Grunion are sardine-sized fish in the Atherinopsidae family of New World silversides. They are found only off the coast of California, USA and Baja California, Mexico (found on both the Pacific Ocean and Gulf of California coasts).
Grunion are known for their very unusual mating ritual. At very high tides the females come up on sandy beaches and dig their tails into the sand to lay their eggs. A male then wraps himself around the female to deposit his sperm. For the next ten days the grunion eggs remain hidden in the sand, but at the next set of high tides the eggs hatch and the young grunion are washed out to sea.
There is also a related species in the Gulf of California, the False Grunion (Colpichthys regis) that looks very similar, but does not have the same breeding method.