Description
Pleurodema somuncurense is an almost entirely aquatic frog with a male snout-vent length of 28 - 35 mm and a female snout-vent length of 30 - 44 mm. The head is larger than it is wide and the snout is round, having a nose that does not protrude. The nostrils are equidistant from the eyes and snout tip. The loreal region is flattened and the canthus rostralis is slightly distinct. The interocular and internarial distances are equal. The eyes are located dorsally and are prominent, having a diameter that is larger than the interocular distance with a horizontal pupil. The tympanum, which has a diameter of about half the eye diameter, is distinct and somewhat covered by the supratympanic fold. The forearm is slender and the fingers are imperfectly fringed, following the relative finger length formula II < IV < I < III. The metacarpal tubercles are weak. Prominent subarticular tubercles are present on the hands and feet. The hind limb is slender. The toes are about ⅓ webbed. The metatarsal tubercles are round and prominent. The nuptial pad is present. The skin is smooth (Cei 1980).

DIAGNOSIS: (How this species is differentiated from similar species)

Originally described as Telmatobius somuncurensis, the species can be differentiated from other Telmatobius species by the markings on the iris and the light dorsal midline stripe, which other Telmatobius species lack (Cei 1980).

COLORATION: (In life and/or in preservative)

The dorsum is brown or yellowish with irregular spotting and a light yellowish medial line. The belly is purplish-yellow, having reticulate spots speckled throughout. There are two circular markings centered on the upper and lower iris. The nuptial pad is pale gray. It is unclear whether this is in life or preservative (Cei 1980).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Argentina

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (4 records).

The species only inhabits the thermal springs on the northeastern slopes of the Somuncurá plateau, Río Negro Province, Argentina, within a range of 10 km² and an elevation range of 600 - 800 m. This is a volcanic area with extreme weather conditions but constant water temperatures at around 20 - 22 °C. (Cei 1980, Velasco et al. 2017). Valcheta Stream is one of the main waterways in the area and is the only place where P. somuncurense occurs. It is 80 km long and begins where tributaries Rama Fría and Rama Caliente meet (Velasco et al. 2019b).

Life History, Abundance, Activity, and Special Behaviors
Pleurodema somuncurense lives under stones or within floating mosses in streams and is not found in the stream beyond 1 m from the bank, where natural vegetation occurs (Cei 1980, Velasco et al. 2019b). Extant Pleurodema somuncurense population sizes are estimated to range from 243 to 4516 individuals (Velasco et al. 2019b).

The breeding season is likely in late spring, in October and early November. During this time, males can be observed to have very large testicles and tadpoles are seen in streams (Cei 1980). No tadpoles have been observed in winter. Pleurodema somuncurense exhibits reproductive behaviors that are characteristic of both prolonged and explosive breeders, including a breeding season of over five months, a lack of association between rain and reproduction, scramble competition, and males actively searching for mates (Velasco et al. 2017).

Amplexus is pelvic and multiple males may attach to one female. Axillary amplexus is rare. Amplexus occurs on the water’s surface with the female grasping onto a rock or vegetation (Cei 1980, Velasco et al. 2017).

Females lay an average of 78 eggs per clutch and can spawn multiple times per season. Breeding sites are completely covered by aquatic vegetation, namely Cardamine cordata and Hydrocotyle bonariensis. Oviposition only occurs in shallow lentic waters close to a thermal spring. Egg clutches are submerged, gelatinous, and string-like, with one end attached to the roots or stems of aquatic vegetation. Egg diameter is about 1.8 mm on average without the gelatinous capsule. The eggs are dark and have a black animal pole with a vegetal pole that begins pale and darkens to black over time. Tadpoles hatch about four days after oviposition (Velasco et al. 2017).

When hunting for prey, P. somuncurense mainly displays sit-and-wait behavior, but may actively search for prey as well. Pleurodema somuncurense has been observed to ambush prey by staying concealed in aquatic vegetation. Though P. somuncurense has a mainly aquatic life, aquatic prey like amphipod shrimps only composes about a quarter of its diet. Other prey include woodlice and ants. Ash has been found in some P. somuncurense stomachs, which comes from grassland fires for livestock management. These fires and livestock negatively impact P. somuncurense populations (see Trends and Threats). Captive individuals were observed hunting for prey before eating them while completely submerged in the water, using their forearms to collect the prey into their mouth (Velasco et al. 2019a).

