Imagine getting inside the mind of a shark: swimming silently through the ocean, sensing faint electrical fields, homing in on the trace of a scent, and navigating through the featureless depths for hour after hour.
We may soon be able to do just that via electrical probes in the shark's brain. Engineers funded by the US military have created a neural implant designed to enable a shark's brain signals to be manipulated remotely, controlling the animal's movements, and perhaps even decoding what it is feeling.
That team is among a number of groups around the world that have gained ethical approval to develop implants that can monitor and influence the behaviour of animals, from sharks and tuna to rats and monkeys. These researchers hope such implants will improve our understanding of how the animals interact with their environment, as well as boosting research into tackling human paralysis.
More controversially, the Pentagon hopes to exploit sharks' natural ability to glide quietly through the water, sense delicate electrical gradients and follow chemical trails. By remotely guiding the sharks' movements, they hope to transform the animals into stealth spies, perhaps capable of following vessels without being spotted.
The project, funded by the Defense Advanced Research Projects Agency (DARPA), based in Arlington, Virginia, was presented at the Ocean Sciences Meeting in Honolulu, Hawaii, last week.
Neural implants consist of a series of electrodes that are embedded into the animal's brain, which can then be used to stimulate various functional areas. Biologist Jelle Atema of Boston University and his students are using them to "steer" spiny dogfish in a tank via a phantom odour. As the dogfish swims about, the researchers beam a radio signal from a laptop to an antenna attached to the fish at one end and sticking up out of the water at the other. The electrodes then stimulate either the right or left of the olfactory centre, the area of the brain dedicated to smell. The fish flicks round to the corresponding side in response to the signal, as if it has caught a whiff of an interesting smell: the stronger the signal, the more sharply it turns.
Atema plans to use the implants to study how sharks track chemical trails. We know that sharks have an extremely acute sense of smell, but exactly how the animals deploy that sense in the wild has so far been a matter of conjecture. Neural implants could change all that. "You get much better information from a swimming shark than from an anaesthetised animal that is strapped down," says Atema. "It could open up a whole new window into how these animals interact with their world."
At the Hawaii Institute of Marine Biology, Tim Tricas is using the implant to investigate what information scalloped hammerhead sharks glean from their electric field sensors. Gelfilled pores, scattered across a shark's head connect to nerve endings that make them sensitive to voltage gradients. Sharks can use these electroreceptors to spot the weak bioelectric fields around hidden prey, such as a flounder buried in sand.
For decades, marine biologists have suspected that sharks might also use these electroreceptors for navigation. Tiger and blue sharks can swim mile after mile in a straight line with no view of the ocean floor and only scattered, changing light coming from above. Some researchers suspect they maintain their heading by using the Earth's magnetic field. When a conductor – in this case the shark – passes through a magnetic field, the interaction sets up a voltage across the conductor. The strength and orientation of that voltage depends on the conductor's angle to the magnetic field. If a shark could detect those changes, it could use its electrical receptors like a compass. The only way to test this, Tricas says, is to monitor electroreception in a freely swimming shark.
The scientists will be particularly interested in the sharks' health during the tests. As wild predators, it is very easy to exhaust them, and this will place strict limits on how long the researchers can control their movements in any one session without harming them. Despite this limitation, though, remote controlled sharks do have advantages that robotic underwater surveillance vehicles just cannot match: they are silent, and they power themselves.