Two closely related nocturnal species of frogs in the same marsh sometimes mate, but the embryos almost always fail to complete development. The mechanism maintaining these frog species is .

Post-zygotic reproductive isolation

Explanation:

Reproductive isolation refers to a series of behavioral or physiological mechanism through which speciation of living organisms is achieved and maintained.

Reproductive isolation can be pre-zygotic or post-zygotic. Pre-zygotic processes are those that act to prevent fertilization and formation of zygotes while post-zygotic processes are those that act after fertilization and formation of zygote between two species.

In this case, the frog species sometimes mate but the embryos almost always fail to complete development. This is a clear case of post-zygotic reproductive isolation mechanism.