Lesser Swimming Frog

This is a small, highly specialized amphibian belonging to the family Hylidae, outstandingly adapted to an aquatic lifestyle. It displays a small to moderate body size, with a snout-vent length ranging from 20 to 45 millimeters, where adult females are consistently larger than males. Its head is relatively small, featuring prominent, dorsolaterally positioned eyes that enable the frog to scan the environment above the water surface while keeping its body fully submerged. The dorsal coloration is highly variable, featuring shades of olive green, brown, or grayish, often patterned with dark longitudinal stripes or irregular spots that provide exceptional camouflage among floating vegetation. Its venter is whitish or pale yellow, characterized by a diagnostic pattern of dark longitudinal stripes on the posterior surface of the thighs, which serves as a key feature to distinguish it from related taxa. The hind limbs are remarkably robust and long, equipped with fully developed swimming webs and a spade-like inner metatarsal tubercle, granting this species superb swimming and diving capabilities.

Geographical distribution: This species is endemic to southern South America. Its geographic range encompasses vast areas of the Rio Grande do Sul state in southern Brazil, the entire territory of Uruguay, and northeastern Argentina, specifically within the provinces of Entre Ríos, Corrientes, Santa Fe, Chaco, Formosa, and Misiones. Additionally, populations have been documented in the bordering wetlands of southern Paraguay, closely following the basins of the Paraná and Uruguay rivers.

Habitat: It is an obligate aquatic organism that populates lentic and permanent or semi-permanent freshwater ecosystems. It is frequently observed in lagoons, marshes, swamps, flooded grasslands, and slow-moving streams, primarily within the Pampa and Espinal biomes. The species requires aquatic bodies with a high density of floating and emergent vegetation, such as water ferns and plants from the genera Salvinia, Pistia, and Eichhornia, which offer physical support, protection from predators, and foraging grounds.

Feeding: This frog exhibits a generalized carnivorous diet, feeding mainly on small invertebrates. Its trophic spectrum includes a wide range of aquatic and terrestrial insects that fall onto the water surface, such as beetles, flies, bugs, and dragonflies, as well as spiders, microcrustaceans, and small gastropod mollusks. It employs an active and ambush hunting strategy, capturing prey with rapid lunges on floating leaves or directly from the surface film of the water.

Behavior: It is a highly active species with predominantly diurnal and crepuscular habits, although its vocalizations peak during nocturnal hours. It spends most of its life floating horizontally with only its eyes and nostrils exposed, or resting quietly upon floating mats of vegetation. When threatened, it displays a remarkable ability to perform hydro-propelled leaps and dive instantly, burying itself in the bottom mud or seeking refuge among dense aquatic roots. During the coldest months of winter, it undergoes a period of torpor or dormancy, remaining inactive in the substrate of deep aquatic bodies.

Reproduction: The breeding season is extensive, running from early spring through late summer, typically between September and March. During this period, males gather on floating vegetation to emit a distinctive advertisement call, resembling the quacking of a duck, to attract females. Amplexus is axillary. Females deposit gelatinous egg masses containing hundreds of small eggs, which are carefully attached to submerged vegetation. The most remarkable feature of its life history is the larval stage; the giant tadpoles of this species exhibit gigantism, reaching up to 75 millimeters in length, which often makes them significantly larger than the fully metamorphosed adults.

Conservation status: It is currently classified as Least Concern (LC) on the IUCN Red List. This status is justified by its broad geographic distribution, stable population trends, and high abundance in suitable habitats. Nevertheless, local populations are increasingly threatened by the loss and degradation of wetlands due to agricultural expansion, chemical pollution from pesticide runoff, and the introduction of invasive predatory fish species.