Leeches belong to the phylum Annelida, which includes terrestrial earthworms and marine sandworms. There are more than 700 species worldwide and they can be found in a variety of habitats including freshwater, marine, and moist terrestrial areas. Leeches are grouped according to the way they feed. The jawed leeches (Gnatbobdellida) have jaws and teeth they bite their host with. Once attached, they secrete a substance called hirudin that prevents blood from clotting, allowing them to feed on the blood of their host. Another group of predatory leeches (Rhyncobdellida) insert a needle-like proboscis into their prey and suck the body fluids out of their victim. A third group (Pharyngobdellida) is also predatory but, unlike the others, they swallow their prey whole. The predatory leeches feed on a wide range of invertebrates including insects, pond snails, and freshwater clams. The parasitic leeches will feed on a variety of vertebrate hosts like fish, turtles, and mammals, including humans.
Leeches move by alternately attaching and detaching their suckers along a surface, moving like an inch-worm. They can also flatten their bodies and swim in an undulating fashion through the water. Leeches detect their prey with a range of different sensory structures, some of which can be very sophisticated. They have simple eyes that are used to detect changes in light intensity and possibly some movement, however they rely mainly on chemoreception to detect taste or smell, as well as mechanoreception to detect movements of potential prey.
Leeches are hermaphrodites possessing both male and female sex organs. Mating involves the intertwining of bodies while fertilization take place. Once this is done, a gelatinous cocoon is produced which contains the eggs. The cocoon is either buried or attached to a rock or other surface and the young emerge weeks or months later as miniature versions of their parents. Leeches die after one or two cycles of reproduction.
Leeches were historically used in medicine to treat a number of ailments in an approach similar to blood letting. Their use in Europe peaked between 1830 and 1850, however shortages led to a decline in their use. Leeches are used today for a number of medical purposes including treatments for arthritis, blood-clotting disorders, varicose veins, and other circulatory disorders, as well as in plastic and reconstructive surgery.
Talk to any avid still water fly fisherman and you will find a fan of the leech – leech imitations are a proven trout catcher and most anglers carry a wide assortment to help ensure a successful day on the water!
Australian Museum https://australianmuseum.net.au/leeches
Chan, B. http://www.riseformflyfishing.com/leeches.htm
Govedich, F.R. and Bain, B.A. 2005. All About Leeches. http://www.invertebrate.us/leech/info/leech.pdf