Barnacles and mussels are filter feeders that attach themselves to rocks in the ocean. They eat by filtering small particles of food from the ocean water as it flows over them. A scientist observed a dense colony of barnacles covering the surface of a rock. The mussels were only able to attach to a few, small crevices on the same rock. Evaluate this observation and explain a limiting factor that is affecting the barnacles and mussels on the ocean rock.