Carl Zimmer‘s latest article describes Dr. Siddall’s subjects as Highly Evolved and Exquisitely Thirsty:
‘You can’t set traps for leeches,’ Dr. Siddall said. ‘We are always the bait. You can turn over rocks. You can turn over branches. But ultimately the interesting stuff is going to come to you.’Turning himself into bait is paying off. Dr. Siddall’s research has shown that the ancestors of leeches were probably freshwater worms that fed harmlessly on the surface of fish or crustaceans, as the closest living relatives of leeches do. Not only do these worms have the most leechlike DNA of any animal, but they also grow the same sucker on the base of their tail that leeches use for crawling.
The leech evolutionary tree suggests that the earliest land vertebrates may have been the first hosts for leeches. Dr. Siddall has identified several major innovations that early leeches evolved as they became blood feeders. They acquired a proboscis they could push into their hosts to drink blood. Later, some leeches evolved a set of three jaws to rasp the skin.
Leeches also needed chemicals that could keep their host’s blood thin so that it would not clot in their bodies.
Leeches have evolved many different molecules for that work that interfere with different stages in clotting, along with other molecules that prevent inflammation. Pharmaceutical companies have isolated some of these molecules and sell them as anticoagulants.
Blood is a good source of energy, but it does not make for a balanced diet. Mosquitoes and other blood feeders have evolved a symbiosis with bacteria that can manufacture the vitamins and amino acids necessary for life.
Leeches appear to have evolved their own partnerships, even producing special chambers in their throats where bacteria can live.