Blind cave fish navigate through the dark with a sixth sense

Humans have five senses: sight, smell, sound, taste and touch. Fish, however, can tap into an extraordinary sixth sense called the lateral line. This sense – which consists of rows of special cells with microscopic hairs – helps fish sense motion and vibrations in the surrounding water. In the pitch black of underwater caves, vision becomes a useless ability to the cave's scaley inhabitants. In fact, many cave-dwelling fishes are blind – such as the aptly named Mexican blind cavefish (Astyanax mexicanus). Without vision, such fish are especially reliant on their lateral line to sense food, friends and foes and, possibly, to create mental maps of their surroundings. However, creating mental maps of underwater caves is no simple task. As the lateral line can only sense objects very close to the fish (within a fin's reach, if you will), navigating through the cave's complex terrain would require fish to extensively explore the habitat. Sofia Marketaki and Fidji Berio of Stockholm University, Sweden, partnered with Valentina Di Santo of Scripps Institute of Oceanography, USA, to investigate whether the Mexican blind cavefish takes on this explorer's challenge to traverse its home using its lateral line.To find out whether the Mexican blind cavefish relies on its lateral line for navigation, Marketaki and colleagues tested whether the fish could find their way through an obstacle course if they couldn't use their lateral line. The scientists filmed each fish as they swam through a series of obstacle courses and compared how much the fish explored the course as they navigated through. Remarkably, the researchers found that fish with a functioning lateral line took their time and opted to thoroughly explore the obstacle course. However, those without a working lateral line sped right through, suggesting that the cavefish take time to explore their surroundings so their lateral line can provide them with precise navigational information.The team also measured how fast the fish swam through the obstacle course and how often they bumped into the obstacles along the way. Fish with their lateral line intact swam nearly 30% slower and bumped into obstacles less often than those that couldn't use their lateral line. Conversely, the speedy fish without a working lateral line not only ran into obstacles more often but also did so seemingly deliberately, potentially so they could navigate the terrain by feeling with outstretched fins and by bumping into things head on. Marketaki and the team deduced that this difference in behaviour reflects a shift in how the fish navigate when the lateral line is lost. It seems that these cavefish traverse their rugged habitats with precise exploration made possible by the lateral line but, if they lose their lateral line, they default to a backup plan of feeling their way around obstacles by gently running into them.Marketaki, Berio and Di Santo made a new and exciting discovery that the lateral line helps these fish navigate through a challenging environment when vision is futile. Further, this study also highlights that cavefish are remarkably resilient and can compensate for a disabled lateral line by shifting from a slow and careful strategy to a fast and reckless tactic. It's almost as though, when their internal GPS goes offline, these fish just put the pedal to the metal and hope for the best.