Marine shells (bi-valves) in water systems are caused by ova being released by mature bi-valves when the water temperature is of the right order. Millions of ova drift with the current until they stick onto a mass that can support their growth. The ova gradually over several weeks develop a shell and grow to maturity. Bi-valves filter their feeding requirements as the water passes through their shells. When mature they release ova, and should the bi-valves be within a piping system, the build up will rapidly accelerate.
Because shell ova are contained in a glutinous jelly like substance, shells can build up in low to moderate flows. Ova at the time of lodgment are extremely susceptible to attack by low chlorine concentrations. Free chlorine in the seawater destroys the glutinous jelly, and exposes the ova directly to further direct chlorine attack, thereby destroying it. However in systems where the ova have developed a shell, it can be very difficult to eradicate. When feeding, the bi-valve shell is open to allow water to flow through, should there be any chlorine in the water it will immediately shut and remain shut. It will continue to open and shut until either the chlorine is no longer there, or the chlorine will destroy the inner flesh and kill it. Therefore shock dosing continuously on a time scale of less than 2 to 3 weeks is ineffective in controlling shell that is already formed in pipe work.