Understanding Waterfowl: The Amazing World of Macroinvertebrates

An important food source for many waterfowl, these tiny creatures have fascinating life histories

© Mike Checkett

By Scott C. Yaich, Ph.D.

You don't have to journey to a faraway galaxy to see an amazing world inhabited by "brain-jacking" parasites and monsters with extendable lower lips that can reach out and snatch unsuspecting prey. You only have to visit your nearest pond or wetland and take a close look at some "duck soup." Stir up the shallow water along the shoreline to create your own duck soup. Next scoop up some water and floating debris, and pour the mixture into a flat pan. Now take a close look. Anyone who still has at least a hint of a child's sense of wonder, regardless of their chronological age, will be delighted by the diversity of tiny aquatic life squirming and swimming in the pan. Study some close-up photos (Google "macroinvertebrate images") and basic life history information, and you will be positively astounded.

An aquatic macroinvertebrate is defined simply as an invertebrate (any animal without a backbone) that lives in the water and is large enough to be seen without a microscope. Although many macroinvertebrates are insects (and not just "bugs," which constitute only one category of insects), many more are from other classes of life such as crustaceans (freshwater shrimp and water fleas), nematodes (worms), and mollusks (snails and tiny fingernail clams).

Aquatic macroinvertebrates are vitally important food for waterfowl. When many people think of "duck food," grains like corn, wheat, and rice immediately come to mind. Most grains are high in carbohydrates and energy, but low in protein. The seeds of moist-soil plants like smartweed and barnyard grass often contain high to moderate levels of energy, but are much richer in fiber. Hard mast, such as some types of acorns, can be composed of almost 20 percent fat. Invertebrates are a rich source of protein and other key nutrients that ducks need at particular times of year. For example, amphipods (scuds) are approximately one-half protein and are among the most important invertebrate foods for certain species of ducks, such as scaup. Northern shovelers are adapted to sifting small, swimming invertebrates from the water column. Early nesting species such as mallards and pintails consume large quantities of midge larvae and earthworms that are prevalent in ephemeral wetlands. These and many other invertebrates are especially important food sources for rapidly growing ducklings, molting adults, and breeding hens, all of which require large quantities of protein to meet their particular nutritional needs.

Another essential nutrient that waterfowl obtain from invertebrates is calcium. Invertebrates with hard exoskeletons, like insects and other arthropods, are good sources of calcium for waterfowl. However, most female waterfowl depend on snails to supply the calcium they need to produce eggshells. Snails can be susceptible to having their central nervous systems hijacked by other invertebrates, most often parasites. This "brain-jacking," as it's called, makes the snails more vulnerable to feeding waterfowl (see sidebar). A parasitic flatworm called a trematode has been shown to cause its host snail to feed later in the morning, when waterfowl also more actively feed. This helps ensure that the snail is eaten by a duck, transmitting the parasite to its final host.

Brain-jacked! In a real-life story that sounds like something from a science-fiction horror movie, some of the invertebrates eaten by ducks get "brain-jacked" by other invertebrates, most often parasites. For example, spiny-headed worms, which are parasites of waterfowl, begin their life cycle in amphipods (scuds), a common waterfowl food. To avoid being eaten, amphipods usually try to hide under vegetation and out of sunlight. However, when infected by the spiny-headed worm, their central nervous system gets hijacked and starts producing neurochemicals that change their behavior. Instead of staying near cover, they swim toward light and up to the surface. This makes them much more susceptible to being eaten by feeding ducks. Some species of spiny-headed worms have cysts that are bright orange and make infected amphipods even more obvious and likely to be eaten by ducks, the worms' final host. Once inside a duck, the spiny-headed worm lives attached to the gut and produces eggs, which the duck excretes into the water. The eggs are then consumed by amphipods, completing the worm's life cycle.

But invertebrates are perhaps most interesting when viewed not simply as duck food but as key players in a complex ecosystem. Each species has its respective role near the base of the food web that is the foundation for all life. Many duck hunters may have noticed what appear to be finely shredded leaves or crop stubble on the windward edge of flooded fields or woods. This is not the result of passive breakdown of the vegetation, but is evidence of the activity of a group of invertebrates called shredders. This group includes scuds and isopods like aquatic sowbugs. They are voracious feeders, mincing vegetable matter into infinitesimal pieces. Grazers such as snails also depend on the detritus that shredders feed on, scraping off the algae and microbes that grow on the surface of the debris.

Collectors, which include some midge and mayfly larvae, are next in the food chain, obtaining their nutrients and energy from more finely shredded vegetation. The smallest pieces of plant matter are consumed by filter feeders like fingernail clams. And as in all living ecosystems, predators prey upon the herbivores. Predaceous diving beetles and dragonfly and damselfly nymphs are all formidable predators in this ecosystem.

Although small in scale, these ecosystems can be easily upset by factors such as introduced species and pesticides. For example, pesticides have been shown to reduce the abundance and diversity of invertebrates in prairie wetlands. In addition, the introduction of minnows to previously fishless wetlands can wreak havoc on these miniature worlds. Small fish can consume a high proportion of the invertebrates in a wetland, and significantly reduce its value to breeding waterfowl. Consequently, while these ecosystems-in-miniature may often go unnoticed, we must take care to conserve them as key components of the larger ecosystems on which waterfowl depend.

FASCINATING SUBJECT Studying aquatic invertebrates is a great way to teach children about nature. Just scoop up a bucket of "duck soup" at your nearest wetland, pour some of its contents into a flat pan, and watch the kids' excitement as you introduce them to a whole new world.

Little Monsters In the tiny but complex wetland ecosystem of freshwater invertebrates, dragonfly nymphs are among the fiercest predators, like miniature versions of Tyrannosaurus rex. Although aerial as adults, immature dragonflies spend their lives underwater as nymphs. One of their most unique adaptations as a predator of other invertebrates and even small fish and tadpoles is their formidable set of mouthparts. Like many insects, they have hard, strong mandibles (jaws) that work side to side rather than up and down to tear their prey into smaller pieces. But what really sets them apart is their labium, or lower lip. The labium of dragonfly larvae is long, jointed, and has specialized structures that help it catch and hold prey. If you can imagine your arm stuck on your lower jaw and folded against your chest, you can visualize how the dragonfly's lower lip works and how it can suddenly reach out and grab prey.