Individuals can be infected with Hydracarina mites and have Leptus encapsulated beneath the skin (Cei 1980).

Pleurodema somuncurense shares the habitat with the endemic fish Gymnocharicanus bergi (Cei 1980). Other amphibian species that occupy the Valcheta Stream with P. somuncurense include Rhinella arenarum and Odonotphrynus occidentalis, the former being the most abundant species in the habitat (Velasco et al. 2019b).

Larva
Tadpoles with small hind limbs are about 40 - 44 mm in total length. The head is slender yet robust and wider than the body. The snout slightly protrudes and has a sucker-like shape. The nostrils are equidistant from the snout tip and the eye. The eyes are large and placed dorsally, having upper and lower menisci present in the iris. The interocular distance is greater than the internarial interval while being less than the distance between the eye and snout tip and the anterior dorsal musculature width. The lips are laterally folded, having many marginal papillae about the edges. The spiracle is sinistral and opens ventrolaterally. The viscera are visible from the ventral view. The cloaca opens ventrally on the ventral fin. The dorsal and ventral fins are lower toward the body and raised posteriorly (Cei 1980).

In life, the skin is dark. The dorsum is brownish gold while the ventral side is a reddish gray. Brownish spots mottle the caudal fins, which are transparent (Cei 1980).

Tadpoles hatch about four days after oviposition (Velasco et al. 2017) and live on the bottom of the stream. As indicated by the protruding mouth and snout, the tadpoles are omnivorous and eat slime (Cei 1980).

Tadpoles beginning metamorphosis have a total length of 46 - 49 mm, and froglets have a snout-vent length of 18 mm (Cei 1980).

Trends and Threats
Pleurodema somuncurense is listed as “Critically Endangered” in the IUCN Red List and is one of the most threatened species in Patagonia. It is also included in the top 100 of Evolutionarily Distinct and Globally Endangered (EDGE) species. The main threats to the species are invasive predatory rainbow trout, chytrid fungus, and livestock overgrazing, all of which contribute to habitat decline and population isolation (Velasco et al. 2019b, Velasco et al. 2018).

As of 2019, seven P. somuncurense populations exist throughout the Valcheta Stream, four of which inhabit Rama Fría and three of which inhabit Rama Caliente. The invasive rainbow trout Onchorynchus mykiss is a predator and the introduced fish Cheirodon interruptus competes with P. somuncurense for trophic resources (Velasco et al. 2019b). In addition to introduced predators, the chytrid fungus Batrachochytrium dendrobatidis threatens P. somuncurense populations. However, at infection levels produced in a laboratory, it may be possible that P. somuncurense is resistant or tolerant of the fungus (Arellano et al. 2018). Together these threats cause species isolation and decline, with rainbow trout inhabiting all stream corridors and reducing the species range to 2 km². Two P. somuncurense population extirpations, the El Destacamento and El Ariete populations, have occurred within recent decades. No individuals were found in El Destacamento after a dam construction in 2009, which resulted in water stagnation and livestock disturbance to native vegetation. Individuals have not been seen in El Ariete since 1969 (Velasco et al. 2019b, Velasco et al. 2018).

Additionally, dependence on terrestrial prey makes P. somuncurense susceptible to human activity that affects prey populations, like intentional fires that promote pasture growth and livestock overgrazing (Velasco et al. 2019b). Insufficient pastures for livestock has also led to livestock trampling on breeding sites. Compared to populations in captivity, wild P. somuncurense populations have a low number of clutches throughout the stream, indicating that breeding sites with the right conditions are scarce (Velasco et al. 2017).

Conservation efforts have been implemented to restore P. somuncurense populations, which involve limiting livestock in the area and removing the invasive toxic rosehip plant (Rosa sp., Velasco et al. 2018). A reintroduction program is also in place in the former El Destacamento habitat (Velasco et al. 2019b). According to population viability models, most current populations could support long-term populations if threats were removed. In the absence of threats, populations that would still be unsustainable in the long-term are affected by inbreeding and stochastic events, and would require the introduction of a mating pair every 17 - 19 years (Velasco et al. 2022).

Relation to Humans
There are no known uses of P. somuncurense by humans (IUCN 2016).

