Subtidal communities of mobile substrates are vegetation-free environments. Most of the underwater areas with sand or mud are not covered by plants, as they need light and hard surfaces to grow, which are rare at this depth. The main characteristic of these areas is darkness. Organisms in such ecosystems feed on organic debris coming from other areas with currents, but also from the excrement and dead organisms of these areas. Many fish in these mobile substrate communities are carnivores (skates, tongues, turbot, etc.). Pelagic fish such as sea bream and sea urchins, in this aquarium, feed on inhabitants of the moving substrate.