Tongues covered in teeth. Eggs like corkscrews. The secret lives of beach creatures revealed

Ah, the beach. There’s nothing like it in summer, is there? The warm sands, the hiss and foam of crashing breakers, the raw smell of salt. There is something about them that has drawn us throughout history. Aboriginal shell middens hidden in dunes, made up of layers of mussel shells, whelks and abalone, hint at the rich resources of the sea. Humans continue to build cities on the edges of the water – as with every Australian capital city bar Canberra – despite these areas being fragile and difficult.

Each beach you visit is subtly different. Some are nothing but sand. Others are covered in lines of seaweed that turn stinky in the sun. Some are backed by dunes dotted with spinifex while some reveal rock pools when the tide slinks out.

Beaches sit on the border between two ecosystems: the land and the vast sea. What you find there often reflects those borders – jellyfish washed up by the surf, plastics blown in from city streets. But beaches also have their own ecosystem, supporting life in cracks of rock and folds of seaweed. “You look at them and your first thought is they are biological deserts. But nothing could be further from the truth,” says Associate Professor Melanie Bishop, a coastal ecologist at Macquarie University. “Between the grains of sand is a whole other land.”

So, how do beaches work? And what is all the stuff you can find on them?

How do beaches work?

A beach is, essentially, three interacting habitats packed into one small sliver of land: the surf zone, the beach proper and the undulating dunes behind it.

Let’s start with the key feature: sand. Hold a handful up to your face and you’ll see it’s composed of thousands of grains of different shapes, textures and colours. Most of this sand is rock, broken down by weather over time. Two minerals, quartz and feldspar, tend to hold up to this weathering best and so make up most beach sand – the quartz, tinted with iron oxide, gives beaches their brown colour. Some sand is fragments of shattered shell; near coral reefs, this makes up most of the beach.

Within those grains, life thrives. “There are more different groups of organisms living on a beach than in the world’s rainforests. It’s just most of them are teeny-tiny,” says Bishop. Some tiny creatures live on the grains themselves. Algae are larger and more numerous, and can migrate up and down the beach; if there are a lot of them close to the surface, they lend the sand a golden or green hue. And like trees, they are producing oxygen whenever the sun shines.

Much as we build sandcastles, it’s the ocean that is always shaping the beach. It can look flat at first, but look again and you’ll see an architectural digest of ridges, ripples, bars and banks. You can’t see it with the naked eye, but the ocean in front of the beach ripples with invisible highways; what you see washed up from that transport network might have come from a long way away.

And everything deposited and dumped on land eventually ends up at a beach, says Jacinta Early, Melbourne Aquarium’s education officer. Take plastic. Sadly, on our suburban beach visit we found more plastic than anything else: face masks, parking tickets, fragments of food wrappers, even a plastic bag floating in a rock pool. “A good rule of thumb is take three: you take three pieces of plastic away each time you come down to a beach,” says Early. “Just help clean it up.”

Still, the (non-plastic) flotsam provides an important environment to many beach dwellers. Lines of seaweed, known as wrack, turn stinky in the sun and turn up sunbathers’ noses. But inside them, tiny animals including bugs and lizards hide from seabirds and the crashing surf, and quickly start eating and decomposing the seaweed, which returns nutrients to the even-smaller creatures living in the sand.

Stroll along the waterline and you will experience thriving nature: small animals going about their lives, shells that once housed scuttling creatures now being broken down into sand, sea treasure carried from near and far by deep currents. Here are a few common finds.

A sea urchin shell

Those holes you see in a sea urchin shell? One is their mouth, and the other is, yes, their anus. When it’s still alive, the urchin’s jaws and muscles allow its protruding toothed mouth to scrape at algae on rocks or reef. On the inside of its hard shell, the creature has its intestines and a hydraulic system that pumps water in and out to move its feet. When they’re alive, an urchin can use tube-like feet sprinkled among its long, sharp spines to locomote. Some urchins have hundreds of feet!

The little dots left on the shell are the spots where the spines once grew.

Urchins are sometimes called sea hedgehogs but their spines drop off when they die. The little dots left on the shell are the spots where the spines once grew.

Often you’ll come across shells on the beach with holes in them; shell necklaces are mementoes of summer holidays. But those holes tell a sinister story: they are the leftover lunch of a cone snail. The snails use little barbs to pierce the shell and then they snack on the creature inside.

A sponge

A sea sponge is really just a simple construction of protein – a lot like a bath sponge, but alive. Early scientists thought they were plants, but they are animals, able to regulate how water flows through them. Sponges attach themselves to rocks and feed by filtering food particles from water that they pump through their tiny pores, in a process called sponging. To do the pumping, they use special cells that have little whips in them, which beat in waves that create a current that pulls water towards the sponge.

Scientists believe sponges may share a common ancestor with humans.

Their elastic skeletons are made from collagen, a protein also found in human skin. In fact, scientists now believe sponges may be the oldest multicellular creatures – sharing a common ancestor with humans.

A moon snail

Its name comes from its round white shell. But don’t be fooled by the romantic associations and the elegant colour – moon snails are predators. They hunt clams, slide on top of them and push them down into the sand. The snail’s tongue, known as a radula, is covered in teeth that can cut a hole in a shell; the snail can even squirt out hydrochloric acid to help break through. The snail then sucks out the clam meat.

Easy to spot because they are often quite large, the moon snail can pump water into hollow tubes to expand its “foot” outside its shell, allowing it to make its way across the sand. (Its foot can be very large; a moon snail’s shell can appear to be propped on top of a fleshy mound.) If threatened, the moon snail jets out this water and shrinks back inside its covering.

