These are grafted or bio-engineered combat modifications appended to the beast's throat, chest, tail, etc. They enhance the creature in combat without inhibiting its ability to carry and use other weapon-symbiotes.
Identifiable as a barbed stinger attached to a tail or tongue, Implant attacks usually deliver corrosive pathogens, parasitic infections, organic acids or lethal toxins. Survivors of such attacks are rare.
A model which fails its saving throw when wounded by a creature with an Implant attack biomorph enhancement suffers two wounds instead of one. Implant attacks do not affect wounds inflicted by bio-plasma.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): When the attacking creature has rending claws or is a monstrous creature ignoring Armour saves outright, there's no save roll to fail — but the ignored save still counts as automatically "failed" for the purposes of triggering the Implant attack's extra wound.