I was on the east side of Vancouver island last weekend and noticed an interesting phenomenon on some barnacle rocks:
What is going on here? Have you seen this before?
I had a thought relating to competition and/or the boundary layer. Barnacles are particle feeders, scooping tasty treats from the water column. They also can come (as seen here) in very high densities. If you and your neighbour are both trying to steal the same nutritious blobs, there’s a problem. If you are just a little bit taller, you can reach out that little bit further and get the scoop. The same explanation fits when you look at fluid dynamics.
Water is a sticky substance, that’s how the meniscus is formed on your water glass or graduated cylinder. This causes a big problem for some aquatic animals, especially small ones, because the water sticks to every surface, including yourself. This creates a layer of highly viscous water covering everything – the boundary layer. This water is slow moving, and for a particle/filter feeder, is quickly emptied off all its foods. Thus animals need to reach out further from themselves to reach the faster moving water that is constantly refreshed with particulate matter/snacks. This is a simplistic explanation, and doesn’t touch on all the effects of a boundary layer on organisms, but will suffice here.
This might explain why some barnacles are so tall in the center, trying to reach out beyond the boundary layer for food.
Neither of these ideas help the barnacles in the grooves, which don’t seem to be unhealthy (although I did not complete a thorough survey to test this). Speculation: Perhaps the winding grooves act as ‘wind tunnels’, increasing the turbulence of the water (a factor which reduces the thickness of boundary layers), producing more flow in the depths of the brain as well….