Surf's up!

Check out these five beach animals with totally surprising hidden lives.

Bees that build sandcastles, mole crabs digging right under your feet, beach hoppers partying in the dead of night, and horseshoe crabs mating by the moonlight.

First up, sand dollars and their heavy metal breakfast.

It's a beachcomber's prize.

But this sand dollar is just an empty husk ... a skeleton.

This is what sand dollars really look like.

Off the coast of California, Pacific sand dollars snuggle up together, like a big pile of purple sea cookies.

They're fuzzy ... almost cuddly.

But look closer.

That fuzz is actually made up of tiny spines ... thousands of them.

Some long and spiky ... others rounder.

Mixed in are miniature tube feet with grabby little suckers on the ends.

They use them to meticulously sift the sand and pass the grains down the line ... until they reach the sand dollar's mouth ... at the very center of its underside, buried under all those spines.

Sand dollars eat sand.

They're after the algae and bacteria that coat the grains.

And these sand dollars can also stand themselves up on their sides to use the long spines around their edges to trap tiny plankton floating by.

So what about that part that looks like a flower with five petals?

It's called the petaloid.

They have special tube feet there that help the sand dollar breathe, absorbing oxygen out of the water.

You can see that same five-point body plan on the skeletons of their relatives, like starfish and sea urchins.

In fact, sand dollars are just a type of flat sea urchin.

But while their cousins prefer the rocky shore - chock full of life and spots to hide - sand dollars don't have such a cozy place to live.

They're at the mercy of what's basically an undersea desert - thrashed and sandblasted.

So being flat is an advantage.

They're sleeker, streamlined against the powerful currents.

And they have another scrupulous solution for staying put.

Not all sand is the same.

Mixed in there are some extra heavy grains.

They're made of magnetite, a type of iron ore. Scientists think that as they grow, young sand dollars sort them out and swallow them ... grain after grain.

The heavy ore builds up inside their bodies and helps weigh them down to the seafloor.

At the California Academy of Sciences in San Francisco, researchers used X-rays of sand dollars to look for it.

See those bright white areas?

Those are the pockets of magnetite.

That's how these tireless little creatures can hack it out here in such turbulent waters, where most other things can't.

Turns out, it takes a lot of work just to lay around.

Where the waves break, there are signs of life underfoot - these expert diggers: mole crabs.

Chaotic ... turbulent ... harsh.

The ocean's edge isn't the easiest place to make a home.

And yet, someone does.

Pacific mole crabs - also called sand crabs.

You've probably seen the little holes they make.

They spend their lives just under the surface of the sand, waves crashing overhead.

That ever-present flux offers a meal.

Each wave kicks up plankton and other tasty morsels.

They peep out from below the sand and use their feathery antennae to catch food right out of the water.

But they can't just sit around gorging themselves all day.

The crabs want to stay in the swash zone - this part here where the waves break and sweep in.

But it's always moving with the tides, up and down the beach.

So the mole crabs have to move with it.

But they're not the only ones out here looking for a meal.

To birds, they're basically beach candy.

Easy pickings.

So mole crabs have become champion diggers.

They can disappear in a flash ... back under the sand to hide.

But they have to be fast.

So, what makes them so good at burrowing?

Researchers at UC Berkeley want to find out.

Back at the lab, they film the mole crabs in action using high-speed cameras.

Mole crabs actually burrow backwards, digging into the sand with their pointy rumps.

But moving densely packed sand is hard work.

The grains stick to each other, making it tough to push through.

So the crabs have become tiny engineers.

They stir up the wet sand with their tails, making it easier to move.

A process called liquefaction.

Then, they push the loose sand up toward the surface by handing it off between their five pairs of legs.

See?

Kind of like a conveyor belt.

The waves never rest, so the mole crabs do this day and night, as they're tossed and tumbled.

But they take it all in stride.

When you survive on chaos, fluidity is what it's all about.

These bees skip the hive life and build their own little sandcastles on the sides of beach cliffs.

Ocean views, anyone?

It might seem peculiar.

Bees living at the beach.

But this is their home.

And they spend the spring building their perfect beach condos.

At their local watering hole, they're not actually having a drink.

They're collecting water as a raw material.

They slurp it into a pouch in their abdomen called a crop.

They can carry one sixth of their weight in water, hauling it to the side of this cliff in Northern California.

Now that's a view!

Back and forth, back and forth, 80 times a day.

They're building their nests from the cliff's mix of sand, clay and gravel, spraying water to soften it up.

See how she extends her proboscis and uses it like a hose?

Then she digs ... and digs and digs.

They're digger bees.

The females build their nests side by side in what's called an aggregation.

The males, most of them have died by now, after mating.

The females work peacefully ... most of the time.

Hey, make your own nest!

Some even make sandcastles, shaping the earth they dig out into a turret.

To do that, they pat down the wet gravel with the tip of their abdomen.

Scientists think the turrets could help keep out large parasitic insects, like other kinds of bees.

This cratered landscape isn't unusual.

Most of the world's bee species - 70%!

- nest underground.

The nest opening leads to a burrow a couple of inches long.

At the bottom, she digs holes called brood cells.

She will lay an egg in each one.

But first she needs to stock up on food for her future offspring.

