The Terrestrial Haemadipsid Leeches and Continental Drift

The haemadipsids are terrestrial blood-feeding leeches with an unusual biogeographic distribution; being found only in the Indian subcontinent, Southeast Asia, Wallacea, Australia, Melanesia, Madagascar, and the Seychelles but neither in Africa nor in South America. All other leech families have a global distribution. No other group of leeches has inspired such passionate accounts by travelers or naturalists. Even North America’s most prolific leech researcher was particularly awestruck by this family where in "the dank tropical jungles, the misty ravines and the showery, forested mountain-sides of this extensive region they are among the most dominant and self-assertive elements" (Moore, 1927: 224). A few travelogues convey the fascination and horror these annelids have instilled in their victims:

"Leeches swarmed with incredible profusion… they got into my hair, hung from my eyelids and crawled up my back." (Hooker, 1854 — Himalayan Journals)

"…they swarm in myriads in every wood… it is impossible to take a single step without being attacked… they are on every bush and tree, from which they drop on the head and neck of the passer-by" (Haekel, 1883 — A Visit to Ceylon)

"There had been leeches on the side of the Refuge Trail — lots we had thought… but at that time I hadn’t seen the Pebu Trail. This must have been the birthplace and ancestral home of the leech… so close together that your eyes had to be focused at your feet to find a place where you could step… I finally compromised with the leeches… letting them get their fill… so long as they kept away from my face and the fly of my trousers…" (Seagrave, 1946 — Burma Surgeon Returns)

"I counted no fewer than ninety-seven of them on my body, most of them concentrated round my private parts!" (Campbell, 1953 — Jungle Green).

These are the only leeches that have a strictly terrestrial habitat preference, most other leeches, of course, being essentially aquatic.

The nature of the global distribution of the haemadipsids has been cause for speculation regarding their evolutionary history since Whitman postulated a Tertiary origin in Northern India where freshwater lakes were thought to have been drained by the Himalayan uplift.

Considerably distant terrestrial blood feeders like Mesobdella gemmata in Chile, Malagobdella spp. in Madagascar, and the Seychellian Idiobdella species naturally caused some consternation for researchers attempting to explain the world’s distribution of this group.

Richardson suggested that this could be partly solved if one recognized the Chilean Mesobdella as having an origin independent of the Haemadispsidae proper. Noting that "the occurrences on two continents, on major island masses, on adjacent islands in archipelagos, and on widely spaced oceanic islands, raises the manner of their dispersal as a major zoological problem", in a more thorough consideration of the problem he tried to include Wegener’s theories of plate tectonics but acknowledged that this would still require an independent origin for the Malagasy and Seychellian taxa.

All agree that long distance oceanic dispersion is not a likely mechanism for extant haemadipsid distributions. Leeches are notoriously intolerant of salt.

The central problem to the solution of the the evolutionary patterns of distributions of terrestrial leeches lies with solving the position of those that remain in Madagascar and the Seychelles.

The Haemadipsidae have a distribution that is suggestive of an origin in the mid-Cretaceous after the southern supercontinent, Gondawana, began its breakup. Current theories put the separation of South America and Africa from the remainder of Gondwana roughly at 150 Ma followed, 30 million years later, by the separation of the Indo-Mascarene-Malagasy section from Austral-Antarctica. The sequence of tectonic events for post-gondwanan plate movements are sufficiently well known so as to provide a generally accepted area cladogram and hypotheses for the relationships of taxa endemic to these regions.

One hypothesis would have the haemadipsid clade’s origin in the late cretaceous (< 120 Ma) in the Indian subcontinent when it was still connected to Madagascar followed by India's collision with the Asian continental land mass and dispersal of haemadipsids through southeast Asia from whence they then further disperse through the Wallacean Archipelago and into Australia. A second, or 'vicariance' hypothesis, has an earlier origin (>120 Ma) for the haemadipsids, with an early isolation of the Indo-Malagasy group from the Australian taxa and a phylogeny that is more or less congruent with the area cladogram.

The principal difference between these two scenarios relates first to where the Malagasy and Seychellian species fall in leech evolutionary trees, and also to whether or not the Australian taxa are highly derived or represent a clade (with the Papuan taxa) that is sister to the remainder. In addition, there are different expectations regarding the relationships of the Indonesian taxa. In either case the Southeast Asian Haemadipsidae are expected to have arrived with the collision of the Indian subcontinent and Asia in the early Cenozoic (~25 Ma), a point supported by their absence North of the Himalayan orogeny (Neseman and Sharma, 1996). Under the dispersal hypothesis, Indonesian leeches should form a paraphyletic assemblage leading to the Australian and Melanesian haemadipsids. Under the ‘vicariance’ hypothesis they should be expected to have separate origins that more or less obey the biogeographic separations outlined by Wallace, Weber and Lydekker (reviewed in Scrivenor, 1941; Burkill, 1943; Schmidt 1954). Preliminary (very limited) data support neither of these hypotheses, though much more of the diversity still needs to be sampled.