A symmetrical cubic discrete coupled logistic equation is proposed to model the symbiotic interaction of two isolated species. The coupling depends on the population size of both species and on a positive constant lambda, called the mutual benefit. Different dynamical regimes are obtained when the mutual benefit is modified. For small lambda, the species become extinct. For increasing lambda, the system stabilizes in a synchronized state or oscillates in a two-periodic orbit. For the greatest permitted values of lambda, the dynamics evolves into a quasiperiodic, into a chaotic scenario, or into extinction. The basins for these regimes are visualized as colored figures on the plane. These patterns suffer different changes as consequence of basins' bifurcations. The use of the critical curves allows us to determine the influence of the zones with different numbers of first-rank preimages in those bifurcation mechanisms.