Possible reasons for amphibian decline

General habitat alteration and loss
Intensified agriculture or grazing
Dams changing river flow and/or covering habitat
Predators (natural or introduced)
Introduced competitors
Disease

Comments

PHYLOGENETIC RELATIONSHIPS:

Based on Bayesian analysis of cyt B, 12S, and 16S mtDNA as well as intervening tRNA, P. somuncurense is sister to P. bufonium and is included in the P. thaul clade. Besides P. somuncurense and P. bufonium, this clade also includes P. marmoratum and P. thaul. All of these species occur in Argentina or Chile (Faivovich et al. 2012).

ETYMOLOGY (Origin or explanation of scientific name):

The species was originally described as Telmatobius somuncurensis (Cei 1969). It was then placed in the Somuncuria genus, which was considered the sister genus to Pleurodema, before being placed into the Pleurodema genus as Pleurodema somuncurense (Faivovich et al. 2012).

OTHER INTERESTING INFORMATION:

The skin of P. somuncurense contains antimicrobial peptides (AMPs) somuncurin-1, somuncurin-2, and thaulin-3. All three have been shown to be effective against Escherichia coli while somuncurin-1 can also inhibit Staphylococcus aureus (Cancelarich et al. 2020).

References
Arellano, M.L., Velasco, M.A., Martínez Aguirre, T., Zarini, O., Belasen, A.M., James, T.Y., and Kacoliris, F.P. (2018). Treatment of adult Valcheta frogs Pleurodema somuncurense for chytrid fungus. Conservation Evidence, 15, 37-37. [link]

Cancelarich, N.L., Wilke, N., Fanani, M.L., Moreira, D.C., Pérez, L.O., Barbosa, E.A., Plácido, A., Socodato, R., Portugal, C.C., Relvas, J.B., de la Torre, B.G., Albericio, F., Basso, N.G., Leite, J.R., and Marani, M.M. (2020). Somuncurins: bioactive peptides from the skin of the endangered endemic Patagonian frog Pleurodema somuncurense. Journal of Natural Products 83(4), 972-984. [link]

Cei, J. M. 1980. Amphibians of Argentina. Monitore Zoologico Italiano. Nuova Serie, Monographia. Firenze 2: ixii + 609. 248-252.

Faivovich, J., N. G. Basso, C. F. B. Haddad, M. T. Rodrigues, W. C. Wheeler, E. O. Lavilla, and D. P. Ferraro (2012). A phylogenetic analysis of Pleurodema (Anura: Leptodactylidae: Leiuperinae) based on mitochondrial and nuclear gene sequences, with comments on the evolution of anuran foam nests. Cladistics 28: 460–482. [link]

IUCN SSC Amphibian Specialist Group. 2016. Pleurodema somuncurense. The IUCN Red List of Threatened Species 2016: e.T20372A85948443. https://dx.doi.org/10.2305/IUCN.UK.2016-1.RLTS.T20372A85948443.en. Accessed on 26 October 2023.

Velasco, M.A., Akmentins, M.S., Kass, C.A., Williams, J.D., and Kacoliris, F.P. (2019a). Diet of critically endangered Valcheta frog, Pleurodema somuncurense (Anura: Leptodactylidae), in the Somuncura Plateau, Patagonia, Argentina. North-Western Journal of Zoology, 15(2), 147-151. [link]

Velasco, M.A., Berkunsky, I., Akmentins, M.S, Kass, C.A., Arellano, M.L., Martínez Aguirre, T., Williams, J.D, and Kacoliris, F.P. (2019b). “Status and population dynamics of the Critically Endangered Valcheta frog Pleurodema somuncurense in Somuncura Plateau, Patagonia.” Endangered Species Research 40, 163-169. [link]

Velasco, M.A., Berkunsky, I., Di Pietro, D.O., Arellano, M.L., Williams, J.D. and Kacoliris, F.P. (2022). The Critically Endangered El Rincon stream frog: Population viability and management actions in Patagonia, Argentina. Aquatic Conservation: Marine and Freshwater Ecosystems, 32(11), 1842–1851. [link]

Velasco, M. A., Úbeda, C. A., Williams, J. D., and Kacoliris, F. P. (2017). Reproductive biology of the critically endangered Valcheta frog, Pleurodema somuncurense (Anura: Leptodactylidae), from Patagonia, Argentina. South American Journal of Herpetology, 12(3), 205-211. [link]

Species Account Citation: AmphibiaWeb 2023 Pleurodema somuncurense: El Rincon Stream Frog <https://amphibiaweb.org/species/9386> University of California, Berkeley, CA, USA. Accessed Apr 18, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 18 Apr 2024.

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