Northern green anaconda

The northern green anaconda (Eunectes akayima) is a disputed boa species found in northern South America and the Caribbean island of Trinidad. It is closely related to Eunectes murinus, the (southern) green anaconda, from which it was claimed to be genetically distinct in 2024. It is one of the heaviest and longest snakes in the world, with one specimen reported by a newspaper to have been 6.3 m long. Like all boas, it is a non-venomous constrictor.

E. akayima is estimated to have diverged from its closest relative between 5 and 20 million years ago, originally separated by the Vaupés Arch. Today, specimens of E. akayima are known from the Orinoco basin and surrounding regions, ranging from Ecuador to Trinidad. Its range is still mostly separate from that of E. murinus, although they partially overlap around French Guiana, with no clear geographical barrier. While allegedly separated through mitochondrial DNA markers, the two species are indistinguishable in morphology.

The anacondas in the region have been known for centuries as akayima by the local Carib people, and this became the formal scientific name proposed for the species. Later studies raised nomenclatural issues about the description, while also calling into question the validity of the clades recovered through mitochondrial DNA, casting doubt on the validity of E. akayima as a separate species.

Previous proposals
Before the discovery of E. akayima, several proposals were made to split a new species or subspecies from the green anaconda (Eunectes murinus), such as Eunectes gigas (Latreille, 1801), or Eunectes barbouri (Dunn and Conant, 1936).

Boa gigas, later Eunectes gigas or Eunectes murinus gigas, was proposed in 1801, at first described from specimens in Guyana and Trinidad but later believed to range from Colombia to French Guiana. It was distinguished by a lighter postocular coloration, as well as differences in scale count in the ventral and subcaudal regions. The coloration was later found to be uniformly distributed throughout the green anaconda's range, while the scale count was found not to differ significantly between the putative subspecies. This showed E. m. gigas to be a color variation rather than a true subspecies.

Eunectes barbouri was described in 1936 from an individual donated to Philadelphia Zoo, believed to originate from the island of Marajó in northern Brazil. The holotype was distinguished from E. murinus specimens by the coloration pattern of its dorsal spots. In the presumed E. barbouri holotype, the latter had a dark border with a light center, while they were uniformly dark in E. murinus. Beyond the holotype, E. barbouri was estimated to have a wider range throughout Brazil, with another attributed specimen originating from Barreiras.

The validity of E. barbouri was debated by some contemporary authors, although others continued to maintain the distinctness of the species. A 1970 expedition on Marajó failed to find any E. barbouri specimens, with the only individuals encountered being E. murinus. Later studies have cast doubts on the diagnostic characteristics used to define the species, with the spot patterns being found to be part of the natural color variation in E. murinus, while a 2022 genetic study upheld the synonymy of the two species.

Discovery
In 2024, research claimed through genetic testing that populations of Eunectes murinus found in northern South America formed a clade distinct from those occurring farther south. The approximately 5.5 percent difference in mitochondrial DNA prompted the northern populations to be described as a separate species, Eunectes akayima, by the researchers. Both wild and captive specimens were tested with blood samples taken. While nuclear DNA failed to show measurable differences between the two species, authors attributed this to the lack of appropriate markers with high enough variation rates, with nuclear genomic studies barely separating E. murinus from the more distant E. notaeus.

Samples were taken in multiple locations throughout South America over the course of 20 years in a study led by New Mexico Highlands University professor Jesús Rivas, measuring both genetic data through blood and tissue samples and anatomical characteristics like scale count, as well as habitat data. An earlier specimen, MCNG 1042, was designated as the holotype. This specimen, collected by Jesús Rivas in 1993 at Hato El Cedral in the Venezuelan Llanos, is now preserved by the UNELLEZ in the Museo de Ciencias Naturales de Guanare.

Several of the specimens were found in a 2022 expedition by researchers working with indigenous Huaorani people, who consider the anaconda sacred, in the Ecuadorian Amazon. The 10-day expedition, organized with Huaorani leader Penti Baihua, was accompanied by the filming of the Pole to Pole with Will Smith series for National Geographic, with actor Will Smith taking part in the expedition. The Huaorani collaborators were acknowledged as co-authors in the discovery paper.

