Major Habitat Type
Tropical and subtropical upland rivers
This ecoregion includes the entire Essequibo River basin in Guyana and part of southeastern Venezuela where it drains the eastern margin of the Guiana Shield (source at 600 m) and low country south of the Orinoco Delta. The Essequibo is separated from the Caroní (Orinoco Basin) in the west and southwest and Branco (Amazon Basin) to the south by moderate divides of little more than 500 m elevation. It is also separated from adjacent small rivers of coastal Guyana by low divides of only a few meters. It is limited to the east by the drainage divide between the Essequibo and Demerara rivers.
Drainages flowing into
Main rivers or other water bodies
The main rivers of the ecoregion are the Cuyuni, Mazaruni, Potaro, Rupununi, and Essequibo.
The ecoregion encompasses a low coastal plain, forested hills, and highlands. Some of the ranges include the Pakaraima, Kanuku, and Acarai mountains, from which the Essequibo River rises. Elevations range from sea level to nearly 2400 m asl.
According to Köppen (1936), the majority of this ecoregion falls within the tropical rainforest (Af) zone, which experiences little seasonal variation in temperature. Mean annual temperatures range from 16 °C in the highlands to over 27 °C in the lower moist forests. Mean annual precipitation ranges between 1000–3800 mm.
The uplands include moderate gradient small streams and rivers with rapids and cataracts. Kaieteur Falls (251 m) on the Potaro River is impressive with a flow rate of 663 m³/s. Along the coastal plain are low gradient rivers, lakes, and swamps with seasonal flooding.
Moist forests cover a majority of the ecoregion, with marsh forests along rivers.
The Essequibo is a biogeographically complex basin that hosts both endemics and species otherwise distinct from neighboring river systems to the east (Atlantic coastal drainages), west (Orinoco), and south (Amazonas). A recent compilation of species using Cloffsca and the CAS database lists over 310 described species, of which 58 are considered endemic.
Description of endemic fishes
The Essequibo contains 58 endemic species including a handful of Rivulus species (Rivulidae), Bryconamericus hyphesson (Characidae), Lithogenes villosus, and Parotocinclus collinsae (Loricariidae). Bryconops colaroja (Characidae) is endemic to the Cuyuní Basin (Chernoff & Machado-Allison 1999) and the monotypic Mazarunia mazarunii (Cichlidae) is known only from the Upper Mazaruni (Kullander 1990). Farlowella rugosa is near-endemic to this ecoregion and the Guianas . Endemic genera are: Acanthocharax, Mazarunia, and Skiotocharax.
Other noteworthy fishes
The only goliath catfish that occurs in this ecoregion is the Laolau catfish (Brachyplatystoma vaillantii).
Other noteworthy aquatic biotic elements
The ecoregion contains the potamadromous barred sorubim (Pseudoplatystoma fasciatum).
The type locality of the enigmatic genus Lithogenes (Eigenmann 1909) is known from a single cataract (Chenapou Falls) on the Upper Potaro above Kaieteur Falls. Lithogenes is alternately considered the most primitive member of the family Loricariidae (e.g. Schaefer 1987) or a species of Astroblepidae (Nijssen and Isbruucker 1986; J. Armbruster, pers. comm.), the sister family to Loricariidae otherwise restricted to high-elevation Andean streams. Another interesting catfish genus, Corymbophanes (Loricariidae), also is known only from the Upper Potaro River above Kaieteur Falls (Armbruster et al. 2000). Corymbophanes contains two species, C. andersoni and C. kaiei, that are morphologically adapted for separate habitats, large-river cataracts and small-creek riffles, respectively. Its primitive position among loricariid catfishes and apparent endemism suggest that Corymbophanes is a relict of the original fauna of the Guiana Shield (Eigenmann 1912; Armbruster et al. 2000). Another relict species is the arapaima (Arapaima gigas) from the bonytongue family, Arapaimidae. Growing more than two meters in length it is one of the largest freshwater fish in the world.
Justification for delineation
This ecoregion lies in the Guyanan ichthyographic province outlined in Ringuelet (1975) and more broadly within the Guyanan-Amazonian ichthyographic region (Gery 1969; Ringuelet 1975). It has a unique composition of endemic Guiana Shield taxa that are considered the most primitive sister groups to many fish taxa in South America.
Level of taxonomic exploration
Fair in larger rivers, still poor in headwaters and small streams.