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I have no idea but here is my best guess.
The sperm recognizes some chem trail from the egg, or receptor on the surface of the egg which the sperm can bind to. This allows the sperm to be swallowed by the egg.
It might not work for interspecies scenarios because the receptor on the egg varies. But a hybrid organism can fail at many stages. For one meiosis may not be able to take place because genetic info isn't compatible.
If meiosis works and a hybrid organism begins developing in the womb, it is possible that it is not viable outside the womb because larger structures like organs do not function correctly.
Finally it is possible that the hybrid organism is viable outside the womb and develops into adulthood, yet when it mates its offspring are not viable so it is the last of its "blood".
