Arapaima gigas, also known simply as Arapaima, pirarucu, or paiche, 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. 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. Overall, it is unclear if Arapaima gigas truly is the only species in its genus.
thumb|thumbtime=1|left|A. gigas swimming in captivity
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%.
It is an ancient fish, belonging to a group of primitive carnivorous bony-tongued fishes. 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.
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. 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. 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. 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.
Physiology
Average lifespan in captivity is 15–20 years. 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. 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.
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). However, there are still large markets of illegal arapaima fishing in the lower Amazon, with about 77% of its harvest coming from unlawful practices. A joint study of the Bolivian government and different research organizations from 2017 points out the necessity to further evaluate the complex environmental and socioeconomic impact of Arapaima in the country.
Oftentimes, introduction comes from accidentally escaped aquaculture groups, or for purposeful control of undesirable prey in parts of Brazil and south-eastern Peru.