There’s more to beaches than just sunning, jogging and surfing.

Sea gulls soar above the sandy beaches of the South Bay and long-legged willets scurry across the sand.

But beneath the sand and in the waters a short distance offshore, there are animals that most beach-goers--caught up with sunning, jogging, walking or surfing--never know are there.

Spiny sand crabs, which look a little like small armored vehicles, lie burrowed deeply under the sand. Olive snails look so much like small, dark stones that people wading in the surf never suspect there are animals underfoot.

And 10 or 20 yards offshore, thousands of sand dollars are embedded upright in the sand, looking like rows and rows of cookies.

The Cabrillo Marine Museum is about to unmask this hidden world of sand-dwelling animals with a new permanent exhibit opening Saturday at 10 a.m. “Life in Shifting Sands” focuses on the area where the ocean meets the land in a crash of heavy surf.

“As a kid, sand crabs and sea gulls were all I knew were there,” said Mike Schaadt, exhibits director at the San Pedro museum. “People will be surprised and aghast at the number of animals that live in an area where there are a lot of energetic waves.”

Said museum administrator Susanne Miller: “Most people know sandy beaches for their recreation value, but they don’t think so much (about) marine life actually being there.”

A 20-by-5-foot mural dominating the exhibit reflects just how rich in life the sandy beaches are. Not only are there the familiar sea birds and sand crabs that youngsters love to catch, but a variety of other crabs, clams and fish, including the gray corbina, that take advantage of wave action to get food dislodged from the sand.

The mural and other displays focusing on specific life forms paint a picture of a hearty group of animals that live with constantly pounding waves.

They have developed tough shells or have become powerful swimmers or burrowers to survive. “They can maintain where they want to be in spite of all that surf action,” Miller said.

One display is devoted to explaining how sand crabs, clams and other animals are able to burrow for protection and still get food.

Waves get special attention in the exhibit, which features a 20-foot-long wave tank that shows the movement of a wave from the time it begins until it breaks on the shore, churning the sand with its motion. The tank also contains flatfish, snails, sea stars and sand dollars, allowing visitors to see how wave action affects these animals.

“Waves continually shape the beach, which is different in the winter than in the summer,” Schaadt said. “In the winter, waves tear the beach apart, drawing the sand away. In the summer, they bring sand in and the beach is replenished.”

The exhibit also includes a tank that permits visitors to turn a wheel and make their own waves, controlling the size and number.

Anyone whose knowledge of sand dollars is limited to those washed up on the shore will be surprised at the live animals displayed in a bubble tank. They are purple and furry with spines that permit them to move and collect plankton for food.

Grunion--those silvery little fish that fascinate thousands of nighttime beach-goers--also merit special attention in the exhibit. A three-minute video explains why thousands of grunion wriggle on the beach between March and August.

It’s not self-destruction, but a mating ritual in which females burrow and lay their eggs and males fertilize them. After a two-week gestation period, the young grunion hatch and go out to sea.

While beach-goers steer clear of smelly piles of decaying seaweed, another museum display--cryptically labeled “Life in Death”--shows how this rot swarms with tiny animals seeking shelter and food. Schaadt said: “We make the point that even though people don’t like it, it serves as a habitat.”

The sandy beach exhibit, which took a year to design and build, is one of several permanent museum installations that are designed to present a picture of Southern California marine habitats.

Schaadt said the new exhibit is aimed at letting people know that a place normally associated with Frisbees and boogie boards is alive with animals.

“We want them to understand that it’s a delicate environment that we can have a great impact on without knowing it,” Schaadt said, adding that he hopes people will be prompted to protect beach life as they enjoy it. “Nature has to have a place, too.”

What: “Life in Shifting Sands.”

Where: Cabrillo Marine Museum, 3720 Stephen White Drive, San Pedro.

When: Saturday-Sunday, 10 a.m. to 5 p.m.; Tuesday-Friday, noon to 5 p.m.

Admission: Free; Cabrillo Beach parking, $5.50.

Information: 548-7562.