Macroinvertebrate Clues

Purpose: This exercise will introduce students to the identification and use of macroinvertebrates as indicators of stream quality.

Summary: The students will be introduced to the types of macroinvertebrates found in stream systems, will learn how to categorize them using anatomical differences (head, thorax, abdomen), will learn how macroinvertebrates reflect stream water quality, and will play a game in which they match pictures of macroinvertebrates to their characteristics.

Background: Many organisms inhabit stream systems. Healthy streams have highly diversified populations, ranging from microscopic diatoms and algae to large fish, birds, and mammals. This diversity provides for various food chains and also can act as an indication of water quality in the stream.

In the water column, some organisms are found on the bottom (benthic), on stones, or in mud or vegetation. Other organisms, such as plankton, may float or swim freely throughout a stream. More plankton is found in waterbodies with slow-moving water.

When producers are plentiful, consumers also flourish. For example, diatoms coating a rock feed primary consumers such as mayflies. They, in turn, feed higher-order consumers such as stoneflies and fish. The streamside vegetation found in riparian areas also supplies terrestrial insects to the food chain, and many aquatic insects depend on this vegetation during emergency and adult stages of their life cycle.

Organisms that are found on the bottom (benthic), lack a backbone (invertebrates) and are large enough to be seen with the naked eye (macro), called benthic macroinvertebrates, can be used as an indicator of stream quality. While we tend to think of these things as "bugs", in reality, there are a number of groupings of macroinvertebrates found in fresh water systems:

• Insects
• Freshwater aquatic worms
• Snails
• Clams
• Crayfish
• Spiders
• Others

In most streams and rivers, the larval insects will tend to dominate the macroinvertebrate community. These organisms cannot move far from their immediate habitat, and thus must adjust to changes in water quality, including the food that's available to them, the kind of stream bottom, predators, the temperature, and the speed of the water. For this reason, these organisms are good indicators of what has happened in the stream during their life cycle of about four to six months. In the Truckee River, regular macroinvertebrate surveys are used to determine conditions in the river.

Bottom dwellers can be categorized by how they eat and where they live. Some insects, called shredders, chew up pieces of dead plants (leaves, grasses, algae, etc.) and reduce the dead material to very fine pieces that are then excreted. Most caddisfly larvae are shredders. Collectors gather and eat very fine pieces of decaying material and the bacteria that live on them. This includes the materials excreted by the shredders. Some act as filters, straining small particles out of the water, such as net-building caddisfly larvae. Other collectors, such as certain mayfly nymphs, gather and eat material that has dropped to the bottom of the stream. Scrapers remove and eat green plants and algae off the rocks they are growing on, and graze on surface plants. Their bodies may be flat, allowing water to flow over them when attached to rocks. Some mayfly nymphs are scrapers. The final category of eaters includes predators. Predators feed on smaller, live, aquatic organisms. Some will sit still and stay hidden, waiting for their prey to approach. Dragonfly and damselfly larvae are predators.

Benthic organisms can also be categorized by their habitat. Some will live in the spaces between rocks and rubble on stream bottoms. These macroinvertebrates are adapted to the fast moving water and high oxygen levels, as might occur in riffles or rapids. Others burrow into or cling to silt, sand, and sediments where the water is warmer and quieter. These organisms are adapted to lower oxygen levels. Other types may cling to leaves, attached algae, or rooted aquatic plants, and can be gathered using nets.

There are two ways to judge water quality using benthic macroinvertebrates. The first is by surveying the variety of organisms. When the water quality degrades, certain creatures will disappear. For example, if the level of sediment in the water increases, it may settle into the spaces between the rocks and ruin homes for caddisflies and other organisms that live in these spaces. The other way is to look for certain "indicator" species. Caddisflies need good habitat, and will be found in areas where the stream water quality is good. Red midges will survive in areas with low dissolved oxygen, and sowbugs like warm water. If you find lots of them, stream water quality is degraded. These presence or absence surveys give a clue to the overall health of the stream.

When water samples are taken for analysis, the results show only the conditions at the time the sample was collected. Macroinvertebrate samples, however, reflect the conditions during the time that they were growing, generally 4 to 6 months. They may give a better picture of the overall conditions of the stream.

In order to determine the relative populations of these organisms, it is necessary to sort them by type. To categorize aquatic macroinvertebrates, it is useful to examine the three main body segments: head, thorax, and abdomen. The head segment includes both head and antennae. The middle part of the body is called the thorax, and includes the jointed legs and the wings. The final section of the body is the abdomen.

