In an odd sort of way, the conch is a symbol and practical tool of "symbiosis" in that it is the means for which the boys live together in harmony.
If you meant, "How is the conch symbolic," I'd say it represents order, law and maybe even camaraderie. Given that it was first used to summon the boys together, it is a symbol of unification, community, and unity. And since it is the means by which each of the boys has the right to speak, it also symbolizes power: political and otherwise. So, it represents law, order, unity, power, and in as much as it is used to keep the boys living together in harmony, it does represent a symbiotic catalyst.
As the order and civilization the boys had established begins to fade, the conch is destroyed and the general result is anarchy. The transition is interesting: kind of a descent from a rudimentary democracy (as long as each gets to speak) into anarchy.
The conch shell is a symbol of power. Whoever holds the conch, holds the power. It is also the one link the boys have to their previous connection to civilization. A world without some semblance of order creates chaos. The conch brings the boys together when it is blown. It establishes the position of leadership for whoever holds it. However, as the boys fall deeper into their inherant savage nature, the conch begins to lose its power. When Piggy gets crushed by the boulder we see the conch break amongst the rocks. When the conch breaks, so does their civilization.
l Parliamentary order and democracy
l It signifies the rules and discipline of the boys on the island. And is their last reminder and hope of civilization
l Throughout the novel, this sacred and powerful object losses its values and importance as the boys enter a stage of savagery
l At first, Ralph was proclaimed chief and he possessed the conch. He used them to call to an assembly, so to them, it somewhat symbolized the power and order amongst them. “There was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerful, there was the conch” (p. 24)