The Grunion’s Spawning Technique

The small fish called the California grunion spawns on the Pacific beaches of California, U.S.A., and Baja California, Mexico. These fish know the precise days and times when they should spawn so their offspring will survive.

Consider: Grunions spawn only during the three or four nights after the highest tide associated with each full or new moon. If they spawned on the nights before the full or new moon, their eggs would be washed out to sea by the rising tides, which erode sand from the beach. However, because they spawn shortly after the tides peak—when the waves start to recede and redeposit sand—the eggs are safe under an increasing layer of sand.

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In addition, during their spawning season in the spring and summer, the high tide at night is always higher than the high tide during the day—which allows the fish to swim farther up the beach and keeps eggs out of reach of the following high tides.

To spawn, grunions wait for a far-reaching wave and ride it onto the beach, swimming as far as possible until they are stranded. As the water recedes, the female digs a hole in the wet sand with her body, partially burying herself tail first. As she deposits her eggs five to eight centimeters (2 to 3 in.) deep, one or more males fertilize them. Then the fish wriggle toward the water and ride the next wave back to the ocean.

The eggs develop in the moist sand but need the agitation of the waves to trigger hatching. Therefore, about two weeks later, at the next peak tide, the baby grunions hatch, although this can wait for four weeks till the following peak tide.

What do you think: Did the grunion’s ability to know when and how to spawn evolve? Or was it designed?