Reactions
The description of Eunectes akayima has been favorably received by Wolfgang Böhme (zoologist), who acknowledged the validity of the genetic divide between the two green anaconda species. However, the study's other result of merging the yellow anaconda species Eunectes deschauenseei and E. beniensis into E. notaeus was criticized as premature by Böhme, whose own study upheld E. beniensis as distinct two years earlier.

Two reviews published in the month following the discovery criticized the original publication. The first critique, by Vásquez-Restrepo et al., only discussed the validity of the name Eunectes akayima on procedural grounds, pointing out violations of the International Code of Zoological Nomenclature. While also raising nomenclatural criticism, the second study by Dubois et al. additionally cast doubt on the reliability of the genetic markers used, and on the existence of a separate northern green anaconda species.

Future studies
Follow-up studies of E. akayima have been considered by the discovering team, aiming to understand the green anaconda's evolutionary history and the split between the two species. The differences in genital configurations of the two species is also an open topic of research. These often vary even among related species of snakes, as non-compatible genitals are often a factor preventing interbreeding between nearby populations and leading to further divergence between species.

Type specimen
While Carl Linnaeus originally described Boa murina in 1758, the precise location of the species' syntypes is not known, with the most likely locations of Suriname and French Guiana being contact zones between both populations. As the required genetic testing is not possible on samples of that age, it is uncertain which species the type specimens belonged to, making the choice of which species to refer to as Eunectes murinus ultimately arbitrary. With the southern clade being the most widespread, discoverers opted to keep the original name for the latter in the name of taxonomic stability. They designated as lectotype for E. murinus a specimen found in the Xingu River of Pará, Brazil in 2011 and now hosted in the Museu Paraense Emílio Goeldi, and established the northern population as a new species, Eunectes akayima.

Etymology
The species' name akayima comes from the local Cariban languages, with akayi meaning "snake" and the suffix -ima describing largeness in a way that elevates the term to a separate category, giving a literal meaning of "The Great Snake". The word akayima and variants (okoyimo, okoimo) have been used by the local Carib people to refer to the northern green anaconda for centuries before its formal scientific description. The term also refers in Cariban languages to the rainbow, likely associated with a feathered serpent in Carib beliefs. The Arekuna, for instance, speak of a rainbow serpent in their creation narrative, believed to be the source of all birds' plumage. Among the Carib peoples, the Wai-wai also give mythological significance to the Okoimo-Yenna or "anaconda people", understood either as relatives or as opposite counterparts to human beings.

The continued use of the name by indigenous people prior to the advent of the ICZN, along with the invalidity of previous proposed splits of E. murinus due to inconsistencies in differentiation, prompted the researchers to accept akayima as the senior synonym for the species.

Nomenclatural availability and validity
A review by Vásquez-Restrepo et al. argued against the validity of E. akayima, pointing out several purported violations of the ICZN in the Rivas et al. article. Notably, they concluded that the oldest available name for this taxon was Eunectes gigas (Latreille, 1801), historically considered the northern subspecies of E. murinus, but for which no evidence of distinctiveness had been provided prior to Rivas et al.'s molecular analyses.

Separately, Dubois et al. criticized the publication for lacking a well-defined species concept, while questioning the validity of the clades recovered through mitochondrial DNA. Their review claimed that the name itself was not validly published under the rules of the Zoological Code (ICZN) as it violated Article 13 of the Code, and labeled the designation as a nomen nudum. This would make the proposed name unavailable, rather than only invalid as Vásquez-Restrepo et al. concluded.

Both studies also considered the designation of the Eunectes murinus lectotype by Rivas et al. to be invalid, with Dubois et al. pointing out that the specimen (only collected in 2011) was not part of the syntypes designated by Linnaeus, and thus not valid as a lectotype. The latter study instead designated as lectotype a specimen collected by Albertus Seba in the Spanish West Indies, referenced by Linnaeus in his original description.

