As the cracks grow bigger and become more frequent a rocky shore turns into a boulder-field. This increases habitat complexity and results in a higher diversity of the fauna. There are still the species of the rock platform and those of the cracks and crevices but to these are added the fauna that specialises in living under stones. The understone fauna can include tunicates, sponges, and bryozoa as well as more mobile crustaceans of all sizes from crabs to amphipods. Surprisingly, the most diverse fauna is not found on the biggest boulders that are highly stable but on medium sized ones that are disturbed occasionally.
Small rocks tend to have a poor fauna .