In Search of Some Macroinvertebrates

When Judy McDonald visited Hilton Pond Center last winter on a field trip with the Environmental Education Association of South Carolina, the first question that popped into her mind dealt with student learning. Judy--a junior high science teacher at Covenant Day School in Charlotte--wanted to know if she could bring students to the Center to search for macroinvertebrates living in Hilton Pond. Since we've never done any formal macroinvertebrate surveys, we responded in the affirmative, and on 9 May Judy showed up with five of her most industrious and inquisitive seventh and eighth graders--complete with dip nets, hand lenses, test kits, sampling trays, field guides, and a boatload of energy.

This was one field trip that almost didn't occur, however, because of family illness and rain that caused it to be re-scheduled from the week before. Then, on make-up day as she was bringing her students to York, Judy's vehicle broke down at the South Carolina I-77 Welcome Center, just as she crossed the border from North Carolina. Luckily, the school had a mini-bus they could deliver that far and--even more fortunately-- Judy has a bus driver's license and was able to continue on to the Center with her crew. This vehicular unreliability cut into the group's sampling time, but they were still able to get a real feel for the kinds of living things that lurk beneath the surface of Hilton Pond.

"Macroinvertebrates" is a very big name for what can be very small creatures-- tiny organisms just visible to the naked eye (hence the prefix "macro"); at that size they certainly couldn't have backbones. (Note: Some macroinvertebrates such as crayfish can be pretty big.) This catch-all group includes a wide assortment of animals from diverse taxa: insects and other arthropods (both larval and adult), worms (round, segmented, and flat), and molluscs, many of which have little in common except their--to coin a term--"macroinvertebratism." Although most school-based macroinvertebrate surveys look at freshwater aquatic habitats such as ponds and streams, these organisms are just as common in marine and terrestrial locales and can even be found floating or flying about in the air.

On their excursion to Hilton Pond Center, the five Covenant Day students concentrated on a narrow microhabitat--the edges of Hilton Pond itself at water depths of less than a foot. To begin, they sampled with pH paper (above right), determining the water registered about 6; thus, like most standing water in the Carolina Piedmont, the pond was slightly on the acidic side of a neutral 7. Then the students used glass thermometers to read the water temperature at a balmy 76 degrees F (24 C). After collecting these baseline data, they got down to the business of stalking their sought-after macroinvertebrates, primarily by scraping the pond bottom with small dip nets or grabbing handfuls of waterlogged leaf litter.

The substrate of Hilton Pond is mostly clay and silt with lots of organic material such as dead leaves, twigs, and other plant parts. Some macroinverts eat this vegetable matter while others hide beneath it awaiting the approach of prey items. When the pond fell victim to our recently ended five-year drought, water levels dropped drastically and terrestrial plants sprouted on its banks. Now, with our abundance of spring rain, this vegetation is submerged, dead, and providing new-found shelter and fodder for a broad assortment of macroinvertebrates.

Working in two groups stationed at different parts of the pond, the Covenant Day students scraped the pond bottom and dumped the mucky contents of their dip nets into white plastic trays that allowed them to separate macroinverts from detritus. Their first few attempts revealed little that moved, but as the students got more proficient they began to uncover all sorts of little critters--most of which the kids had never seen before. Carefully they transferred each organism into a clear plastic box with a magnifying glass in its lid--all the better to observe tiny creatures that were hard to see otherwise.

The most common macroinvertebrates netted by students were scuds--not the dreaded Iraqi short- range missiles, but miniscule semi- transparent shrimp-like animals that swim on their sides, sometimes too swiftly to follow with a hand lens (drawing, upper right). These plentiful quarter-inch (6mm) crustaceans are a valuable food source for baby fish and are reasonably tolerant of water pollution in Piedmont ponds affected by run-off from nearby agricultural and commercial sites.

Also present in many student samples from Hilton Pond were small aquatic snails with exceedingly thin shells--so thin the snails' innards were visible, as were their copious droppings (above left). The snails themselves were hard to find, however, since they were only about 1/8th inch long (3mm) and were nicely camouflaged against the dead, wet, dark brown leaves. Surprisingly, when we tried to photograph these molluscs, one crawled upside down with its muscular foot gliding along on the UNDERside of the water surface. Such is the strength of surface tension and the small size of the snail.

The biggest macroinvertebrate the students hauled in was a true denizen of the pond--a predator capable of taking on everything from mosquito larvae to tadpoles and even small fish. This 3/4" (20mm) brawler (below right) was a juvenile insect that would one day grow up to be a dragonfly. It was hard for the students to imagine that this spindly legged, flat- headed, fuzzy- looking, stout-bodied nymph would someday become a graceful four-winged dragonfly with a long, slender body and colorful compound eyes.

The students discovered several other macroinverts too small to photograph without a microscope adapter, including a red worm with side appendages on each segment that made it look almost like a miniature centipede. An unidentified roundworm nearly as thin as a human hair writhed about when touched, and some samples contained tiny ball-shaped water mites whose larvae are parasitic on dragonflies. With the field guides at hand these organisms weren't identifiable to species, but that wasn't the most important part of the lesson. What really mattered was that the Covenant Day students were out in the field doing "real science" and learning a lot about the diversity of animal life as they searched for macroinvertebrates in Hilton Pond.

We're just glad they had that extra bus and a dedicated teacher who is also a licensed bus driver.

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