They often cling to seagrass (posidonia) or other grass on rocks and sometimes gather in large groups.

The head consists of two parts: a roughly triangular anterior lobe—the prostomium—and a posterior ring-like portion—the peristomium.

The scientific Latin genus name Nereis derives from the Ancient Greek Νηρηΐς : Nērēís (stem Νηρηΐδ- : Nērēid), a sea nymph.

They are dioecious (individuals are male or female) and they release their haploid gametes into the water, a process called spawning.

Moreover, upon fertilization and mitotic divisions of the zygote, Nereids form a larval stage which is similar to that of molluscs – i.e. a trochophore larva.