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The beach is constantly on the move. As tides, winds, and currents move water across the beach surface, sand grains shift. On calm days, the movement is subtle, but during storms, the sediments churn in nature’s rock tumbler. An ever-shifting substrate means that no algae or animals can live on the beach surface. Animals such as snails, clams, and worms burrow down, but the real action is between the grains of sand, in the interstices, where a booming metropolis of microscopic organisms slither and crawl in the film of water.

This interstitial community, called meiofauna, is rich and diverse, and includes individuals from most animal phyla. And although they’re small, these animals are as complex in their physiology as those millions of times their size, from worms to wombats.

To survive in a sandy mobile home, which alternates between a marine environment when the tide is in and a terrestrial one when it’s out, meiofauna have a variety of adaptations.

Note: we have taken a few liberties for visual clarity. In reality, most meiofauna lack pigmentation and are transparent. As well, the organisms and the sand grains are not to scale. Consider that in just one millimeter, there could be about 22 of the smallest meiofauna laid end to end. When you’re that tiny, even a sand grain is a mighty hill to climb.

Click on the creatures in this interactive image to see what it takes to live in the sand.

Tardigrade

Challenge

Hostile life conditions, such as desiccation

Solution

Tardigrades (aka water bears) live in a variety of habitats, including the ocean, lakes, clumps of moss (thus their other aka, moss piglets), and even gutters, and they’re known for their ability to withstand some of the harshest environments on the planet (and in space, too). These toughies go dormant during extreme stress, such as dehydration, radiation, and freezing, and use a variety of strategies to withstand duress. To endure radiation, for instance, they have a protein that protects their DNA from damage. To stand extreme cold, they replace water with a sugar called trehalose, which stabilizes the animals’ cells.

Rotifer

Challenge

Finding food

Solution

A rotifer filters food out of the water using a corona, a crown of beating cilia, creating a vortex that drives food toward its mouth. Meiofauna use a variety of appendages and strategies to filter food—often just bits of detritus—from the water.

Nematode

Challenge

Narrow spaces

Solution

Many types of meiofauna have an elongated shape to help them fit between the grains of sediment. Nematodes are long, slender, and highly flexible. To move, they contract muscles that run lengthwise along their bodies, resulting in a thrashing movement.

Gastrotrich

Challenge

Mobile environment

Solution

Meiofauna have developed all sorts of strategies for holding on in their constantly shifting habitat. Some use hooks or claws, but others, such as gastrotrichs, use adhesives and a dual-gland system. One gland secretes an adhesive to temporarily anchor the animal; the other secretes a releasing compound for when it’s ready to make a move.

Ciliate

Challenge

Dark conditions

Solution

Most meiofauna lack eyes, so must use other methods to sense their environment. Some ciliates use gravity organs—small vesicles each carrying a crystal that moves with gravity—to help with orientation and balance.

Flatworm

Challenge

Narrow spaces

Solution

To squeeze through narrow spaces several meiofauna, such as this flatworm, have a highly flexible body. Despite its name, a flatworm’s body shape is actually quite spherical, rather than flat, but the broadened shape allows for better exchange of oxygen and nutrients, and provides a wider area for traction as it squirms through the sediment.

Kinorhynch

Challenge

Locomotion

Solution

Depending on the species, meiofauna use cilia, bristles, muscles, hooks, and more to swim, wriggle, or crawl through the sand. But some, such as this kinorhynch (aka mud dragon), uses its spine-covered head. The kinorhynch forces body fluids into its head, which, in combination with the action of its powerful muscles, everts a rosette of spines. The spines hook the sand grain surface and the animal pulls itself forward.

Ostracod

Challenge

Reproduction

Solution

Meiofauna have developed a variety of strategies to ensure that sperm meets egg. Some males produce bundles of sperm, which they attach to the substrate or transfer directly onto the skin of their partner. The most direct method, of course, is copulation. Many species are hermaphrodites—being both male and female increases the chance of successful reproduction when a potential mate wanders by. Ostracods, a type of crustacean, have perhaps gone the furthest to ensure successful reproduction—males have two penises and females have two genital openings.

Solenogaster

Challenge

Abrasive environment

Solution

All that sand and grit makes for a very abrasive environment. To combat potential damage to their bodies, many meiofauna use a hard cuticle as a sort of armor. Solenogasters, a type of shell-less mollusk, are covered in spine-like spicules that give them a fuzzy appearance. Flattened, the spicules form a tough cuticle. When raised, they’re a formidable barrier.

Loriciferan

Challenge

Micro habitat

Solution

It’s easy to forget that meiofauna are just as complex as their macro cousins. They are multicellular animals with highly specialized physiology, anatomy, behavior, and life cycles—it’s just that everything is miniaturized. Loriciferans, for example, have as complex a feeding, nervous, musculature, and reproductive system as an animal many times their size.

Polychaete

Challenge

Finding food

Solution

As in the macro world, meiofauna take certain roles in their ecosystem: scavenger, herbivore, carnivore, predator, prey. Polychaete worms are a highly varied group, but many are active predators that use their impressive jaws to snag food. As they move through the sand grains, they help aerate the sediments.