Benthos

All organisms found at the bottom or on the seabed. E.g. macro-algae are attached to the bottom, they are part of the benthos (unlike unicellular algae or micro-algae that float passively and are therefore part of the phytoplankton). Molluscs, crustaceans and worms are also part of the benthos, although their eggs are planktonic. It is the same with echinoderms.