What is a "Macro"?

If you have ever spent any time around anyone who works in or around water, you may have heard them say they are looking at macros. It is even more confusing when they list all sorts of critters as "macros" - flies, worms, crayfish, spiders, snails, even jellyfish. What are they studying?

"Macro" is short for macroinvertebrate.

Before we define "macro," let us define invertebrate. A vertebrate is the scientific name for "backbone" or "spine." As mammals, we have backbones- they help us move. Without them—and because we don't have anything else—we would be unable to move. Putting the prefix "in-" before many words makes the word opposite, so invertebrate means "without backbone."

Three Things Macros Have in Common

Go back to the critter list. You might notice that they have shells or exoskeletons (outside skeletons), segmented muscle bodies, or no structure at all, just blobs that float. That is the first thing these critters have in common—no backbones! They have something else, just not a spine.

The "macro" part is the second thing they have in common, and there is a good chance you already know that "micro" means something that is small (like microchip or microscope). For our purposes, to see something "micro" you need help to see it (like a microscope). "Macros" are still small, but they are alsobig enough to see with just your eyes. So "macroinvertebrates" are small, spineless organisms that are visible without special equipment or gear.

The third thing macros have in common are ecosystems or places where a whole bunch of different plants and animals live and use each other. Macros fit into the ecosystem by being big enough to eat and break down a lot of organic matter like leaves, feces (poop), and other smaller organisms but small enough that fish, birds, and mammals use them as food. When they are missing from the food web, we notice.

By studying which macroinvertebrates are in a stream or a pond, scientists and students can get a snapshot of water health without expensive lab tests. Healthy populations of macros usually signal clean water, while polluted waters might have fewer or different species. This makes many macroinvertebrates what scientists call indicator species or organisms whose presence or absence are good at telling us something about water quality. This is what is meant by doing a macro study.

For more information on macro studies, see how to complete a kick net stream study.

Types of Macros

Freshwater macroinvertebrates come in many shapes and sizes, from crawling insects to squirming worms and tiny clams. Each group plays a different role in the ecosystem, and learning to recognize them helps us understand how streams and ponds work—and how healthy they are. Here are the main types you're likely to find in Pennsylvania waters.

• Insects: Many aquatic insects spend their youth underwater as larvae or nymphs before becoming winged adults, like dragonflies, mayflies, and stone flies. They are key food for fish and birds. Some species are sensitive to pollution, making them great water quality indicators.
• Crustaceans: These are hard-shelled animals like crayfish, scuds, and aquatic isopods (like pill bugs or roly-polys). Crustaceans help break down plant material and provide food for larger animals.
• Worms: Aquatic earthworms and segmented worms often live in the mud at the bottom of streams, where they recycle nutrients by breaking down organic material. They are also able to survive in low-oxygen environments, which means if there are a lot of certain species like sludge worms (Tubifex tubifex), it could mean poor water quality.
• Mollusks: This includes clams, mussels, and snails. Some (like certain snails) have lungs, while most others have gills. They help keep the water clean by filter feeding or taking food out of the water. They are also an important wildlife food source.
• Arachnids: Tiny water mites live in streams and ponds, while larger fishing spiders (Dolomedes spp.) hunt near or even on the water’s surface—sometimes catching insects, tadpoles, or even small fish.
• Leeches: Though worm-like, leeches belong to a different group of animals. Some feed on the blood of other animals, while others are predators or scavengers that eat small invertebrates or decaying matter.
• Flatworms: Also known as planarians, these are simple, soft-bodied animals that look like a cross between a worm and a fish. They glide along rocks and vegetation using tiny hairs on their undersides. Flatworms are sensitive to toxins and are often an indicator of moderately clean water.
• Hydras: Also known as cnidarians, these tiny freshwater relatives of jellyfish are present in Pennsylvania waters, but are rare. They are more likely to be seen in still or slow-moving freshwater, such as ponds, lakes, or the calm edges of streams. Because they’re uncommon, they aren’t used for water quality testing—but they’re fascinating because they’re among the simplest animals with stinging cells.

Where to Find Macros

Each of these freshwater habitats supports different kinds of macroinvertebrates based on oxygen levels, flow speed (how fast the water is moving), water temperature, and how much food is available. Most macroinvertebrates are at the water's bottom, clinging to rocks, plants, or sediment or on the water's surface. However, some float in the water column, especially in slower-moving or still waters. In all waters, leaf packs (piles of fallen leaves), twigs, or submerged logs are hotspots for biodiversity and food sources.

