INTRODUCTION:

The rough-toothed dolphin, Steno bredanensis, is unique among Delphinidae as the gradual slope of the front portion of its skull forms a long, narrow beak with no separation between the melon and the snout. The species is distributed in the tropical and subtropical waters of the Pacific, Atlantic, and Indian Oceans. In Mexico, the species' distribution is based on records of strandings and sightings along the Pacific coasts of Baja California, Baja California Sur, Nayarit, and Sonora; and off the coasts of Veracruz and Yucatan in the Gulf of Mexico. This paper reports the live stranding of a juvenile male of the species in the community of Cerro Hermoso in Villa de Tututepec de Melchor Ocampo, Oaxaca, in September 2012. Measurements of the cranium and periotic-timpanic bone complex as well as the vertebra count of the stranded individual are presented here in the first published record of this species in the state.

METHODS:

Identification guides and general descriptions of the species were used to identify the species of the stranded individual based on two prominent characteristics: 1) the gently sloping beak with no clearly defined melon, and 2) the individual's body. An external examination was performed in order to identify the possible cause(s) of the stranding. Prior to the necropsy, 13 body measurements were taken. The skeleton was then obtained using bacterial maceration to permit measurement of the periotic-timpanic bone complex and cranium and determination of the dental (based on the alveolar count) and vertebral formulas.

RESULTS:

The cause of the stranding is not determined. As is characteristic of the species, the specimen presented hollow, conical teeth with a roughened surface created by ridges in the enamel. On the left side, the upper and lower dental series consisted of 21 alveoli each, while the right upper and lower dental series consisted of 22 alveoli each. The body measurements are presented in Figure 2, cranial and periotic-timpanic bone complex measurements in Figure 3. The vertebral formula was as follows: 7C, 13T, 16L, 29Ca (total = 65 + 25 chevrons in the caudal region). The vertebrae 1C and 2C were fused.

DISCUSSION AND CONCLUSIONS:

Our understanding of the biodiversity of the order Cetartiodactyla in the state of Oaxaca is still in its infancy; sporadic studies and opportunistic records of strandings have confirmed the presence of eight species of the family Delphinidae. This report on S. bredanensis increases the total number of marine mammal species recorded in the state and highlights the need for increased research and monitoring efforts aimed at understanding the ecology and diversity of the marine mammals that inhabit the waters off the coast of Oaxaca.