Eggs and sperm are typically released directly into the water through the mouth from the gonads located on the meridional canals beneath each comb row. The eggs are fertilized externally and develop into tiny ciliated larvae. Cestid and lobate comb jelly larvae develop cydippid-like paired tentacles that eventually become reduced as the adult body form takes hold. Cydippid ctenophores retain these tentacles throughout the lifespan. Beroid comb jellies, on the other hand, never pass through a tentaculate phase.
Comb jellies are common residents of both nearshore and open sea habitats.
Densities of open ocean species have probably been greatly underestimated since many delicate species are destroyed by trawls and other collection methods. More ctenophore species favor tropical seas, but Arctic waters may harbor very high densities of types like Pleurobrachia , Beroe and Bolinopsis .