Leech Facts

Leech Profile

Outside of the world of fishing, worms don’t get nearly enough attention. This is partly because they’re kinda gross, but also because they all look fairly similar and none really taste or look all that good. 

But from an ecological and biological perspective, there’s a lot to take in: four major phyla of worms exist and species range from microscopic to 55 metres long. Within Annelida, we have the groups of collared worms, such as those often used on hooks, and their rather frightening cousins, the leeches. 

And even among leeches, there’s an incredible amount of diversity.

Leech Facts Overview

Habitat:	Terrestrial, arboreal, marine, freshwater,
Location:	All continents and seas except for terrestrial Antarctica
Lifespan:	Up to 3 years
Size:	From 1cm to 45 cm long.
Weight:	Up to 80g
Colour:	Usually brown, green or black. Sometimes reds and yellows
Diet:	Varied, often blood or fluids, sometimes snails, worms, invertebrate prey
Predators:	Birds, fish, amphibians
Top Speed:	Slow, but can swim well
No. of Species:	700+
Conservation Status:	Least Concern to Vulnerable (IUCN)_

Leeches are ancient, somewhat horrific relatives of the friendly garden worm. Separated by hundreds of millions of years, they took the dark path and turned out the be incredibly good at it.

These incredible blood-sucking, skin-piercing, sometimes-hunting blobs of muscle and goo have spread to pretty much every ecosystem on Earth and while most have no intention of doing anything good for the world, some are still helping in the realm of medicine. 

Interesting Leech Facts

1. They’re annelids

Annelids are the most familiar group of worms to most of us. This is a big phylum with multiple different wormy groupings nested in it. Annelids are some of the most abundant animals on Earth and have been putting our wormy animals for over 500 million years. 

It was around this time that several of the commonest annelid worms diverged. The tube worms in the ocean with their hairy appendages are annelids, but more local to us, and so are the worms found in the compost pile. 

And these compost makers are members of the group of worms known as the clitterates – worms with collars. This group also contains the leeches – worms that for some are equally familiar, and not nearly as pleasant. 

But being pleasant isn’t the only way to be interesting, and leeches have a lot of redeeming features. ¹

2. They’re extreme

There are more than 700 described species of leech, and they can be found almost everywhere. They also come in all colours and sizes, but one thing that really stands out about this group of worms is how tough they are. 

Leeches can be found in extreme environments, and have adapted well to handle them. Leeches are found in hot, cold, dry, wet, fresh and salty habitats, which means there’s hardly anywhere on Earth you can go to avoid them. ²

3. They’re mostly freshwater

The vast majority of leeches are where most people expect them to be: in fresh water. Most are no more than 2.5cm long and feed on the blood of their hosts, which they access using a jawed mouth that is lined with teeth. 

These leeches can release special chemicals that reduce pain, prevent inflammation and prevent clotting so that the host barely notices they’re there as they fill up on blood. Leeches in freshwater are surprisingly good swimmers, and they use an eel-like wiggle to get through the water. 

But this is only where most of the leeches can be found. There are still hundreds of species that don’t live in water. 

4. Some are terrestrial

All jawed leeches appear to have originated from a terrestrial ancestor, and some likely never evolved to enter the water at all. The Haemadipsidae family of leeches is exclusively terrestrial, and they experience some of the most extreme conditions of any leeches. 

One way they adapt to life on land is to produce copious amounts of mucous to stay hydrated and a high tolerance to desiccation.

Land leeches are found in Asia, Africa, Australia and North America, and terrestrialism evolved independently among other branches of leech, too. Most of these are predators of the friendly earthworms, but others feed on blood like their freshwater cousins. 

5. Some are marine

The Piscicolidae family of leeches are one of the few groups of leeches that can handle salt water, and they are primarily parasites of bony fishes. They are likely to have evolved from freshwater ancestry and moved to the marine to exploit open niches there.

Even the polar regions aren’t safe from these parasites, and fish are commonly found with numerous leeches attached to them. 

6. Some are terrifying

The largest leech grows to 46 cm long, and 10cm thick. These monsters are grey and slimy, and have no jaws, but still manage to suck blood. Instead of a bite, they use a piercing proboscis, which somehow makes them even more frightening, and each of its 34 body segments has its own ganglia for movement and perception. 

Of course, an abomination like this is going to be hermaphroditic, and when stressed, this giant spews mucous everywhere to get away. 

The hypodermic needle attached to its face can be 10cm long and shoots a clot-preventing enzyme into the puncture wound. They can consume 15ml of blood every minute with this monstrosity, and common hosts include cows, rabbis, and humans.

These leeches can be so big and so numerous, that it’s said that they have killed cows. ³

7. They can be prolific

Leeches can get a bit carried away when the conditions are right, and gather in numbers that are just unacceptable. Densities of 10,000 per square metre are not unheard of, which conjures up images of a space into which nothing should ever set foot. 

But thankfully, not all leeches are bloodsuckers. Some are active hunters. ⁴

8. And predatory

Predatory leeches, like the inexcusable giant leech, have a proboscis instead of a toothy jaw. This acts like a spear gun and pierces prey foolish enough to come close. 

This strategy feeds the leech on snails, worms and sometimes insects. Usually, the victim is swallowed whole, à la snake, but sometimes they’re polite enough to digest and suck up the inner tissues through a straw instead. ⁵

9. They can be medicinal

As much as they make us uncomfortable, leeches have a long history of helping us, too. Back when all of your troubles could be fixed by just bleeding a bit, leeches were used in phlebotomy. Medical leeches, Hirudo medicinalis, are the largest leech in the UK and were harvested en masse to supply a market of quackery that turned out to have some legitimacy to it.

Bloodletting isn’t used for half the number of things it was once professed to cure, but it does come in handy sometimes, especially when done through a leech, which has anti-clotting properties and can increase blood flow to various areas. 

These leeches were collected so much that they almost went extinct, and are still threatened today. ⁶

Leech Fact-File Summary

Scientific Classification

Kingdom:	Animalia
Phylum:	Annelida
Class:	Clittelata
Subclass:	Hirudinea

1. Dr Luke Parry (2020), “Discovering the Earthworm’s Half a Billion Year Old Cousin”, St Edmund Hall.
2. Anna J. Phillips (2020), “Leeches in the extreme: Morphological, physiological, and behavioral adaptations to inhospitable habitats”, NIH.
3. Spooky (2024), “Living Nightmare – The World’s Largest Leech Can Grow Up to 18 Inches Long”, OddityCentral.
4. Sawyer, Roy T (1990), “In search of the giant Amazon leech”, Science Direct.
5. Animalogic (2021), “Predatory Leeches Exist, And Are Horrific”, YouTube.
6. “Medicinal Leech Recovery Project”, Freshwater Habitats Trust.