During tests in the Pacific, fish did not flee even when the device came closer than one meter to them, the US researchers report in the journal 'Science Robotics'. The robot 'SoFi' (short for 'Soft Robotic Fish') can therefore observe marine animals in their natural environment without disturbing them, emphasize Robert Katzschmann and colleagues from the Massachusetts Institute of Technology (MIT) in Cambridge.

Normally, underwater robots move forward by means of propellers or water jet propulsion. "However, these propulsion systems generate considerable turbulence and have the potential to frighten marine life and prevent close-up observations," say the researchers. They therefore designed an underwater robot that moves like a fish: by moving its tail fin back and forth. A hydraulic motor sets the soft rear end in motion and can be set to different speeds.

A crucial component in 'SoFi' is the buoyancy unit: a piston in it regulates the volume of the enclosed air. If the air is compressed, 'SoFi' sinks; if it can expand, the robot rises. The movements are supported by two adjustable side fins. SoFi' carries a small camera at its tip, which enables it to take pictures of marine life. Divers can control SoFi by remote control with a range of up to 20 meters.

The fish robot is 47 cm long and weighs 1.6 kg. To accommodate the drive motor, sensors and other technology, Katzschmann and his team had to modify or develop many new robot components. But the effort seems to have paid off: Film footage shows that 'SoFi' is not perceived as a threat by sea creatures. The researchers are therefore confident that their robot will provide new insights into various marine ecosystems. 'SoFi' can be used in water depths of up to 18 m; further down, the water pressure becomes too strong.

Teething troubles of 'SoFi'

However, the team admits that some "teething problems" still need to be resolved: the tail movement causes the entire robotic fish to move back and forth, which is evident in the camera footage. Nevertheless, the researchers already have further plans: they want to use SoFi "firstly to study the behavior of marine life over long periods of time without human intervention in the scene, secondly to test whether SoFi can be used to influence the behavior of marine life, and thirdly to create robotic swarms". This would make it easier to analyze the dynamics of shoals of fish, for example.