The sacks look like strange, sandy collars, and people often stomp on them, thinking they are jellyfish.

These clam-sucking gastropod molluscs do have an amorous side. They breed near the shore, laying their eggs in large semicircular sacks, as big as 15 centimetres, which the female glues together in a kind of sand-mucous mix. The sacks look like strange, sandy collars, and people often stomp on them, thinking they are jellyfish – “but you’ve just destroyed the egg sack of a moon snail,” says Early.

A shark egg

Not many eggs are shaped like corkscrews. But then, there are not many creatures like sharks. Most sharks give birth to live young, all ready to wriggle into the swim of their new life, but some sharks lay eggs – and don’t wait around to sit on them.

Corkscrew-shaped eggs, like the empty one pictured here, from a Port Jackson shark, are designed to be grasped by the mother-shark’s mouth. “They grab the egg with their mouth and shove it underneath a little rocky outcrop. So it acts like a screw and wedges there,” says Early.

The eggs are encased in a leathery watertight casing. When the baby sharks emerge – typically after about 10 months of gestation – they have to be ready to go because they’re not getting any parental assistance.

Dead man’s fingers seaweed

This is dead man’s fingers, a native seaweed with long fronds that looks a lot like, well, broccoli. Often attached to rocks, during low tide this seaweed drapes across the rock face, thus the ghoulish name.

Seaweed is to the ocean as plants are to the land: an absolute foundation species on which whole food webs are built. The word refers to an enormous diversity of species of algae (including kelp), many not particularly closely related, from complex plants to simple multi-celled organisms; some are a few millimetres long while others can grow as long as 50 metres.

Some species have air-filled bubbles along their leaves, which hold them up in the water closer to the light.

Like plants, most seaweeds have roots at their base (known as a holdfast) to grip the seafloor, although unlike plants they don’t use these roots for picking up nutrients – everything a seaweed needs is dissolved in the water around it.

Leaves, called blades, pick up the sunlight that filters through the water and use it to power photosynthesis, which generates energy for the plant and removes carbon dioxide from the water, replacing it with oxygen. Between about half and 80 per cent of the atmosphere’s oxygen is manufactured in the sea.

Because light is important, some species such as mermaid’s necklace have air-filled bubbles along their leaves, which hold them up in the water closer to the light. Light also explains seaweed’s distinct colour. At the surface green algae thrive but, further down, the water filters out certain wavelengths. The deepest seaweed is a ruby red, which absorbs blue light – the wavelength that best penetrates the ocean’s depths.

A cutt

A cuttlebone looks absolutely nothing like the animal it comes from. A cuttlefish looks like a squid, with a large head with a big brain relative to their size (they may be quite intelligent, and scientists have studied them holding off on eating standard crab meat when they know something better, like a live shrimp, is coming up). They have two eyes, eight arms and two longer tentacles with pouches for storing the crabs, fish, worms, or even other cuttlefish that they grab hold of. They also have three hearts.

The cuttlefish can adjust the amount of gas in the shell, moving up or down the water.

But back to this object above. You’d be hard-pressed to guess its function. The cuttlebone, as it is known, is an internal shell made of aragonite, the same material used to make most shells, and is full of small holes that give it a soft, spongy inner-layer. Rather than providing protection the internal shell is used for buoyancy; the cuttlefish can adjust the amount of gas in the shell, moving up or down the water. It is, essentially, a cuttlefish floatie.

A mussel shell

Bivalve molluscs, such as this mussel, have hinged shells (each side is called a valve) which the animal lives inside. Strong muscles hold the shell closed when needed. Bivalves filter water through their gills, picking out pieces of food as they go. When we pick up a mussel (or eat one) they seem like gelatinous blobs, but they are surprisingly nimble. They can protrude a pointy limb and use it to pull themselves along the ocean floor, for instance, and burrow.

If you look closely at the edges of a mussel shell, you can see growth rings, just like on a tree.

Mussels and other bivalves make their own shells. A layer of tissue called the mantle secretes proteins and minerals that harden. As the animal grows, its shell grows; if you look closely at the edges of a mussel shell, you can see growth rings, just like on a tree, that allow scientists to work out the mussel’s age.

On the inside of many bivalve shells you’ll find a colourful coating called mother of pearl or nacre. On a microscopic level, nacre looks like little bricks of argonite, the same stuff used to make cuttlebones. But while cuttlebones are dull in the sun, nacre is iridescent. This is because the microscopic brickwork creates multiple overlapping reflections of light; the reflected lights all bounce off each other, creating a colour that shifts as you move the shell.

A blue blubber

The most common species of jelly you are likely to see on beaches around Australia is the blue blubber, Catostylus mosaicus, like this one. The blubber’s bell is creamy white or brown in southern waters, and further north takes on a blue hue. The colour is down to the algae that lives symbiotically inside the jelly’s cells, converting sunlight into energy. The creature’s tentacles sting – they are painful but not seriously dangerous to humans – and can capture tiny crustaceans.

Bluebottles, Physalia physalis, are commonly seen washed up on Australia’s beaches in summer, too. As with most jellies, they are bizarre: each is actually a colony of four separate “individuals” that work together to survive. The tentacles are counted as one individual, the digestive system and reproductive organs are another two, and the float holds everything together.

The box jellyfish, Chironex Fleckeri, is usually found in northern Australia. It has a box-shaped bell trailed by dozens of stinging tentacles. The creatures are pale blue or transparent and therefore almost impossible to see – not good when they are perhaps the most venomous sea creature, with a sting that can kill in minutes. They prey on crustaceans and, unlike most other jellies, are capable of swimming towards their prey.

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