Leeches vary also in their prey and what they do when they find it.
Many are bloodsuckers, often of fish or amphibians. If you look at the underside of a healthy tadpole, you can see the heart pumping red, white, red, white as the muscle contracts and relaxes. I have seen a tadpole heart pumping white, whiter, white, whiter after an encounter with an aquatic leech. Other species eat worms or insect larvae. Some species of leech larvae remain on the mother as a brood for some time after hatching.