The evolution of parental care in freshwater leeches

Summ. The life-history strategies of a selection of the most common Europea n freshwater leeches (Euhirudinea) are described. On the basis of this info rmation and results from the literature, the probable phylogenetic developm ent of parental care in the Euhirudinea is reconstructed. The jawless worm leeches (Erpobdellidae) secrete a protective cocoon, cement it to the subst rate and sometimes ventilate it before they leave the egg capsules. This be haviour represents the most ancient state in leech evolution. Members of th e jawed Hirudinidae deposit desiccation-resistant cocoons on land. All know n Glossiphoniidae (leeches equipped with a proboscis) have evolved the habi t of brooding the eggs and young. These unique parental care patterns withi n one family of extant freshwater leeches can be arranged schematically in a series of increasing complexity which may reflect the evolution of broodi ng behaviour. Glossiphoniid leeches of the genus Helobdella, which have a w orld-wide distribution, display the most highly developed parental care sys tem: they not only protect but also feed the young they carry. This results in the young being much larger when they leave the parent and, presumably, in higher subsequent survival. Isolated cocoons of all aquatic leeches are rapidly destroyed by predators, primarily water snails. In erpobdellids (b ut not glossiphoniids, which protect the cocoons) a large portion of the co coons are lost due to predatory attacks. We conclude that the major selecti ve pressure driving the evolution of parental care in leeches may have been predation on eggs and juvenile stages.