaculture ponds allowed fish to escape into the watershed and establishing populations outside of containment.
Current distribution in Bolivia is characterized by 70 registered distribution points, including small rivers, lagoons, and streams between the Beni and Madre de Dios (Peru) rivers. Historical reports of arapaima sightings started north near the border of Peru, then traveled downstream as populations began to establish and spatially separate. In less than 30 years, the species has invaded the Madre de Dios and Beni watersheds.
The species has been introduced to parts of East Asia, both for fishing purposes and accidentally. The fish are found in flooded forest areas where they reproduce during the wet season; they relocate to lakes after water levels drop. It is also distributed by ornamental aquaculture and commonly pet-traded in Europe, North America, South America, and Asia. There are fish farms in Thailand and Malaysia.da Costa Amaral, A., Lima, A. F., Ganeco‐kirschnik, L. N., & de Almeida, F. L. (2020). Morphological characterization of pirarucu Arapaima gigas (Schinz, 1822) gonadal differentiation. Journal of Morphology, 281(4-5), 491–499. It is particularly prominent in Java and Sumatra regions in Indonesia. Studies found some single individuals in freshwater or dead in shallow water. One finding recorded a total of 22 individuals in Surabaya, which is the second largest Indonesian city.
A 13-million-year-old fossil of arapaima (or very similar species) has been found in Colombia, in the Villavieja Formation, which dates from the Miocene epoch.
Physiology Average lifespan in captivity is 15–20 years. There is no sufficient evidence on lifespan in the wild.
A. gigas grows rapidly, reaching about 10kg in its first year, then at moderate growth rates once reaching sexual maturity, around 5 years of age or 1.65m length.Lima Sobrinho, P. S. de, Silva Gomes, V. D., Amâncio, A. L. de L., Jordão Filho, J., Cavalcanti, C. R., & Silva Neto, M. R. da. (2020). MEDIÇÕES MORFOMÉTRICAS APLICADAS NA AVALIAÇÃO DO PESO CORPORAL DE PIRARUCUS (Arapaima gigas). Arquivos De Ciências Veterinárias E Zoologia Da UNIPAR, 23(1cont). https://doi.org/10.25110/arqvet.v23i1cont.2020.7105(translated to English)Queiroz, H. L. (2000). Natural history and conservation of pirarucu, Arapaima gigas, at the Amazonian Várzea: Red giants in muddy waters [Doctoral dissertation, University of St. Andrews]. Morphology changes occur as an individual undergoes the transition from water-breather to air-breather 8–9 days after hatch. During the transition to air-breathing, the structure of the gills changes, making them better adapted for ion absorption, but less able to undergo gas diffusion. Once developmental changes in the gills take place, the lamella is less recognizable. The adult gills are made up of smooth, column-shaped filaments. The kidneys have an important role in nitrogenous waste excretion in this species and are enlarged in adult fish.
Phenotypic sex of A. gigas juveniles is not differentiated, meaning the gonad cells can produce both sexes. In females, only the left ovary was developed which differs in size during the reproductive cycle. It increases as maturation occurs, then reduces after spawning. Females have a genital pore on their ventral (under) side that connects the reproductive organs with external environment to allow for mating. In males, only a left testis was developed, which was small and did not differ in size during the reproductive cycle. Both reproductive organs are unpaired and situated near the swim bladder. In some cases, right gonads were found but atrophic.
Both males and females have a gland-like secretory organ on the head, mostly used for communication with offspring and is therefore more prominent in males—who provide the majority of parental care. The secretion is made of 400 substances and consist of hormones, proteins, peptides and likely pheromones which also provides nutrients for fry (freshly hatched fish).
Ecology
Breathing A. gigas requires breathing surface air to supplement the oxygen it derives from the use of its gills, and as such, is dependent on surfacing every 5–15 minutes to loudly gulp air at the surface. As in other species in the genus, a modified swim bladder that contains lung-like tissue is used for this purpose.
Feeding The species are specialized carnivores, feeding primarily on fish. Invertebrates are also part of their diet. Juveniles prefer insects and fish larvae until fully grown. Seasonal growth is observed due food selectivity and strong seasonal changes in food availability. They also consume birds, mammals, fruits, and seeds on the water surface. Prey fish families include Callichthyidae, Loricariidae, Pimelodidae, and Heptapteridae.Castello, L. (2008). Lateral migration of Arapaima gigas in floodplains of the Amazon. Ecology of Freshwater Fish, 17(1), 38–46. https://doi.org/10.1111/j.1600-0633.2007.00255.x
Migration The species undergoes seasonal lateral migration between river channels and floodplains driven by low depth and low current velocity. The lake and forest habitats seem to be preferred. When water levels are high, A. gigas fish migrated to flooded forests with increasingly rising levels. When water levels decline, fish begin to migrate towards lower levels of flooded forests, then to lakes where they stay when it is physically isolated due to low water levels. Migration to the flooded forest is a mechanism for feeding and parental care, with prey (for both adults and offspring) having increased densities in this habitat at the high-water season. During high water levels, A. gigas populations have high densities in lakes, due to increased survival rates (physical safety), feeding (adequate feeding rates), and promoting reproduction (courtship and mating activities).
Spawning Sexual maturity is reached at 5 years, and female fecundity increases with each year. Spawning occurs in lakes and river channels during the time of low water levels (August to March) in small, separate clutches with an average of less than 500 eggs in each batch. Parents have been observed using their mouths to dig holes in the substrate to create a nest for eggs. Small, batched spawning strategy is thought to derive from the unpredictability of the environment, as it lowers the total loss of fertilized eggs due to natural events. In the event of reproductive failures, pairs may breed again for better success. Males remain to protect the offspring for three to six months. Body mass and muscle yield does not differ during sexual maturation cycles, unlike many other fish species. Red pigment among males is thought to be used for sexual selection as it is a proxy for age. Conservation