These worms are commonly referred to as leeches. Leeches are very diverse in physiology; some freshwater species live in water with very low salt concentrations, while others can tolerate concentrations that are more severe than seawater. They can also survive severe fluctuations in oxygen levels.
Because of the different habitat occupancy and lifestyles, leeches come in different shapes and sizes. There are a few characteristics that set them apart from all other worms. All have both an anterior and posterior sucker and are divided into 32 post-oral segments. The type and placement of eyes are used for classification.
The musculature of leeches is modified from that of other worms. They posses the circular and longitudinal muscles of the other groups, but also have a set of diagonal and dorsoventral muscles. The result is a reduced coelom (and a loss of septa separating the coelom at each segmental juncture) as this space is taken up by the extra sets of muscles. Setae, which are used by other worms to anchor themselves while burrowing, are absent in the leeches. These structural changes are reflected in leech locomotion. Leeches crawl and swim, but are not capable of burrowing. In order to crawl (often called looping) the leech uses its anterior and posterior suckers to alternately anchor itself to the substrate.
All leeches have a flattened body shape and a relatively high surface area exposed to the water relative to their body volume. They absorb oxygen through the body wall, so there is no need for the leech body to waste energy developing gills to obtain oxygen. If there is a low oxygen content in the water, they attach their rear sucker to the substrate and make intense waving motions with their body. This causes a water flow so more oxygen reaches the leech.
When you finish a swim in your lake and you get out to find a leech clinging to your toe, what do you do? Just reach for a shaker of salt and let osmosis do its job. Leeches, like other freshwater animals, have a higher concentration of salts in their bodies than the water around them. Leech bodies are used to this balance, and have mechanisms dedicated to pumping out excess water that tries to enter their cells. When salt is poured on them, the water tries to reach higher concentration of salts that is now present outside the leech. The water inside the leech leaves the body to dilute the salt outside. The cells in the wither up, spelling the end of the leech.
Blood sucking leeches have tiny bacteria in their guts that break down the blood into components that the leach can absorb. This is a symbiotic relationship that benefits both parties.
All leeches are hermaphroditic, as all individuals have both male and female reproductive organs. Female and male reproductive organs are not active at the same time in the lifecycle; they must find a partner with which to copulate. They can, however, simultaneously exchange sperm with a partner and store it for later use. Unlike other annelid groups, leeches cannot reproduce asexually or regenerate damaged body segments. Most predatory leeches breed only once and then die. Blood feeding leeches are more likely to reproduce several times in a year.
Between two days and many months after copulation, depending on the species, eggs are laid inside an albumin-filled cocoon created by the clitellum of the egg donor. The eggs hatch inside the clitellum and the juveniles develop while feeding on the nutritive albumen, before being released into the environment.
Contrary to popular belief, not all leeches are blood sucking terrors. In fact only a few species are ectoparasites, and even fewer are parasites on humans. The majority of leech species are predators on oligochaete worms, molluscs and larvae of chironimid midges. Although leeches are predators and ectoparasite, they are still food for some higher predators. Fish, birds, snakes, amphibians, and to a lesser extent insects and gastropods, are all voracious predators on leeches.
The saliva of blood sucking leeches has amazing properties. Human saliva carries out only a few tasks, the most important of which is lubrication of food to ease swallowing, and some basic chemical breakdown. Leech saliva also has some digestive powers, but it is also uniquely adapted to aid in the act of extracting blood.
When a leech first latches onto your finger, it uses its mouthparts to break the skin to gain access to blood. It has a better chance of having a good meal if it's not noticed. The saliva released into the wound acts as a painkiller. The next step is the extraction of blood. Ordinarily, blood clots as soon as a wound is made. The leech ensures the free flow of blood with another saliva component that acts as both an anticoagulant—which stops the red blood cells from sticking together to block the wound—and a vasodialator—which causes the vessel that the leech has tapped to open wider for a better blood flow. In this way the leech ensures an abundant flow of blood into its mouth.
Worms in this order include the common earthworm, Lumbricus lumbricus, and look much the same as well. They have no appendages, a simple brain, and long bodies. There are tiny bundles of hairs on every segment that are characteristic of each species and is also the basis for their name ("Oligo"= few, "chaeta"=hairs). These hairs are used both as basic touch sensors and as anchors in the sediment to facilitate movement.
Oligochaetes are segmented, laterally symmetrical animals without appendages. Most freshwater species are shorter than 10 centimeters (cm) in length, although some species can reach 70 cm. The longest annelids in the world occur terrestrially in Australia and reach lengths over 3 meters (m).
Since most worms live deep within sediments, they have no need for eyes. They feed continuously on organic matter in the surrounding soil or sediments, so they also have no need for visual receptors to seek prey. Members of the freshwater Naididae are the only worms to have even rudimentary eyespots. Since they have the ability to swim and are exposed to predators and competitors, eyespots are not a bad idea. Other groups have photoreceptors that will detect shadows of potential predators, but they are extremely simple and can only distinguish between light and lack of light. Some species spend time with their posterior ends out of the sediment to obtain oxygen. This would leave them exposed to predators if they didn't have mechanoreceptors on their tails that sense movement around them. When they sense the approach of a predator they pull their body back into the safety of the sediment.