• Streams and Rivers: These range from fast-moving riffles to slower, deeper pools. Riffles, with their shallow, rocky bottoms, are rich in oxygen and tend to be cooler, providing habitat for mayflies, stoneflies, and caddisflies. In contrast, pools are deeper, slower-moving, and often warmer, where dragonfly larvae, worms, and snails thrive in the calmer waters.
• Ponds and Lakes: These still or slow bodies of water offer diverse habitats, such as muddy bottoms, aquatic vegetation, and shoreline zones. Snails, water beetles, and damselfly larvae thrive among plants, while worms and midge larvae are common in soft, silty bottoms. In these nutrient-rich environments, macroinvertebrates play a key role in maintaining the health of the ecosystem by breaking down organic material and providing food for larger animals.
• Wetlands: Wetlands, including marshes, swamps, and bogs, are areas with standing water, either year-round or seasonally (ephemeral). These habitats have lots of different organisms, including mosquito larvae, water striders, aquatic beetles, and (shrimp-like) fairy shrimp and scuds. Wetlands are crucial nursery habitats for many species, providing food and shelter, especially in seasonal cycles.

Roles of Macros

Macroinvertebrates play a key role in freshwater food webs by connecting the energy from plants and organic material to larger animals like fish, birds, and amphibians. They fall into several main feeding groups, each with a special job.

• Shredders: These macros tear apart dead leaves and plant matter in the water. While they do feed on plant matter, their shredding also makes it easier for other organisms to feed. Examples of shredders include stonefly larvae and some caddisflies.
• Scrapers: Also known as grazers, these macros feed on algae growing on rocks and plants. This helps control algae growth and keeps surfaces clean. Examples of scrapers include mayfly larvae and snails.
• Collectors: There are two major types of collector macros—gathering and filtering. Gathering collectors, like midge larvae and some mayflies, pick up particles from the bottom. Filtering collectors, like freshwater clams and net-spinning caddisflies, strain food from the water. Both types recycle nutrients and keep water clean.
• Predators: These macros hunt and eat other macros. They help balance populations and are a food source for larger predators. Predator organisms include dragonflies, damselfly larvae, some aquatic spiders, and water beetles.

So…Again, What is a Macro?

Macros are more than just small, spineless critters. They are essential workers in our freshwater ecosystems—cleaning, recycling, feeding, and signaling the health of the water. Whether you are wading in a pond or flipping rocks, take a closer look. The more we understand these tiny creatures, the better we can protect the waters they call home—and the ecosystems we all depend on.

Other Extension Resources

• Monitoring Streams with Visual Assessments – Danielle Rhea
• Stream Connections: Benthic Macroinvertebrates, Native Fish, and PA Geology – Katie Bartling
• How to Complete a Kick Net Stream Study – Jenn Fetter
• Benefits of Large Woody Debris in Streams – Danielle Rhea
• Algae in Pennsylvania Ponds – Susan Boser
• Watershed Friendly Mosquito Control – Beth Yount

Works Referenced

Allegheny College (n.d.). Aquatic macroinvertebrates fact sheets. Creek Connections. Retrieved April 12, 2025.

Delaware River Basin Commission (n.d.), Living resources: Macroinvertebrates. Retrieved April 14, 2025.

Leaf Pack Network (n.d.),What are benthic macroinvertebrates? Retrieved April 10, 2025.

Macroinvertebrates.org (n.d.), Atlas of common freshwater macroinvertebrates of Eastern North America. Retrieved April 14, 2025.

Maine Audubon. (2022).Maine Stream Explorers: Basic macroinvertebrates guide (3^rd ed.).

Maryland Department of Natural Resources. (n.d.). Freshwater benthic macroinvertebrates. Retrieved April 11, 2025.

National Park Service (n.d.). Aquatic macroinvertebrates: Introduction and distribution. U.S. Department of the Interior. Retrieved April 14, 2025.

Stroud Water Research Center. (n.d.). Macroinvertebrate identification key. Retrieved April 14, 2025.

U.S. Environmental Protection Agency. (2021). Macroinvertebrates – Water quality parameter factsheet.

West Virginia Department of Environmental Protection. (n.d.). Save Our Streams field guide to aquatic invertebrates. Retrieved April 10, 2025.