Variações morfológicas e geográficas no sincrânio do peixe-boi da Amazônia Trichechus inunguis (Natterer, 1883)

Abstract

The Amazonian manatee (Trichechus inunguis) is endemic to the Amazon basin and the only freshwater sirenian. Apomorphies features relative to other Trichechidae derived from its adaptation to freshwater environment and it is distributed from the headwaters of rivers in Ecuador, Peru and Colombia to Amazon River estuary in Brazil. While studies of the morphology of this species, no cranial ontogenetic study was developed over its distribution. Morphological studies in Trichechidae have been conducted through the conventional morphometry, which cannot always describe or represent the variability of form from organisms. Geometric morphometrics is a toolbox that is been widely applied in studies comparing the shape of organisms taking into consideration the biological shapes and analyzing their variation statistically. To understand the ontogeny and morphology of this species, the aim of this study is to analyze the variation in size and shape of the Amazonian manatee skull throughout its ontogeny and morphological variations in sexual and geographic levels. A total of 80 skulls and 102 mandibles were used to collect morphometric geometric data in two and three dimensions. These skulls and mandibles were grouped into three age classes: calves, young and adult according to their origin and then were analyzed geometrically, seeking cranial changes between individuals of age groups and between specimens from different rivers of the Amazon basin. For this skulls and mandibles were scanned and after a geometrical treatment, configurations generated were analyzed statistically by components of shape and size. Amazonian manatee skull presents positive allometry and no sexual dimorphism was observed in this species. The morphological changes were observed in shape of the skull and mandible, which is influenced by size of the structures. The observation of the pattern of closure of supraoccipital - exoccipital synchondrosis and the classification proposed by Amaral et al. (unpublished data) were used to determine three age classes from shape of skulls and jaws. Specimens from white water rivers presented themselves different from other rivers from black and clear water. Within the group of specimens from different rivers of white water, the group originating from Japurá river presents different from the other groups, which are: Amazon, Solimões and Javari rivers.