Leeches are hermaphrodites and therefore each individual is both a male and a female at the same time! When ready to mate, leeches have several different options. Some species mate in such a way that both individuals will become fathers and mothers at the same time (Motobdella montezuma does this), others are more selective and will resist becoming a mother, but will freely mate to become the father of a brood, and some can even become both the mother and father of their own offspring (Helobdella)! After mating, eggs are produced and placed within a cocoon. Most leeches then abandon the cocoons once they are ready. However, prior to abandonment, they do provide food for the developing eggs and young by filling the cocoon with a nutrient fluid. This fluid nourishes the eggs and subsequent embryos and is designed to last until the young are old enough to break out of the cocoon and fend for themselves. Once the young hatch from the cocoon, they are fully capable of caring for themselves and have no further contact with their parents.
Motobdella montezuma attaches its cocoons to aquatic vegetation at a depth where they are safe from ducks and other waterfowl. One aquatic leech family, the Glossiphoniidae, employs an altogether different parenting strategy. The word “leech” is generally thought to be synonymous with selfishness and exploitation, but this is definitely not the case with the glossiphoniid leeches, who are actually quite devoted parents. A variety of parental care behaviors have evolved within this family, ranging from the brooding of egg clusters in an external nest, to brooding of eggs and young on the parent’s body (Helobdella), to keeping the eggs and young within an internal marsupial-like pouch. In each of these three different types, the parents protect the eggs from predators and also ventilate the eggs by undulating their body across them in order to insure that the eggs are adequately oxygenated. After the eggs hatch, the parents will continue to ventilate the young, protect them from predators, and will also begin to provide food for them either by capturing and providing prey (snails, oligochaetes, mosquito larvae, etc.) or by transferring nutrients across the body wall to the developing young in a manner reminiscent of a “placenta.” Some glossiphoniids, including Helobdella, have even progressed to the point where they have taken the next step, becoming “social” leeches. These species spend much of their time living together in aggregations of two or more individuals and have been observed sharing food with each other and even caring for juveniles that are not their own. Living in groups probably helps with capturing prey and also reduces the chance that a single leech will be killed by a predator, but it may also ensure that offspring will survive if the parent dies.