Lake Look: Leeches

Leeches are related to earthworms. Once you get past the initial revulsion for the animals, you can begin to see a grace and elegance in the orange spotted flattened worms. Both earthworms and leeches are hermaphroditic. Like earthworms they have segmented bodies and lack a structural skeleton. However in earthworms the number of segments varies while leeches all have 34. Where earthworms can add or regenerate body segments, leeches can not. Earthworms have round bodies while most leeches are flattened.

North America hosts perhaps 79 species of leeches with more species in the north than in the south. Worldwide there are 680 species described. The vast majority live in freshwater habitats but about 15% inhabit marine environments, and less than 15% live on land. Some nominally aquatic species will traverse moist ground on occasion. They have been found up to a quarter mile from a waterbody. Leech collectors know to keep their prey well fed because the leeches have a habit of crawling out of aquaria and wandering around the floor.

Traditionally leeches were collected for blood-letting for medicinal purposes. Leech saliva produces a substance, hirudin, that prevents blood from clotting. This practice was always more common in Europe than in the United States, perhaps because the European species of leech used was better able to draw blood from patients than its American counterpart. At their peek, 25 million leeches were used in France in 1846 and 7 million in London hospitals in 1863.  Leeches are still sometimes used, particularly to maintain blood flow when limbs have been amputated but there is hope of reattaching them.

Today leeches are mostly used as fishing bait. In a 1997 radio interview a Minnesota leech collector explained how he and seven employees would spend endless hours crouched in boats harvesting 15 tons of leeches by hand during their five month collecting season. They would then send leeches around the country to bait and tackle shops. 

Leeches inhabit a broad array of freshwater habitats but seem to be most abundant in areas with warm water, little wave action, less than six feet depth, and plants, stones, or debris for concealment. Lake Champlain offers a host of such habitats particularly in sheltered bays and areas around docks. They are less common in areas of mud, clay or sand bottoms, where there is nothing for them to hold onto. Under ideal circumstances, there can be more than 4,000 leeches per square foot. However, since they are nocturnal and well-hidden, casual searching won’t reveal them.

The epithet ‘blood-sucking parasites’ has frequently been applied to leeches, but parasitism is not their only means of feeding. Some species hunt insects and other small invertebrates which they swallow whole. Others scavenge for their food like those munching on fish guts at the end of the dock. Some species switch between different feeding strategies. A wide variety of organisms can host parasitic leeches such as frogs, fish, birds, turtles. One study of map turtles in the Lamoille River found 73% of females and 29% of males had at least one leech attached (females tend to be larger, so there’s simply more area for the leeches to attach to).

When leeches affix to a host they first do so with a suction cup located at their rear end. They explore with their head, seeking a place of thin skin or preexisting abrasion where they can cut into the flesh. A second suction cup around their mouth then attaches and they make a quick incision with three teeth, leaving a y-shaped mark. In addition to the anti-coagulant, they also inject an anesthetic that makes the cut painless. They can feed until they have taken in up to five times their body weight and then they detach on their own. Once sated, a leech may not need to feed again for a long time; captive specimens have gone two years without eating. 

The bite of a leech is harmless unless it gets infected, but if the thought of letting a leech have its fill is too uncomfortable, they can be detached by sliding a knife, fingernail, credit card or other thin object between the animal and the cut. Other folk remedies like fire, salt, or insecticides can cause the leech to vomit and drastically increase what would otherwise be a very low risk of infection. The leech’s anesthetic will make removal as painless as the initial cut, but the anticoagulant can cause the wound to continue bleeding for a short while after removal.