Bottlenose dolphin ecotypes of the western South Atlantic: the puzzle of habitats, coloration patterns and dorsal fin shapes

Abstract

Phenotypic variations occur in several cetacean species, including common bottlenose dolphins Tursiops truncatus, which can also be distinguished as coastal and offshore ecotypes. In the western South Atlantic, these ecotypes have been described based on skeletal morphology and genetics. However, there is still no clear description to recognize them in the field. Here we searched for external diagnostic patterns that may facilitate their visual distinction and investigated their habitat use. We examined dorsal fin shapes and coloration of photo-identified dolphins distributed in a wide geographic range off the coast of southern and southeastern Brazil. A strong differentiation in the dorsal fin shape was observed, with a more falcate shape for offshore dolphins. We also found that offshore individuals have a darker color pattern, while coastal dolphins show 2 wider striped bands at the throat region and a longer rostrum, revealing that the ecotypes can be well distinguished in the field. We also detected differential habitat use. The coastal ecotype inhabits shallow waters (up to 18 m deep) close to the shore (up to 3 km). The offshore ecotype has a wider distribution and more flexible habitat use. It was usually found in coastal and deeper waters (maximum depth of 758 m and >200 km from the coast). Although we observed a small area of overlap in the distribution of the 2 ecotypes, both forms were not seen together. Therefore, our results reinforce the presence of a parapatric distribution and distinct morphology between the ecotypes, supporting their prior description as different subspecies.