Life in the Sand

Life comes in all sizes! There are things we can see, like ladybugs, whale sharks, and strawberries, but there are also microscopic organisms that we cannot see with just our eyes, like bacteria and mold spores. However, there is also a realm between these two worlds. Thousands, if not millions, of different kinds of animals about the size of a pencil tip or smaller, live between the grains of sediment found in oceans, lakes, rivers, and caves. These animals are called meiofauna.

Meiofauna are Diverse

Meiofauna are extremely diverse, with tens of thousands of different species from branches across the tree of life [1]. Most groups of animals including crabs (crustaceans), snails and clams (mollusks), and worms (annelids) have meiofaunal species too. These species have evolved many times from their larger-bodied relatives. In addition, juvenile (young) stages of some larger animals can be small enough to live among sand grains. These young animals are called temporary meiofauna. Temporary meiofauna have complex life cycles with adaptations to survive life as a larva in the water, as a meiofaunal juvenile in the sediment, and then as a larger adult. There are other animals, like mud dragons (kinorhynchs) and hairy bellied worms (gastrotrichs) that only exist as meiofauna. Most permanent meiofauna hatch as a miniature version of their parents and live their entire lives between grains of sand.

Although meiofauna can be seen with the naked eye, you need a magnifying glass or microscope to tell them apart. Once you get a closer look, it is remarkable how complex and diverse these tiny animals can be (Figure 1)! Some meiofauna are covered in spines, others have many hairy legs. Some have tails that make sticky glue, and others have mouthparts that extend far out of their bodies and are used like a harpoon to collect food. These are just some of the diverse adaptations that help meiofauna live in a world where sand grains are the size of boulders.

What Do Meiofauna Have in Common?

Meiofauna must squeeze through tight spaces, dodge predators, and avoid being washed away in storms or being crushed by moving sand grains. To do all this, meiofauna have become highly adapted to their environment. Many have worm-like bodies that allow them to wriggle through tight spaces. To prevent being washed away, they have ways to stick to sediment, like producing mucus or having specialized sticky structures called adhesive glands. Because sand is constantly shifting, meiofauna need protection from being crushed by the sand-grain boulders. Many meiofauna have either internal or external protective structures that can act like armor. Some even have spines for defense.

Because life in the sand tends to be dark, many meiofauna rely on specialized structures to sense the world around them. Meiofaunal annelid worms, for example, have tentacle-like structures near their heads. They use these tentacles to feel around ahead of themselves. Other meiofauna have a gravity-sensing organ called a statocyst—a small, fluid-filled chamber that holds a tiny, rock-like ball called a statolith. When the animal moves, the statolith rolls around and touches sensory cells in the chamber. These cells send signals to the animal, helping it figure out which way is up or down in the darkness.

Adaptations like a worm-shaped body, adhesive organs, and statocysts can be found in meiofauna that are not closely related, and distantly related species can look similar. When two species from unrelated groups evolve similar adaptations or features, it is called convergent evolution. For example, the reason birds, bats, and butterflies all have wings is because they help the animals do the same thing—fly. Convergent evolution can also occur when animals live in similar habitats. Many meiofauna that might not be closely related look similar because they evolved to live between the grains of sand.

How and Why Do Scientists Study Meiofauna?

Scientists are still discovering and describing new species of meiofauna, and there are likely tens of thousands of species yet to be described. The scientists who identify and catalog new species of organisms are called taxonomists. Taxonomists also study how different species are related, to better understand the diversity of life. They often travel all over the world to find new species.

The first challenge taxonomists face when studying meiofauna is separating animals from the sand. Because the animals and sand grains are the same size, meiofauna can’t be filtered out with a net or a sieve. Instead, scientists use the difference in density to separate the relatively light animals from the heavier particles of sand. You can try this method too, by following the directions in Figure 2 the next time you are at the beach.

Once the meiofauna have been separated from the sediment, scientists use microscopes to get a closer look (Figure 3). Taxonomists examine their anatomy (what body parts and organs they have) to identify species and describe new ones. Another challenge in studying meiofauna is that distinguishing differences between animals is more difficult when animals are very small. Because meiofauna often have shared characteristics, they can be difficult to tell apart from each other.

