Aquatic Life: Worms

Worms are divided into three groups: flatworms, roundworms, and segmented worms. Most flatworms are parasitic but some are free-living and belong to the class Turbellaria. The best known of these are Planarians.   These worms can survive in poor quality streams and are usually found in slow-moving waters. They have a feeding tube through which they eat very small invertebrates. Of course, planarians aren't that big themselves, measuring around 2 to 3 centimeters at the most. Planarians make good science projects because of their regenerative abilities and primitive light sensing organs.

 

The nematodes or roundworms are mostly parasitic, but some are free-living. Many of these are widely distributed through many different freshwater ecosystems and contribute significantly to benthic productivity. Some species are carnivores, some herbivores, and some feed off detritus. Roundworms are also a source of food for many aquatic organisms. Nematodes are found mainly in sediments of lakes and streams. One commonly seen roundworm is called a horsehair worm.   Named because they were commonly seen in horse watering troughs along with horsehair, these nematodes are found not only in bodies of water but also in puddles, and practically any wet area that an insect might visit. They can grow as long as 14 inches. The adult form lives in water. These actually parasite insects. The aquatic adult worms will come out on dry land, bury into the soil, mate and lay eggs that are eaten by crickets, cockroaches, grasshoppers, etc. The larvae grow to maturity inside the insect and when the insect gets near water, the worm comes out and spends its adult life in the water.

 

Segmented worms are classified as such simply because their bodies are segmented. An earthworm is the most common example of a segmented worm. In aquatic ecosystems, segmented worms exist but most of the time is an indicator of poor water quality. A good example is a group of worms called Oligochaetes. Oligochaetes are a diverse group, occurring in a spectrum of waters, from unproductive to extremely eutrophic (nutrient heavy, oxygen deficient) lakes and rivers. The best known of this group are the tubifex worms, which are commonly found in soft sediments, rich in organic matter. Several species typically live in sites that receive organic pollution. Tubifex worms feed on detritus and decaying matter and can persist in low oxygen levels due to a red blood pigment that helps them extract the oxygen from the water. Because of the red pigment, they are often called "bloodworms." Tubifex species often occur in high densities in sewage lagoons, making the bottom appear pink.

 

Another group of oligochaetes are the naidid worms. These are common both in running and standing water. Like tubifex worms, most are sediment dwellers, but some are found among aquatic plants. Naidids feed on detritus, algae, plants, planktonic creatures, and some are snail parasites. While not always an indicator of poor water quality, high numbers of particular species can indicate low oxygen levels and low productivity, which very often suggests some degree of pollution.

 

Perhaps a favorite of all the worm species is the leeches, because leeches are really interesting critters. While not necessarily considered as indicator species as many tubifex worms are, leeches very often persist in slow moving or still waters, in similar conditions as oligochaetes. Leeches can be found in shady areas of ponds or in dark places under rocks, logs, and debris on the bottom. Leeches are not often found in clear fast-moving streams but do occur. While the leeches we most commonly think about are blood parasites, many are non-bloodsucking. Most leeches feed on invertebrates such as snails, worms, and insect larvae. Some will eat fish and amphibian eggs, and others feed on detritus on the bottom of whatever particular body of water they occur. Many of the bloodsucking leeches attach themselves to fish, frogs, and turtles. While bloodsucking leeches are reviled, there is very little to fear from them. They don't require a significant amount of blood when they feed and are not disease transmitters like many blood parasites. This quality is what makes them so useful in the medical field.