Leeches are often misunderstood. Some species are true bloodsuckers that are capable of attacking humans, but these make up only a small proportion of the leeches. Most leeches feed on the soft tissues of their prey.
Some species are adapted to feeding on a specific animal, such as a fish or turtle or duck. However, the most successful leeches are opportunistic feeders: they try to feed on any animal that they come in contact with.
Leeches are hermaphroditic, which means that each leech has two complete sets (one male and one female) of reproductive organs. The eggs are fertilized internally, before they are laid.
Some leeches attach their large, yolky eggs directly onto their belly and then shield the eggs using their body to protect the developing embryos. However, for most species, as the eggs are excreted, the leech also excretes a protective cocoon around them. The cocoon is attached to a hard surface.
Some species do both: they lay cocoons and also shield the cocoon with their body and wait for the babies to hatch. The young then cling to the underside of that parent for several weeks, until they are able to venture into the world on their own. The illustration offers a peek at the underside of a brook leech that is carrying a brood of newly hatched juveniles.
Leeches occur in freshwater and marine environments worldwide. A few tropical species are terrestrial. Leeches are generally about 1 cm (0.5 in.) long but bloodsuckers and bait leeches often attain 10 cm (4 in.) at maturity.
There are 650 species in the world, 70 of which are found in Canada. They are very common in every province and territory. Freshwater leeches are found almost everywhere, except for Antarctica. Marine leeches are found in all the oceans.