  We use data from 1222 late-type star-forming galaxies in the SDSS IV MaNGA
survey to identify regions in which the gas-phase metallicity is
anomalously-low compared to expectations from the tight empirical relation
between metallicity and stellar surface mass-density at a given stellar mass.
We find anomalously low metallicity (ALM) gas in 10% of the star-forming
spaxels, and in 25% of the galaxies in the sample. The incidence rate of ALM
gas increases strongly with both global and local measures of the specific
star-formation rate, and is higher in lower mass galaxies and in the outer
regions of galaxies. The incidence rate is also significantly higher in
morphologically disturbed galaxies. We estimate that the lifetimes of the ALM
regions are a few hundred Myr. We argue that the ALM gas has been delivered to
its present location by a combination of interactions, mergers, and accretion
from the halo, and that this infusion of gas stimulates star-formation. Given
the estimated lifetime and duty cycle of such events, we estimate that the
time-averaged accretion rate of ALM gas is similar to the star-formation rate
in late type galaxies over the mass-range M$_* \sim10^9$ to 10$^{10}$
M$_{\odot}$.