Genetic history
The divergence between Eunectes akayima and E. murinus has been estimated to have occurred between 5 and 20 million years ago, during the Miocene. Molecular clock analyses were calibrated using the earlier split between the geographically isolated Sanziniinae (found in Madagascar) and the rest of Boidae, along with fossil evidence. Approaches vary depending on the scenario considered for the aforementioned split, with scenarios involving one land bridge, two, or none providing different hard minimums for the Boidae divergence, and thus different calibrations for the split between the two species. Another method, only considering fossil evidence, led to the most recent estimate for the split between E. akayima and E. murinus, placing it between 5 and 11 million years ago.

The split between the two green anaconda species has been claimed by the discovering team to parallel other such north-south splits in South American fauna, such as between the northern caiman lizard (Dracaena guianensis) and the Paraguay caiman lizard (Dracaena paraguayensis), or between the Orinoco mata mata (Chelus orinocensis) and the Amazon mata mata (Chelus fimbriata). This was attributed by the authors to the rise of the Vaupés Arch between the Andes and the Guyana Shield, which created a geographical barrier between populations in the Proto-Orinoco and Proto-Amazon river basins. Modern E. akayima populations can be found farther south than the Vaupés Arch, and there is currently no geographical barrier between the two species.

A more recent divergence inside E. akayima has been dated to c. 3 Ma, splitting populations in Venezuela south of the Orinoco Delta. This is believed to coincide with the beginning of the Quaternary glaciation, as the sequestration of water by the ice caps led to wetlands receding, while forests grew and separated the Orinoco and Amazon river basins once again.

Description
Eunectes akayima has been described as one of the world's heaviest and longest snakes, with one specimen described in a newspaper article quote as being 6.3 m long. Unconfirmed reports from native Huaorani people speak of individuals reaching 7.5 m and 500 kg.

While the two are allegedly distinguished by their mitochondrial genomes, no morphological differences have yet been recognized between E. akayima and E. murinus, making them cryptic species. Morphological measurements such as scale count in several locations have been found to lie in the same ranges in both species.

Ecology and behavior
Northern green anacondas are ambush predators, and are among the apex predators in the swamps, rivers and other wetlands of northern South America, spending most of their time submerged in shallow waters. They hunt by waiting for prey to come nearby, with the buoyancy of the water helping them to rapidly leap out and take hold of the prey with their strong jaws. Like other boids, they are non-venomous constrictors, subduing their prey by wrapping themselves around and asphyxiating it, often crushing the prey before swallowing it whole.

Prey of the northern green anaconda include large animals such as capybaras, caimans and deer. It is a keystone species in its ecosystem, whose presence impacts the habits and migration patterns of other species in the surrounding environment. Despite popular beliefs, there have been no confirmed records of E. akayima hunting or eating humans.

Distribution and habitat
Eunectes akayima is found in northern South America. The precise distribution area is not yet known, but based on samples taken, the species is known to occur in Venezuela, French Guiana, Suriname, Guyana, Ecuador, and the island of Trinidad. Its range is also believed to include parts of Colombia and Peru, as well as northern Brazil.

Contact regions have been discovered, such as French Guiana and likely Suriname, where populations of both green anaconda species overlap with each other. While specimens of both have been found in nearby localities, notably on opposite riverbanks from each other, the two species have not been found to interbreed.

Conservation
The possibility of E. akayima being a separate species reveals a higher conservation risk than previously believed, despite the green anaconda being originally assessed as Least Concern by the IUCN due to its broad range. Lack of precise knowledge about population distribution make the assessment of both species' true status difficult, while the different ecological habitats and threats faced by both species mean specific conservation programs must be established for each of them. Notably, the northern green anaconda's smaller range makes it much more vulnerable than its southern neighbor.

As with other anaconda species, main threats include conflict with humans, as well as habitat degradation and fragmentation caused by agriculture, climate change and oil extraction in the region. Researchers noted the importance of monitoring population numbers for the species, as well as studying the effects of oil spill-related petrochemicals on the snake's reproductive biology. Their high sensitivity to changes makes them an indicator species for environmental health, highlighting the importance of assessing their populations. It is believed that 20 to 31 percent of the northern green anaconda's habitat has been lost to deforestation, with the number estimated to reach 40 percent by 2050.