Acoustic underwater tags are key devices to study marine animals and obtain information regarding their behaviour. This information is essential to increase our knowledge of oceanic species and implement efficient conservation policies. At present, all the acoustic tags have a unidirectional communication protocol, which introduces important limitations for their localization such as range measurement, and in situ reconfiguration. To solve these issues and improve the current state-of-the-art of acoustic tags, a new bidirectional tag device has been developed. This new tag will allow new studies and will open a wide tracking capability by using autonomous underwater vehicles and range-based algorithms. The main characteristics of the tag communications scheme such as the chosen modulation, its implementation and the communication protocol are presented on this paper. Also, the simulation software and the conclusions that lead to the initial prototype design are presented.