All aquatic macroinvertebrates begin their lives as eggs. Some animals will not change much as they grow, but just continue to get larger, as human beings do. Examples include water beetles and leeches. Other insects change dramatically, or metamorphose, such as mayflies and caddisflies. They may go through several stages that look quite different. Some will have a larval stage, some a nymph stage, and others may have both. This complicates your survey, as you must learn to identify the different stages. Larvae tend to look worm-like, while nymphs are more like the adult in appearance.

Materials:

1. Beginner's Key to Macroinvertebrate Groups
2. Handout with labeled pictures of macroinvertebrates
3. Unlabeled pictures (cut apart and give each group 10 pictures)
4. Macroinvertebrate Eaters handout
5. Freshwater Invertebrates handout
6. Index cards

Procedure:

1. Provide an introduction to macroinvertebrates. First, ask if students know what the word "macroinvertebrate" means. Then, explain where benthic aquatic macroinvertebrates are found (habitats) and what they eat (shredders, collectors, scrapers, predators). Give examples of each.
2. Ask students if they can name the three segments of a macroinvertebrate (head, thorax, and abdomen), and show approximately where they would occur on one of their classmates.
3. Using the labeled pictures, show and name the different segments. What differences exist between larval, nymph, and adult forms?
4. Divide students into groups. Give each group 20 index cards and one set of 10 unlabeled pictures. Have students cut apart the pictures and glue or tape each picture to a separate index card.
5. Have students write the name of the macroinvertebrate on the back of each card.
6. On a blank set of cards, have students write a description of each of the 10 macroinvertebrates. Include information on appearance, habitat, and sensitivity to water pollution. Write the name of the macroinvertebrate on the back.
7. Tell students to shuffle the description cards and lay them in a rectangle. Next, shuffle the picture cards and make a pile, picture side up. Taking turns, students will draw the top picture card and attempt to match it to the appropriate description card. If the student correctly matches the description to the picture, ask them to name the organism. If they can correctly match name, picture, and description, they may pick up the cards.
8. Continue to take turns until all the cards have been picked up.

Macroinvertebrate Eaters

Shredders: Chew up pieces of dead plants (leaves, grasses, algae, etc.) and reduce the dead material to very fine pieces that are then excreted. Most caddisfly larvae are shredders.

Collectors: Gather and eat very fine pieces of decaying material and the bacteria that live on them. This includes the materials excreted by the shredders. Some act as filters, straining small particles out of the water, such as net-building caddisfly larvae. Other collectors, such as certain mayfly nymphs, gather and eat material that has dropped to the bottom of the stream.

Scrapers: Remove and eat green plants and algae off the rocks they are growing on, and graze on surface plants. Their bodies may be flat, allowing water to flow over them when attached to rocks. Some mayfly nymphs are scrapers.

Predators: Feed on smaller, live, aquatic organisms. Some will sit still and stay hidden, waiting for their prey to approach. Dragonfly and damselfly larvae are predators.

Beginner's Key to Macroinvertebrate Groups

GROUP 1: These organisms are not tolerant of pollution. If your sample consists mostly of these organisms, the water quality is generally good to excellent.

• Riffle beetle
• Stonefly
• Caddisfly
• Snail
• Mayfly

GROUP 2: These organisms will be found in a wide variety of water quality conditions. They are tolerant of some level of pollution.

• Dragonfly
• Sowbug
• Crayfish
• Filtering caddisfly
• Blackfly
• Scud
• Dobsonfly

GROUP 3: These organisms are more tolerant of pollution, and when found to be the largest segment of the population, generally indicate fair to poor water quality.

• Midge
• Pouch snail
• Aquatic worms

(Need to add pix of each group)

Freshwater Invertebrates

Mayfly nymphs (Order Ephemeroptera)

Mayflies are insects that spend most of their lives in streams, emerging briefly as adults ("ephemerally") to mate and lay eggs. Gills are often visible along the abdomen. If an animal has three tails it's a mayfly; but some mayflies have two tails. Mayfly nymphs are strong swimmers and move like dolphins. As immature nymphs many mayflies feed on algae; as adults they do not eat. Mayfly diversity declines as streams are degraded; mayflies are particularly sensitive to mine waste.

Stonefly nymphs (Order Plecoptera)

Stonefly nymphs are typically found on or near stones in the stream. They are rather primitive and may have been among the first insects to develop flight. Adult males and females emerge from the water to mate and locate each other by drumming with their abdomens. Stoneflies move like turtles and many are predators that hide and stalk their prey between stones and cobble. Stoneflies look similar to mayflies but are stockier. Diversity of these animals declines rapidly at the first signs of human disturbance.

