Sexes are separate in most bivalves and paired gonads lie in the upper part of the foot region intermixed with elements of the digestive system. For this reason it is difficult to obtain reliable measuremens of gonad size; hence, gonad indices are unavailable. Paired gonoducts release gametes into the exhalent water flow just before it enters the exhalent siphon. Fertilisation leads to the development of a veliger larva that swims and feeds in the plankton for several weeks. This transforms into a bivalved form known as a pediveliger, that is the settling stage. All developmental times shown in the schematic are, of course, temperature-dependent.