Chelus fimbriata (matamata)

Morphological description and ecological aspects of Chelus fimbriata (Testudines, Chelidae), an aquatic turtle from the South American Neotropics

Chelus fimbriata , commonly known as the matamata turtle, is an aquatic species belonging to the Chelidae family, widely distributed in slow-moving freshwater bodies in the Amazon and Orinoco basins, including regions of Brazil, Colombia, Venezuela, Peru, Ecuador and Bolivia. This Neotropical turtle is distinguished by its highly specialized morphology, the result of an adaptive evolution to turbid aquatic environments with black water and slow currents.

The shell of Chelus fimbriata is flattened, rough, and irregularly contoured, with cryptic coloration that varies between shades of brown, green, and grayish, providing excellent camouflage among submerged leaf litter. Its total length can reach 45 cm in adults, with a weight of up to 15 kg, although this varies by sex and region. The shell surface features three poorly defined longitudinal keels and irregularly contoured vertebral and marginal scutes. The plastron is noticeably reduced and narrow, with indented margins, allowing for ample mobility for the neck and limbs.

The head is one of the species' most distinctive structures. Extremely flattened and elongated, it is adorned with multiple dermal projections and cutaneous flaps (stripes, barbels, and appendages) that, along with its coloration, give it the appearance of rotting plant litter. The eyes are positioned dorsolaterally, with papillose eyelids, allowing surveillance from a submerged position. The snout has a tubular extension at the tip of the face, which functions as a breathing tube, allowing the turtle to remain motionless and hidden with only its nose emerging from the surface.

The upper jaw lacks teeth, as is typical in chelonians, but is modified into a sharp, horny beak. The extremely long and flexible neck can be folded laterally into an S-shape, a characteristic common in Chelidae. This cervical elongation is associated with its suction-feeding strategy: Chelus fimbriata is an ambush predator specialized in small fish, crustaceans, and aquatic invertebrates. Prey capture occurs through rapid mouth expansion and cavitation, which generates a water current that draws the prey into the oral cavity.

The limbs are robust, webbed, and equipped with strong claws. They are adapted for locomotion in aquatic environments, although terrestrial movement is possible, albeit clumsy. The tail is short and robust, without ornamentation. The species is primarily nocturnal or crepuscular in habit and tends to avoid daytime activity, spending most of its time motionless among submerged vegetation.

From a reproductive perspective, Chelus fimbriata has an annual cycle, with oviposition occurring during the dry season. The female lays between 12 and 28 hard-shelled eggs in nests excavated in the riverbanks. Incubation can last between 150 and 200 days, depending on environmental conditions, and the young emerge during the first rains of the following wet season.

Phylogenetically, Chelus fimbriata belongs to the subfamily Chelinae and has historically been considered the only species in the genus Chelus . However, recent molecular genetics and cranial morphology studies have proposed the existence of a second species, Chelus orinocensis , restricted to the Orinoco basin, suggesting a more complex evolutionary divergence history than previously assumed.

Approximate measurements of the Turtle:

• 1:1 Scale Youth Complete
• Length 167  mm
• Height 49 mm
• Width 84 mm
• Shell length 100 mm

• 1:1 Adult Scale Complete
• Length 425 mm
• Height 124 mm
• Width 213 mm
• Shell length 255 mm