Planktonic larvae, also known as "meroplankton" are the most common early life stage of marine invertebrates, many of whom live on the bottom as adults. Some examples are crabs, clams, sea stars, barnacles, shrimp, worms, sponges, corals, and sea urchins. These animals, except for crabs and barnacles, release sperm and eggs into the water column when they reproduce, and the eggs develop into planktonic larvae. Crabs copulate and barnacles pseudocopulate rather than release sperm and eggs into the water column. Some of the larvae (depending on the species) may still have some yolk that they can use for nutrition. They are called lecithotrophic larvae. Others must find and capture food, and are called planktotrophic. These juvenile stages living in the plankton are very vulnerable to being eaten themselves. Critical to their success is finding the proper environment to metamorphose into the adult stage. Most worms, for example, require a soft muddy bottom, while barnacles require a hard substrate. The larvae go to the bottom, and if the substrate is not proper, they can go back up and remain in the plankton additional time. Some species, such as oysters, increase the chances of larvae finding a suitable environment by having the larvae attracted to adults of the same species, which will increase their chances of finding a suitable environment for settling and metamorphosis.