Telmatobius macrostomus, also known as the Lake Junin (giant) frog or Andes smooth frog, is a very large and endangered species of frog in the family Telmatobiidae. This completely aquatic frog is endemic to lakes and associated waters at altitudes of  in the Andes of Junín and Pasco in central Peru. It has been introduced to slow-moving parts of the upper Mantaro River, although it is unclear if this population still persists.

Together with the closely related Amable Maria frog (T. brachydactylus), it is sometimes placed in the genus Batrachophrynus.

Ecology and appearance
T. macrostomus is completely aquatic, and restricted to lakes (notably Lake Junín) and waters that are directly connected to them like rivers, streams and canals. It has also been introduced to the upper part of the Mantaro River, but it is unclear if this population persists. It is found both in large marshy lakes and smaller deep lakes. This smooth-skinned, dark brownish frog generally ranges from shallow water to a depth of , but prefers places with extensive aquatic vegetation and  deep. In its native habitat the water temperature typically is between . Captives have been kept in aquariums for more than 5 years at around . It feeds on aquatic snails (especially Physidae), amphipods, aquatic insects (especially Baetidae mayfly larvae) and small fish.

T. macrostomus is among the largest frogs in the world  and the largest exclusively aquatic frog, a title sometimes incorrectly awarded to its somewhat smaller relative, the Titicaca water frog (T. culeus). T. macrostomus measures up to  in snout–to–vent length, and  in outstretched length. It can weigh up to about , and the hindlegs alone can surpass  in length. Very large individuals are rare; most adults have a snout–to–vent length of  and weigh less than . Females grow larger than males. The tadpoles also grow very large, up to about  in length.

Conservation and threats
Once common, this frog is now rare and considered endangered by the IUCN. During a survey in 2012 it was only found at about 10% of the visited sites and in low densities, leading to the suggestion of recognizing it as critically endangered. The primary threats to the species are capture for human consumption, predation by introduced trout, pollution from mining, agriculture and human waste, and water fluctuations caused by the Upamayo Dam. These factors have also contributed to the decline of the native Orestias pupfish, historically an important part of the diet of the frog, but not recorded in a recent study of its diet. Although not yet detected in T. macrostomus, some of its relatives have strongly declined due to diseases like chytridiomycosis and Ranavirus. Mass deaths have occurred in T. macrostomus, with a particular large one in 1996, but the reason for this is unknown.

Much of its range is within reserves, but protection is insufficient. Local and national groups, partially funded internationally, have started monitoring and conservation projects for this species, and education for the people living in its range. In 2008, an experimental captive breeding program was initiated at two centers at Lake Junin and it resulted in a few thousand tadpoles, but this project ceased in 2012. Following progress and success with the captive breeding program for the related Titicaca water frog, it is planned to expand it to the Junin species at Huachipa Zoo in Lima.

See also

 Goliath frog (Conraua goliath) – the world's largest frog from Africa
 Helmeted water toad (Calyptocephalella gayi) – a very large aquatic to semi-aquatic species from Chile

References

macrostomus
Amphibians of the Andes
Amphibians of Peru
Endemic fauna of Peru
Endangered animals
Endangered biota of South America
Taxonomy articles created by Polbot
Amphibians described in 1873
Taxa named by Wilhelm Peters