When the time comes for barnacle youngsters to settle down and set up home, it's all about location, location, location. ‘Juvenile and adult barnacles cannot move’, explain Kiyotaka Matsumura and Pei-Yuan Qian from the Hong Kong University of Science and Technology, so their chances of contributing to the next generation depend entirely on their choice of location: settling close to nearby clusters of barnacles is ideal. The duo explains that established barnacle residents release odours to attract mobile barnacle cyprids (the last life stage before barnacles settle down). However, these scents only work over short distances: could barnacle cyprids also rely on vision to guide their location life choice? Matsumura and Qian explain that the larvae have compound eyes and sophisticated sensory processing – absent in earlier and later life stages – which the youngsters could use to identify a prime location, so they decided to test how much barnacle cyprids rely on vision when identifying a location (p. ).
Using a series of experiments where barnacle cyprids were allowed to select settlement sites in the presence of adult barnacles, the duo discovered that the cyprids preferred to settle near to barnacles, even when the cyprids could not smell them, and they could easily distinguish between barnacle-sized pebbles and barnacles in a transparent box. Next, the duo tested the cyprid's colour preferences, and found that the youngsters strongly preferred to settle on red surfaces. And when the team checked the colour of adult barnacle shells, they found that the shells produced bright red fluorescence. Having carefully extracted the fluorescence from the shells, Matsumura and Qian placed a vial of the fluorescent fluid into an aquarium with the cyprids and found that it was twice as attractive to the barnacle youngsters as vials of fresh seawater.
So, barnacle youngsters use vision to identify prime locations near barnacle adults, guided by the residents' vivid red fluorescence.