Sometimes scientists use DNA to tell similar meiofauna apart. DNA is like a blueprint for building an animal, studying DNA can help answer a lot of questions about how and where meiofauna have evolved. Comparing DNA among different animals helps taxonomists determine how closely related they are and identify new species. However, getting enough DNA from small animals like meiofauna is hard. Scientists have been improving methods to extract DNA and determine the information encoded in the DNA. Thanks to advances in technology, scientists today can sequence DNA from a single meiofaunal animal. Sometimes scientists find meiofaunal animals from two or more far-away places that look identical but, when they study their DNA, they find the species are more genetically different than humans and chimpanzees. These similar-looking but genetically different species are called cryptic species.

Scientists also study meiofaunal behavior, such as how the animals move and eat. These researchers use very small, specialized tools. For example, to catch and move these tiny animals, scientists use small sticks with eyelashes or cat hairs attached. Scientists put meiofauna in clear sand and have even built tiny treadmills to observe how they respond to water flowing through the sand (Figure 3B) [2].

Figuring out how many species of meiofauna there are, how to identify them, how they are related, and how they behave is important for understanding the whole ocean. In some parts of the ocean, you can find thousands of meiofauna in a single teaspoon of sand. These animals grow very quickly, from babies to adults in just a few months. Because they are abundant and grow quickly, meiofauna are important players in moving nutrients through ecosystems. When organisms in the ocean die, they eventually sink to the sea floor. Many meiofauna eat these leftovers, recycling their nutrients back into the food web. After eating this material, meiofauna themselves become important food for young fish and crabs. Meiofauna help keep the seafloor healthy by stirring water down into the sediments, bringing oxygen and nutrients to microbes living there, which recycle this material. Without meiofauna, less of this valuable food would be recycled, and the sea floor would be more like a trash can and less like a recycling bin.

Meiofauna can also help scientists understand how healthy an environment is. Because they are small and cannot move very far, meiofauna are used as bioindicators for environmental and climate change [3]. Just like a doctor uses a thermometer to help diagnose a sickness, scientists can use meiofauna to diagnose the environment. This can be done by counting the number of each species or by studying DNA and counting the different sequences. The species or sequence counts can be compared to the diversity in samples taken from different places or from the same place at different times. For example, after an oil spill in the Gulf of Mexico, scientists compared the diversity of meiofauna at sites across the region before and after the oil spill, to determine how the oil spill affected the meiofaunal community [4]. They found fewer species of meiofauna after the oil spill, but after a year, the diversity increased again.

Meiofauna Matter

The seafloor is filled with diverse and interesting meiofauna that live between the grains of sand. Within this secret world of sand, animals of all shapes glide and stick through small spaces, navigating their “meio-scopic” environment. Next time you play in the sand or walk along the beach, remember that you are stepping on thousands of little animals specially adapted to survive in the shifting sands. Figuring out how many species of meiofauna there are, how to identify them, how they are related, and how they behave is important for understanding the whole ocean.

Glossary

Sediment: ↑ Tiny pieces of dirt, sand, rocks, and organic matter that settle at the bottom of rivers, lakes, or oceans.

Adaptation: ↑ A change in a plant or animal that makes it better able to live in a particular place or situation.

Adhesive Glands: ↑ An adhesive gland is a special structure on the surface of the animal that produces special glue that helps meiofauna stick to sand grains.

Statocyst: ↑ A tiny organ in some animals, like jellyfish or crabs, that helps them sense balance and know which way is up or down.

Convergent Evolution: ↑ When different animals or plants evolve to look or behave similarly, even though they are not related.

Taxonomist: ↑ A scientist who names and classifies living things into groups.

DNA: ↑ Deoxyribonucleic acid, is a molecule inside cells that carries the instructions for how living things grow, develop, and function. It acts like a blueprint for building our bodies.

Cryptic Species: ↑ Different species that look almost identical but are not the same.

Bioindicator: ↑ A living thing, like a plant or animal, that shows how healthy the environment is.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work was funded in part with federal funding from the Department of the Treasury under the Resources and Ecosystems Sustainability, Tourist Opportunities, and Revived economies of the Gulf Coast States Act of 2021 (RESTORE Act) to KD, KH, and KK made available through the Alabama Center of Excellence. Support was also provided by the NSF CAREER grant 1844910 to KD and CAREER grant 1846174 to KK. We thank the participants of the 2022 Smithsonian Marine Station Meiofauna Diversity and Taxonomy Workshop, the participants of the 2022 Biodiversity and Integrative Taxonomy of Invertebrates course at Friday Harbor Labs, and the participants of the 2023 Dauphin Island Sea Lab Meiofauna Diversity and Taxonomy Workshop for permission to use their photos in the figures.