Oxygen uptake occurs through the body wall of most worms. They possess and extensive capillary system near the surface of their outer skin, which provides a large surface area for oxygen exchange. Larger worms have haemoglobin molecules that they use for oxygen transport in the same way as vertebrates. Since the majority of freshwater worms are small and live in sediments they do not require much oxygen to survive. When oxygen levels are low, worms such as Tubifex tubifex stick their posterior end out of the silt and swing it around in an attempt to contact more oxygen.
Oligochaetes have a metanephridial (very simple kidney) system to sort out the products for excretion and the salts they reabsorb for future use. Many can also absorb required salts through their skin.
Oligochaetes are hermaphrodites, having both female and male reproductive parts. Most species rely on copulation with another individual for reproduction but there are many examples of asexual reproduction including parthenogenesis.
After mating, eggs are released into a cocoon, formed by several segments near the anterior end of the body, and the sperm fertilizes the eggs within. The edges of the cocoon close while the eggs are developing. Upon hatching the ends reopen, and tiny worms escape to the outside environment.
Aquatic oligochaetes are benthic dwellers in permanent lakes. They feed continuously on the sediments as they burrow through them. These sediments are largely composed of organic matter, including plant matter, decomposing organisms, faeces and the bacteria that grows on all three. Oligochaetes rarely stop ingesting, digesting what they can—their favorite and most nutritious bits are the bacteria—and get rid of the rest as quickly as they can to make room for more.
Some oligochaetes feed largely on the feces (recolonized by bacteria) of other worm species. These fecal connoisseurs migrate to the same areas as their preferred meal providers.Because worms are softbodied animals, many other organisms, such as fish, crustaceans, and leeches, exploit them as easy prey.
Certain species like Tubifex tubifex create cysts to protect themselves from dessication if they are stranded above the waterline. Some species can survive 14 days with no moisture at all, and for up to 70 days if a little moisture is a added periodically by rainfall.
Members of the genus Lumbriculus create cysts which are resistant to freezing as well as drought.
There are fewer than 50 known species of freshwater polychaetes in the entire world, and only 13 species occur in North America. Polychaetes have two different lifestyles; some are active crawlers and swimmers, but most are sessile tube-dwellers.
These worms are reminiscent of centipedes because they bear paddle-like legs on each segment. However, these appendages are not jointed, but are merely extensions of the body wall. Each appendage bears tiny hairs that aid in traction or current creation. This large number of hairs is the basis for the name of these worms ("Poly"=many, "chaeta"=hairs).
The polychaete head is much more developed than that of oligochaetes or leeches. It contains antennae, eyes, and feeding palps (in some species tentacles are substituted for feeding).
Large polychaetes possess fan-like gills often associated with their appendages for oxygen exchange. They are not needed in the smaller species that rely on simple diffusion of gases to survive.
Most polychaetes have a well developed blood-vascular system that is driven by body movement and accessory "hearts". Some species lack an efficient blood system so the septa that divide the coelom are reduced and circulation is a result of the movement of coelomic fluid. In smaller species there is no need for any respiratory pigments to help carry oxygen. However, some of the larger burrowing species that aren't exposed to high oxygen concentrations have pigments, such as haemoglobin, dissolved in their blood.
There is not much known about the capabilities of freshwater polychaetes to regulate their body salt content except that they keep a high concentration in the fluid of the coelom and that nephridia, often occurring in each segment, play a role in excretion.
Although many marine polychaetes exhibit asexual reproduction, all of the freshwater species reproduce sexually. Most polychaetes have separate sexes, but crawling/swimming species tend to be hermaphroditic. The free-swimming larval stage, typical of the marine forms, is suppressed in all freshwater species.
Adults of one freshwater species, Nereis succinea, change shape just before mating. Their eyes get bigger, and their limbs and cirri start to thicken. When the mating cue is given, there is a mass migration to the surface of the water where the mating takes place.
Sessile species often brood their young inside their tube to protect them from premature dispersal and predators.
Swimming polychaetes are equipped with a jaw-like proboscis that can be extended to grab prey. Even though they are fairly efficient hunters of smaller worms and other soft bodied animals, these polychaetes are usually omnivorous. Many of the tube dwellers are filter-feeders. By moving their appendages within their tubes, these animals create a current that draws smaller water-borne particles into the mouth. The other sedentary groups are deposit feeders, using feeding tentacles to scoop up sediment and draw it into their mouths.
As polychaetes are largely ocean dwellers, it is not surprising that the freshwater species are found in rivers that lead to the ocean or in lakes that were connected to marine waters in the recent past. The mobile polychaetes are found both in rivers and lakes, usually on top of soft sediment. The sedentary groups are found in lakes; some build tubes in soft substrates, while others build calcium based tubes on shells, rocks, or aquatic plants.
Polychaetes are very efficient at regenerating missing tentacles, body segments, and even heads that have been eaten by a predator. The swimmers, whose bodies are less differentiated than those of tube dwellers, are better at regeneration. The nerve cord is very important in initiating regeneration. Some marine polychaetes can even rebuild themselves from a single segment.