Theromyzon tessulatum

This leech is found in freshwater habitats in Europe.^[4] It is common in the coastal flat parts of Holland which are visited by the waterfowl that act as its hosts.^[2] It is uncommon in Britain, where it has a discontinuous range, probably being distributed by movements of its bird hosts.^[3] It has also been recorded in North America from a number of locations.^[4]

When parasitizing ducks and other waterfowl, T. tessulatum invades their mouths and respiratory passages. At other times, it is free-living in freshwater habitats. The adult broods its egg capsules on its under surface, where there may be as many as two hundred eggs.^[3] When the young are ready to emerge, the adult uses its sensilla organs to detect vibrations in the water and will move towards any disturbance that might be caused by a potential host. Suitable hosts are those with body temperatures of between 37 and 40 °C (99 and 104 °F). Although generally nocturnal and negatively phototactic (avoiding light), when the brooding adult is ready to release its young, it may swim in open water in the middle of the day.^[4] If it finds a suitable host, it attaches itself with its sucker to the nasal passages, the respiratory tract or inside the mouth.^[2] This leech is unique in that the adult attaches to the host but does not itself feed on it; instead, the young that it was brooding transfer to the host to take their first blood meal.^[4]