The Evolution of Parental Care in Freshwater Leeches

Abstract

The life-history strategies of a selection of the most common European freshwater leeches (Euhirudinea) are described. On the basis of this information and results from the literature, the probable phylogenetic development of parental care in the Euhirudinea is reconstructed. The jawless worm leeches (Erpobdellidae) secrete a protective cocoon, cement it to the substrate and sometimes ventilate it before they leave the egg capsules. This behaviour represents the most ancient state in leech evolution. Members of the jawed Hirudinidae deposit desiccation-resistant cocoons on land. All known Glossiphoniidae (leeches equipped with a proboscis) have evolved the habit of brooding the eggs and young. These unique parental care patterns within one family of extant freshwater leeches can be arranged schematically in a series of increasing complexity which may reflect the evolution of brooding behaviour. Glossiphoniid leeches of the genus Helobdella, which have a world-wide distribution, display the most highly developed parental care system: 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 (but not glossiphoniids, which protect the cocoons) a large portion of the cocoons are lost due to predatory attacks. We conclude that the major selective pressure driving the evolution of parental care in leeches may have been predation on eggs and juvenile stages.