Fish, Moon And Tides: The Grunion Story (1963)

On certain nights during spring and summer months, sandy California beaches are the scene of a spectacular display put on by small silvery fish called grunion. By the thousands, these fish come out of the ocean to lay their eggs in the wet beach sand. Of all the fishes in the world, only grunion show such spawning habits. There's nothing very unusual about their appearance. They are just small silvery fish about 5 to 6 inches long. However, their breeding habits are most unusual. In addition, they can live out of water for over 20 minutes, much longer than their close relatives. Grunion spawn on sandy beaches along the Pacific coast from central California south into Baja California in Mexico. Another species occurs in the Gulf of California. Grunion spawn only on three or four nights following the new moon and full moon phases. And on these nights for only two or three hours after the time of high tide. Now, let's see what happens during a typical grunion run. The run begins about 20 minutes after high tide with a few fish occasionally stranding themselves on the beach. The numbers gradually increase. However, the quantity of fish on the beach varies greatly during a run. There may even be periods in which no fish can be seen. The heaviest part of the run occurs between 1 and 2 hours after high tide. At this time, there may be thousands of fish on the beach. Although vast numbers of fish are out of water at one time, there are very few casualties. Now, let's see what the fish are doing on the beach. As we look closer, we can see that some of the fish are burrowing into the soft sand, tail first. These are the females who deposit their eggs about 2 in below the surface of the sand. The males do not dig in, but curl their bodies around the female and deposit their milt next to her body. The milt flows down the side of the female to fertilize the eggs in the sand. When all her eggs are laid, she flops or wiggles from side to side to free herself from the sand. After her task is finished, she wriggles toward the water or waits on the sand for a wave to carry her back into the ocean. The female starts to dig in only when there are male fish nearby. If no males are close, she returns to the ocean to come in again on a later wave. The female shivers and sinks into the semi-fluid sand and begins to twist her body from side to side as she bores into the sand deeper and deeper. When her body is 1/2 to 3/4 buried, she releases her eggs. During this period, she remains motionless or moves her body slowly from side to side. There are many more males than females on the beach at any one time because the males come in many times on one run and for several nights after each moon phase. The females may come in several times before they lay their eggs, but once they have spawned, they do not return to the beach until 2 weeks later. After the spawning season starts, both males and females spawn every 2 weeks and an individual female may spawn four to six times each season. She may lay from 1,000 to 3,000 eggs at each spawning. Thus, one female may lay as many as 18,000 eggs in one season. A large percentage of these eggs hatch, but very few fish reach adult size. Most are probably eaten by other fish. The spawning fish are 1 and 2 years old with a few 3-year-olds. During the open season when grunion fishing is permitted, many people of all ages come to the beaches for grunion hunts. Though small, grunion are considered a special delicacy by many people. These strange fish are protected by law. They may be taken only with bare hands. No special gear of any kind may be used. The grunion hunt is a mad scramble with every man for himself. Usually everybody ends up soaking wet. This group has more than just sport in mind. They are helping to collect fish from which we will obtain eggs for laboratory study. Male and female grunion are put in separate buckets. The living fish are gently squeezed to remove the eggs from the female. After the eggs are removed, the living fish are returned to the ocean. Grunion eggs, a clear bright orange in color, are about 2 mm or 1/16 of an inch in diameter or about the size of a pinhead. The eggs will not develop unless they are fertilized by this white liquid called milt produced by the male grunion. When the milt and eggs come together, fertilization takes place and development of the embryo begins immediately. After rinsing the eggs in seawater, we pour off the excess liquid and then put the eggs between paper towels moistened in seawater. We will put more than 1,000 eggs on this towel. The moist paper towels produce about the same conditions as the moist sand of the beach and allow us to sample the eggs easily. Then we take the eggs to the laboratory where they are stored in a cool place. We have done artificially the same thing the fish do in nature. Here the milt deposited by the male may be seen around the female's body. The milt runs down over the eggs which the female is laying and fertilization takes place under the sand. We can find the fertilized eggs by digging a few inches into the sand. They lie in a ball and will be covered deeper by sand left by succeeding waves. In this relatively cool damp place, the baby grunion