Zygote is formed by the fusion of the male and female gamete. The sperms and ova can be released in the water or any other environment depending on the type of species.
The clams also releases their gametes in the water. The sperms and eggs that belong to the same species can successfully unites to form the zygotes. This also represents the gametic isolation.
When the food hits our tongues it triggered saliva (chemical digestion) then our mouths chewing is mechanical. So therefor your answer is The Mouth. Hope this helps <3
