In the depths of time, stretching back over a billion years, the story of the leeches unfolds. These ancient creatures have witnessed the rise and fall of countless species, adapting and evolving to inhabit every corner of our planet. From the icy waters of the Arctic to the lush undergrowth of Borneo’s rainforests, leeches have proven themselves to be the ultimate survivors.
One such leech, the marine Mysidobdella borealis, is a master swimmer, capable of traversing vast distances in the open ocean. Its streamlined body and powerful muscles allow it to navigate the currents with ease, seeking out its next meal with single-minded determination.
In contrast, the medicinal leech Hirudo medicinalis is a graceful swimmer, its movements fluid and precise. This species has been used in medical treatments for centuries, its saliva containing powerful anticoagulants and anti-inflammatory compounds.
But not all leeches are swimmers. The Haemadipsa zeylanica, which lurks in the humid forests of Borneo, and the arboreal Haemadipsa pieta prefer to remain on land, waiting patiently for their prey to pass by. The same is true for Xerobdella lecomtei, Placobdella translucens, Placobdella pieta, Glossiphonia complanata, Hemiclepsis marginata, Helobdella stagnalis, and Theromyzon tessidatum, each adapted to its own unique habitat.
The leech Pontobdella muricata has a peculiar relationship with swimming, only taking to the water when it is hungry. Once it has fed, it loses its ability to swim until its next meal. Similarly, Placobdella omata and Placobdella parasitica only swim as juveniles, before their first feeding, while Placobdella hollensis only swims as an adult.
Leeches are also highly sensitive to light, with most species avoiding intense, direct sunlight. For some, like Piscicola geometra, the predatory cave leech Dina absoloni, the hematophagous Leiobdella jawarerensis, and the deep-sea dweller Bathybdella sawyei, even the faintest ray of light can be lethal.
The dietary preferences of leeches are as varied as the species themselves. Haementeria ghiliani feeds readily on mammals but reluctantly on reptiles, while Theromyzon tessulatum exclusively targets birds. Some leeches, like Erpobdella octoculata and Haemopis sanguisuga, are predators, feeding on small animals. Others, like Bassianobdella dawbini, parasitize their fellow leeches, such as the similar-looking Gaddardobdella elegant.
Perhaps the most peculiar feeding habits belong to Marsupiobdella africana, which attaches itself to the grasping fingers of the Xenopus laevis toad, and Hemiclepsis marginata, which feeds exclusively on tadpoles.
Temperature also plays a crucial role in the lives of leeches. Erpobdella octoculata stops feeding below 1.5°C and above 12°C, while the cold-water specialist Placobdella pieta and the polar leech Johanssonia arctica are most active at very low temperatures. The European medicinal leech Hirudo medicinalis, on the other hand, is drawn to warmth, seeking out objects between 33-40°C.
These diverse adaptations and behaviours showcase the incredible resilience and versatility of leeches, which have existed on Earth for over a billion years. Fossils from the Ediacaran period, dating back 2.6 billion years, contain the remains of annelids, the phylum to which leeches belong. While it is challenging to distinguish leeches from other annelids in these ancient imprints, the discovery of all major invertebrate groups in the Cambrian period, 570 million years ago, confirms the presence of leeches in Earth’s distant past.
As we continue to explore the world around us, we are sure to uncover even more fascinating species of leeches, each with its own unique story to tell. From the depths of the ocean to the canopies of the rainforest, these ancient survivors continue to adapt and thrive, their billion-year journey a testament to the enduring power of life on Earth.