will grow in the eggs. The eggs we took to the laboratory in paper towels develop just as those in the sand on the beach and we can study the growth of the baby grunion by looking at an egg every day under the microscope. Here we are looking at a single grunion egg about 15 hours after fertilization. This is the egg membrane which protects the egg like the shell of a egg. The yolk, the large darker ball in the center, contains many oil droplets and supplies the food from which the baby grunion will grow. Here is the first sign of what will become the baby grunion, merely a cap of cells on the side of the yolk. At 3 days, the embryo or baby grunion has started to form. We can see the head and the eyes. The body is curled around the yolk. Notice that the oil droplets are becoming fewer and larger. When 4 and 1/2 days old, the embryo is better formed. And the heart, just under the head, has started to beat. There are also weak movements of the tail. By 5 and 1/2 days, the heart is larger and the circulation of blood is clearly seen. The blood flows over the yolk to carry food to the embryo. By the 7th day, the embryo is much larger. We can see the head with its large eyes and the body curled completely around the yolk. By the 8th day, the embryo is almost ready to hatch and it wiggles often. It has increased greatly in size and we can plainly see that the yolk is now much smaller. The embryo has grown into a baby fish. When the embryo is 9 days old, it looks like a ball with two big eyes. We have a complete baby fish tightly curled inside the egg. The eggs are now ready to hatch, but they will not hatch until agitated or shaken. We do this by placing some of the eggs in seawater and stirring them vigorously. This action duplicates what happens to the eggs when they are washed free from the sand by waves. If the eggs are not agitated, they will not hatch even if covered by water. The baby grunion remains alive, however, and can hatch even after 3 weeks in the egg. Here is a group of mature eggs about 2 and 1/2 minutes after they have been shaken. Some of the eggs have already hatched and more fish are popping out of their shells every moment. Within about 5 minutes, almost all of the eggs have hatched. At this time, our hatching tank resembles Times Square on New Year's Eve. The empty egg membranes float like balloons among the baby fish as they dash in all directions at once. Now, we're going to show you a close-up of the grunion eggs as they hatch. Don't blink your eyes or you won't see them hatch. Watch the egg at the tip of the arrow. In case you missed the hatching, here is another chance. In a fraction of a second, the grunion twists and pops out of his shell tail first. If we are to understand some of the reasons for this interesting activity of the grunion, we must recall some facts about ocean tides. Remember that there are about two low tides and two high tides every 25 hours. This diagram represents a cross-section of the beach as the level of the ocean rises and falls. If we follow the rise and fall of the ocean tide with a white line, we get a tide trace, which is a picture of the height of the tide at any particular time. This is a tide trace for a period of about 1 month at a beach where grunion spawn. Blue bands in this drawing represent night, and yellow bands represent day. You can see that tide heights vary from day to day, and that the highest tides occur at the time of full moon and at the time of new moon. In spring and summer months, on beaches where grunion spawn, the highest tides occur only at night. The grunion, with little variation, spawn on the second, third, and fourth nights after the full moon and after the new moon. This means that they are spawning at a time when the succeeding high tides are lower than the ones on which they spawn. They also spawn after the time of high tide or at a time when the tide is declining. We know from other studies that sand is carried away from beaches when tides are increasing, but sand is deposited on beaches when tides are declining. Thus, we can see that grunion spawn on nights and at that time of night when their eggs will be covered deeper by sand. If they spawn when tides were getting higher, their eggs would be washed away by succeeding tides. We have also seen that it takes about 9 days for their eggs to develop. They must spawn at a time when their eggs will have 9 days in the sand before they are washed free. Eggs which are laid on the second, third, and fourth nights following the highest tides will probably hatch from one to three nights before the next highest tide, as shown by the three arrows on the right of this diagram. If grunion spawned later than these three nights, their eggs would be washed out before they were ready to hatch. In addition, we have shown that the eggs hatch within a few minutes after they are agitated, but not until they are agitated. In nature, this agitation comes when the eggs are washed free from the sand by waves of rising tides. The exact mechanism controlling the precise timing of the grunion is not yet known, but precise it is. For unless the grunion maintain their timing, they would have little chance of reproducing themselves. And reproduce they do, in a strange and fascinating way.