Grunion are small marine fish that are about the size of sardines and belong to the family Atherinopsidae. The grunion is a slender fish that is pearl gray in color on its belly and side but a greenish blue on its back. Its most distinguishing physical characteristic is a striking silver-blue band on its sides. Adult grunion are about 6-7 inches (15-17 cm) long, with females being somewhat larger than males. Their usual life span is three to four years, but according to researchers, comparatively few live longer than three.
Geographically, the primary range of this fish is in the coastal waters of the Pacific Ocean along the western edge of the United States. There are only two species: the California grunion, or Leuresthes tenuis; and the gulf grunion, Leuresthes sardinas. Each inhabits a different region. The California species ranges from roughly Monterey Bay in California south to Punta Abreojos on the western coast of the Baja peninsula in Mexico, and the gulf species inhabits only the Gulf of California along the eastern coast of the Baja peninsula. Isolated small populations, however, are occasionally found south and north of these points.
For habitat, adults prefer the relatively shallow inshore waters near the open coast and in bays. These fish like to live at or near the surface and, as far as researchers know, they do not migrate. Their spawning behaviors are perfectly adapted to the surf zone near sandy beaches. Not much is known about the diet of grunion. They have no teeth, so scientists assume that grunion feed on very small organisms such as animal plankton.
The grunion's spawning season varies slightly from year to year but generally lasts from early spring through early fall. Grunion, unlike other species of fish, come completely out of the water to deposit and fertilize their eggs on wet, sandy beaches. Eggs are laid in the sand only during full or new moons when the spring tide cycle is at its highest. This cycle is so regular that the spawning date can be predicted a year in advance.
Males and females swim in on waves and remain stranded on the sand when the water recedes. Females tunnel tail first into the sand to lay eggs, and the males discharge sperm directly onto the eggs as they are being laid. This process takes about 30 seconds, and both sexes immediately return to the ocean. Wave action keeps the eggs buried under the sand. In about 15 days, the eggs hatch, and waves carry the larvae to the ocean.