Nematodes are nonsegmented roundworms that are abundant in freshwater lakes. Nematodes often comprise 15% of the total biomass on lake bottoms. This reflects the incredible abundance of these organisms, since most only grow to a maximum length of a centimeter.
Life stages of these species are often as complex as they are abundant. Some are free living their entire lives while others are only free living as adults or juveniles. At other stages some are parasitic on invertebrates, vertebrates, or even plants.
Most freshwater free-living nematodes are about 1 mm in length though parasitic forms are often even smaller. Their body wall is covered by a cuticle that is four layers thick. As the worm grows it moults, sloughing off the outer layer. At the same time another layer is created on the inside. The pseudocoel is small in the free-living forms but tends to be much larger in the parasitic forms. All freshwater nematodes bear a spinneret at the tip of their hind end that secretes a sticky mucous which anchors the worm in place whether it be a on rock or inside an intestine.
Non-parasitic roundworms are adapted to swimming along lake and stream bottoms. In fact "swimming" may not be an accurate word to describe their motion. Nematodes have only longitudinal muscles for movement, unlike segmented worms (like earthworms) that also have circular muscles to help with locomotion. Movement is therefore limited to a side-to-side flailing that pushes them forward.
Nematodes have a pair of amphids (one on each side of the body) which are structures at the anterior end of the worm that were once considered to aid in equilibrium. Now, they are seen as chemosensory structures, perhaps for the purpose of detecting food. Due to the wide variation their structure, amphids are also used to classify horsehair worms taxonomically. Some freshwater species have separate light sensors referred to as ocelli or pseudocelli. They are seen as pigmented spots also situated at the anterior end.
Most nematode species are aerobic, meaning that they need oxygen to survive. However, some species can survive short periods of anoxia, and a few can live without oxygen indefinitely. There are not any specialized systems for acquiring oxygen for any member of this phylum. Because they are small animals with a large surface area, they can exchange gases through the skin surface with enough efficiency to survive.
Nitrogenous waste is eliminated though the body wall in the form of ammonium ions. Osmoregulation, and excretion of other metabolites is controlled either by excretory gland cells, an excretory canal system, or a combination of both. These structures are unique to the nematodes.
Nematodes usually possess separate sexes and all fertilization is internal. In most freshwater species the males have spicules that enter the female's vagina where the sperm is released Other species specialize in "traumatic fertilization" in which the male simply punctures the cuticle of the female with his spicules and releases the sperm directly into her body cavity.
Feeding structures and strategies are determined by the lifestyle of the nematode. Species parasitic on plants have stylets within the stoma (mouth) that can be extended outward and into the plant tissues. These stylets are used like straws to suck out nutritious plant juices. Species parasitic on invertebrates have a slightly modified stylet of the same nature to pierce the cuticle of their prey, for the purpose of feeding on haemolymph. Nematodes that feed on microorganisms need only a small tubular stoma to engulf their prey. Finally, the predators have an enlarged stoma with either a spear-like structure or rows of pointed teeth.
Predaceous nematodes are often the worst enemies of other nematodes. This is understandable since both have roughly the same oxygen and pH requirements, so they therefore live in the same places on the lake bottom. Other predators include crayfish, turbellarians, and nemertean worms. Freshwater nematodes are often infected with protozoan diseases and microsporidia.
Freshwater nematodes survive in very diverse environments. Many species that exist in Canada are apparently found all over the world. Some species can survive in snow pools while others occur in hot springs. Aphelenchoides sp. can survive in a temperature of 61.3°C, the highest temperature tolerance by any multi-celled animal on the planet. Most nematodes have drought-resistant stages in which the roundworm becomes inactive. This attribute is most common in the juvenile stages as this is the most sensitive period for many freshwater species. A steady supply of food and oxygen are necessities for health and growth and when these become unavailable the quickest defense is to dry down until conditions improve. Some species have been known to survive in a dried state for up to 25 years before being reanimated in water. These dessication-resistant stages are the primarily means of dispersal. Flash floods or high winds can carry these nematodes to different areas. There is even the possibility of transport in mud that is attached to animals that frequent different water bodies for drinking or bathing.
Eutely is a phenomenon found in a few organisms, including nematodes, wherein each member of a species has exactly the same number of cells. For example, males of the species Caeonorhabditis elegans have exactly 1031 cells, while females have 959 cells, almost half of which are designated to the nervous system.