In previous chapters, it was shown that the plants and animals on the shore occupy distinct zones or habitats in which they can survive and obtain the resources they require for growth and reproduction. They thus occupy a specific ecological niche. The ecological niche concept will be explored in the succeeding section. While a limited number of animal species exhibit direct development in which the juveniles hatch directly from the egg, other species have pelagic larvae that need to settle at an appropriate level on the shore in order to maintain viable populations. For sessile species, the choice of a settlement site is irreversible. Hence, such species have evolved behaviors that will ensure that they will settle at the right level on the shore. Other species such as mussels and limpets settle low on the shore and subsequently migrate to occupy the zone in which the adults are found. Most algae reproduce by forming microscopic life cycle stages that are released into the water column and later settle on rocky substrates. If they settle at the appropriate level, they will grow to give rise to the adult plant.