Grunnion React to Moon Phases, by Cole Reeves

As the sun sets off the California coast, sky above and sea below are burnished with its soft orange glow. Beneath these gently sunlit waters, a certain little planktivorous fish pursuits its tiny prey, bright flashes in the delicately luminous sea from the silver stripe along its length. As the sun continues to set and the moon begins to rise the fish continues to pursuit its prey, for the fish is preparing for a time, now imminent, during which strenuous effort will be required. Above the grunion the moon, either luminously full or bathed in the stygian gloom of the new moon, heralds a change that will dictate a radical alteration in the behavior of the mature grunion, for the full or new moon coincides with the amplitudes of the tides reaching a zenith; this tidal zenith intimates within the mind of the grunion, however dimly the thought is realized, that the apical moment of the life history of the grunion is at hand and the time to perform the peculiar spawning ritual for which the grunion are known is soon to come. The grunion senses the descending onrush of water returning to the sea from the shore, and being attuned to its force it can sense that the spring tides are in their full strength, leading to a cascade of altered behavior associated with the spawning of the grunion.

In ordinary times, the grunion at night form relatively small groups, but during the time of the grunion’s spawning the grunion form huge schools of extraordinary density. As the grunion, in their densely packed ranks, venture forth towards the shore the brown pelicans and seagulls that had mustered their own ranks in anticipation of just this event sprung from their ambush in the shoreside rocks to descend on the bright silvery huddle. The pelicans and gulls plunge into the sea, taking great mouthfuls of fish, but the grunion appear in such density that their number is hardly depleted. As the gulls and the pelicans continue their assault from above, the grunion face another threat from below as the totoaba strike from the depths. The totoaba, a species of particularly large croaker, lunge voraciously and the grunion form boils at the surface as they flee its eagerly grasping jaws. As the grunion near the shore, they halt just outside the surf zone at which point a few male grunion head for the shore alone to allow the surf to carry them in and across the wave tossed beach. These grunion are called scouts, and for good reason, for if they are interrupted in their expedition the runs often do not materialize. However, if the scouts do return unmolested, the grunion run begins in earnest and the grunion move together, allowing the waves to carry them inshore, then actively fighting against the wave as it returns to the sea so that the grunion may remain onshore to perform the actual act of reproduction and the at first incomprehensible ritual that precedes it.

Grunion Spawning. Jeff Foott/Discovery Channel Images/Getty Images

The grunion run, to an outside observer looking at the writhing mass of fish illuminated in the light of a flashlight or a headlamp, looks like a perfectly chaotic bedlam of flopping silver bodies wriggling in the wet sand, thrashing about without any discernible pattern. There is however, a set mating ritual performed by the grunion, beginning with the male grunion bumping its head against the operculum or pectoral fins of the female fish. This signals to the female fish the amorous intentions of the male fish, and in response the female digs its way into the soft sand tail first to create a cavity so that it may lay its eggs therein. The male fish, though far more often several male fish wrap themselves around the female fish, which holds itself upright throughout, and squeeze the female to help the female fish release its eggs. The moment in which the female releases its eggs is proceeded almost instantaneously by the male grunion releasing their milt, which runs down the body of the female to reach the eggs in the cavity below as the female writhes in the soft sand to help the sperm reach the eggs below. The female subsequently extricates herself from both the sand and the tightly clinging males; as she leaves the sand closes around the eggs forming a protective capsule thereby and the male grunion, who often wrap around the female grunion in such numbers that they become buried in the wet sand and must struggle to extricate themselves, similarly untangle themselves. The entire act lasts an average of thirty seconds after which the female grunion, having expended all its eggs, returns to the ocean and the male grunion remain on the beach to seek the attention of further female fish. The actual act of spawning having concluded, the grunion run, which on average lasts just under an hour and a half, ends rather abruptly. Although there are fish coming and going throughout this chaotic period, the density of the fish remains fairly constant until at the very end when the density of fish drops precipitously.

With the coming of the next full or new moon, the final chapter of this unique saga begins to unfold as the next spring tide washes over the now fully developed grunion eggs. The progeny of the previous spring tide’s grunion run feel the ocean water move in and over the restful cavity of damp sand in which they had been incubating and an instinct as primal as their parents’ comes over them. The larval grunion emerge from their eggs and between the spring tide eroding the cavity in which they had been given form and the onrush of water making the sand itself a more tractable medium for movement, the grunion reach the surface of the sand where the tide’s embrace carries them out to sea.

Bibliography

Clark, Frances N. 1925. Fish Bulletin No. 10. The Life History of Leuresthes tenuis, an Atherine Fish with Tide Controlled Spawning Habits. Scripps Institution of Oceanography Library, San Diego.


Griem, J. & K Martin. 2000. Wave action: the environmental trigger for hatching in the California grunion Leuresthes tenuis (Teleostei: Atherinopsidae). Marine Biology (2000) 137: 177.


Leet, W. S. CDFG (California Department of Fish and Game). 2001. California's living marine resources: A status report. Sacramento, California.


Martin, K.L.M., Moravek, C.L., & Walker, A.J. 2011. Waiting for a sign: extended incubation postpones larval stage in the beach spawning California Grunnion Leuresthes tenuis (Ayres). Environ Biol Fish 91: 63-70. doi:10.1007/s10641-010-9760-4

Muench, K. A. 1977. Behavioral ecology and spawning periodicity of the Gulf of California grunion, Leuresthes sardina. The University of Arizona.

Taylor, M. 1984. Lunar Synchronization of Fish Reproduction. Transactions of the American Fisheries Society 113:4, 484-493.


Thomson, Donald A. and Muench, Kevin A. 1976. Influence of Tides and Waves on the Spawning Behavior of the Gulf of California Grunion, Leuresthes sardina (Jenkins and Evermann). Bulletin of the Southern California Academy of Sciences 75:198-205.


Walker, Boyd W. 1949. Periodicity of Spawning by the Grunion, Leuresthes tenuisL, an Atherine Fish. Scripps Institution of Oceanography. Master’s Thesis. UC San Diego, San Diego, California.