Answer:
Parasitism.
Step-by-step explanation:
Parasitism, in biology, is defined as a relationship between different species, formed by a parasite and a host. So, basically the parasite lives in or on the host, causing it harm. Parasites can be describe as predators that eat less than one prey unit, that is, they feed them selves slowly, they don't kill the prey instantly as actual predators do.
So, in this case, we are talking about leeches, which are defined as parasitic worms which lives in freshwater environments. They feed themselves by sucking blood from the host.
Therefore, the relation between these two species is Parasitism.