This flower is a favorite for nectar.

And check out the pollen on this bee.

Wait a minute!

That's not our digger bee.

That's a yellow-faced bumblebee.

She'll sting you if you mess with her.

Our bee is a bumblebee mimic.

She doesn't sting.

The real bumblebee has a bright-yellow band on the bottom of her abdomen.

Our bee has a band higher up.

By imitating a stinging bumblebee, she scares predators away.

Back from foraging, our digger packs pollen and nectar into each cell and lays an egg on top.

The larva that hatches out will have a ready-made meal.

Then she tears down her turret, bit by bit.

She uses it as mortar to seal her nest closed and keep her eggs safe.

After finishing a couple of nests, the bee's brief, hardworking life comes to an end.

The beach will be her final resting place.

And next year, the ever-shifting sand will bear witness to her young emerging from their nests.

Once a year, under a full moon, millions of horseshoe crabs emerge from the ocean for a romantic rendezvous.

When they're done, they leave their babies behind in the sand.

These delicate, otherworldly creatures are just starting their lives, twitching and twirling inside their translucent homes.

They're baby horseshoe crabs, preparing for the moment they can break free.

It all started two weeks ago, when the tides were at their highest.

Horseshoe crabs emerge from the briny deep.

And head for the shore with only one thing on their minds.

A springtime spawning spree.

These Atlantic horseshoe crabs gather by the millions along the east coast of North America - from Maine to the Gulf of Mexico.

The crowds here in Delaware are some of the biggest.

The males show up first, hanging out on the beach and in the shallows of the bay.

They scramble to latch on to the backs of passing females.

The females can be the size of dinner plates, way bigger than the males.

Success!

They take advantage of the fleeting high tides to haul their precious cargo as high up the beach as possible.

The females muscle their way up the beach, with the males in tow.

The greatest danger for these horseshoe crabs is flipping over, exposing their vulnerable underside.

And it only takes a small wave to do it.

Under the hood, horseshoe crabs' legs end in pincers.

But mature males have something special on their front legs.

This "clasper" that looks like a little boxing glove with a hooked finger.

It's perfect for gripping the back of a female's shell.

To flip back over, the animals use their long, spiky tail, called a telson.

It looks a little scary, but horseshoe crabs don't sting.

Horseshoe crabs have been making these high tide treks under the glow of the full moon since before the dinosaurs.

That's more than 400 million years ago.

Not everyone finds a date.

Especially those younger males.

But they hike up the beach anyway in hopes of getting in on the action.

This female is dragging around two admirers.

When a female finds a good spot, she digs and digs into the wet sand.

The female lays roughly four thousand eggs in one go.

The male that clung to her all this time is in the best position to fertilize the most eggs.

All those single dudes crowd around too ... vying to fertilize the rest.

As the party winds down, the grownups start heading back to sea.

But they'll return for more of these high tide soirees throughout the season.

They leave their fertilized eggs behind, buried in the damp sand.

Each one is smaller than a pea.

Over the next couple of weeks the embryos inside develop.

By the time the high tides return these larvae are ready to hatch.

Jostling of the waves stimulates them to break out of their shell.

This is their chance.

They have just a few hours to scramble into the turbulent surf, before the high tide recedes.

They'll mature beneath the waves for roughly a decade before they're ready to return as adults themselves ... ... where they'll take their turn in this ancient dance of the moon, tides and sand.

Moonlight parties aren't just for horseshoe crabs.

Beach hoppers throw their own night ragers - and the dancing is acrobatic.

As the sun sinks behind the waves, these performers awaken and get ready for their all-night show.

They take cues from the tides, the moon, and their appetite, emerging from sandy underground burrows.

You might know them as sand fleas, but they don't bite and they aren't fleas.

They're called beach hoppers.

These crustaceans are as small as an ant or as large as a cricket.

Their eyes are made up of hundreds of cells called ommatidia, but they don't see much detail - just blurry shapes, light and dark.

They're drawn towards shadowy blobs on the horizon.

They hope it's kelp, their favorite food.

When they find it, they eat and eat.

Sometimes they even eat one another.

This large beach hopper is piercing the other right behind its eye, holding it in place with its claws.

For protection, they dig burrows about a foot deep where the sand is damp and cool.

And males will fight over control of burrows, especially if there are females inside.

Deep in the night, the beach hopper acrobatics build into a dazzling show.

A powerful flick of their curled-up tail launches them skyward.

They do not stick the landing.

A beach hopper can jump as high as your knees, dozens of times the length of its body.

It's a quick way to travel ... towards food or mates.

Or to get out of harm's way.

Beach hoppers' diets are mostly beach wrack - anything natural that washes ashore.

Wrack is an essential source of nutrients for sandy beach ecosystems.

These shredders break down the wrack into smaller parts.

It's the first step in sending nutrients into the food chain.

When predators like shorebirds or insects eat beach hoppers, they can carry these nutrients further inland.

Without these hungry acrobats, beaches the world over would be strewn with rotting seaweed.

After a night of fighting and feasting, they leave only silhouettes behind.

As the sun rises, the beach hoppers retreat to their burrows, just beyond the tide's reach.

The performers need their rest.

Another spectacle is just a night away.