Caddisfly larvae (OrderTrichoptera)

Caddisflies use silk (like butterflies) to build cases from gravel, twigs, needles, or sand. Different species build distinct cases, but they often lose them when removed from a stream. Caddisflies are insects that emerge to mate as winged adults. Caddisfly larvae make a living in a variety of ways: some capture food in nets, others scrape algae or shred leaf litter. Free-living caddisfly larvae do not build cases; many are predators and need to move quickly to capture other animals for food. Some caddisflies are very sensitive to human disturbance; others are tolerant.

Adult mayfly, stonefly, caddisfly

All three of these groups leave the water to mate as winged adults. Large swarms of mating mayflies and caddisflies often occur when all the individuals of a single species emerge at the same time. Stoneflies crawl out of the water and mate on the ground. The females of all three groups fly upstream and drop their eggs onto the water or dive into the stream to attach them to rocks or leaves.

Riffle beetles (Order Coleoptera)

Riffle beetle larvae are specially adapted to cling to smooth rocks in fast-flowing water (riffles). After emergence, adults fly for a short time but return to the water to feed in the same habitat as the larvae. Both the larvae and adults are rather small, dark-colored, and tend to drift to the bottom of a sample so they may be hard to see. Riffle beetles collect and gather a variety of different foods.

Fly larvae (Order Diptera)

There are many species of true flies, but you are likely to recognize three main groups or families. Midge larvae (or chironomids) are very small, often C-shaped, and have a spastic squirming movement. They are often attached to debris by their tiny legs. Black fly larvae (or simuliids) are dumb-bell shaped and soft. They attach themselves to the substrate and prefer soft sediment. Crane fly larvae (or tipulids) are large and fleshy with very short "tentacles" at one end. 

Aquatic worms

Flatworms (planaria), roundworms (nematodes), and freshwater earthworms (oligochaetes) are properly called worms; but don't confuse them with the soft-bodied larvae of flies, for examples, which are not. Nematodes and oligochaetes are long and thin and writhe like snakes. Note that these animals do not have legs. 

Crustaceans

Amphipods (or "scuds") are very fast swimmers that look like shrimp. They have many appendages and look fuzzy. High proportions of these animals are present in very degraded sites. Isopods (or sowbugs) are usually found creeping through leaf litter. 

Molluscs (Classes Gastropoda & Pelecypoda)

Most snails and limpets eat algae they scrape from rocks. Check to see if the animal is still in the shell. As larvae, freshwater mussels (or clams) may hitch a ride by attaching themselves to migrating fish. Mussels are very sensitive to sediment because they feed by filtering stream water through their shells. Mature mussels indicate an undisturbed site and may be up to 40 years old.

Source:

Fore, L. S. and A. Wildrick. 1998. Guide to Freshwater Invertebrates.

http://www.seanet.com/~leska/Online/Guide.html

Resources

A Community Water Quality Monitoring Manual for Waterwatch Victoria, web resource, www.vic.waterwatch.org.au/fortheteacher/manual/index.htm (see section on macroinvertebrates under "Biological Surveys"

Field Guide to Aquatic Macroinvertebrates, 2003, Izaak Walton League, www.iwla.org

Fore, L. S. and A. Wildrick. 1998. Guide to Freshwater Invertebrates.

http://www.seanet.com/~leska/Online/Guide.html

Macroinvertebrate Key, web resource, www.people.virginia.edu/~sos-iwla/Stream-Study/Key/MacroKeyIntro.HTML

Monitor's Guide to Aquatic Macroinvertebrates, 1992, Loren Larkin Kellog, The Izaak Walton League of America, www.iwla.org

Name That Bug, web photographic guide for identification of stream insects, mollusks, and crustaceans, www.rayswords.com/bugs/pages/intro.htm

Streamkeeper's Field Guide, 1996, Tom Murdoch and Martha Cheo with Kate O'Laughlin, Adopt-a-Stream Foundation, www.streamkeeper.org

Vocabulary Words

Abdomen	The rear body section of some invertebrates.
Collectors	Animals that feed on dead or decaying organic matter.
Head	The front body section of invertebrates.
Larvae
Macroinvertebrate	Animals that have no backbone and are visible without magnification.
Metamorphosis	A change in body form during growth and development.
Nymph
Predators	An organism that captures and feeds on other organisms.
Scrapers	Animals that remove and eat green plants and algae off the rocks they are growing on, and graze on surface plants.
Shredders	Animals that chew up pieces of dead plants (leaves, grasses, algae, etc.) and reduce the dead material to very fine pieces that are then excreted.
Thorax	The middle body section of some invertebrates.

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