Bivalves can be dioecious, monoecious, or protandric. External fertilization allows the release of reproductive product from the gonad into the open water where immatures can develop and disperse. It is the process of external fertilization that makes it difficult to control the spread of pest species such as the zebra mussel. Immature bivalves develop into larval trochophores, and become adults when the veliger larva settles on a substrate. The advantage of having morphogenesis from one form to another is the reduction of competition between juvenile and adult members for food and space.
In contrast, some species brood their young in their gills before releasing them to pursue trochophore and veliger stages. Others enter the glochidium stage, which parasitizes fish by hooking onto their gills or body surfaces. After a period of growth during which nutrient was obtained from fish tissues, the larva releases itself from its host and begins filter feeding.
In the case of some classes, spawning is triggered by certain environmental chemical cues. For example, the mussel Dreissena polyphora's spawning is believed to be triggered by phytoplankton blooms, which are the main food source for this filter feeding species. The hormone serotonin also plays a role in the regulation of spawning.
The veliger is an immature stage in the development of organisms in the Mollosca. This form is typically taken on by classes Gastropoda and Bivalvia. This form is characterized by bilobed swimming structure called the velum which has a fine row of long cilia. These cilia are used for locomotion and for bringing food particles suspended in the water towards the mouth.
This is also the stage in development that researchers have chosen to pinpoint as a means of control to prevent further spread of pest species. These minute molluscs can go undetected in waters ways and are permitted to settle and grow to adult size. Thus, this stage is a dispersal mechanism used to increase the chance of successful fertilization and proliferation of the species.