COACTOR

ˌkōˌakˈtȯ(r); kō|ak|tȯ(r),  ̷ ̷ˈ ̷ ̷tə(r) noun

Etymology: co- + actor

: an organism that participates in ecological coaction ; especially : one (as a predator) that actively initiates coaction with another — compare coactee