Critter Corner No. 26 – Leeches by George Sly
While birds may lay claim to the “star power” at Goose Pond FWA, let’s remind ourselves that there is a whole world of smaller animals, many of which lack a backbone, that are of major ecological importance there.
Let us focus on one group of invertebrate animals which most people look at with fear, revulsion, or both. Readers who, like me, are old enough to remember the Humphrey Bogart movie African Queen likely recall the scene which gave many of us our first introduction to aquatic leeches. In North America there are over 60 species of leeches. These are difficult to distinguish from one another. Thus, we will consider them as a group.
Leeches are related to earthworms. A key anatomical feature which reveals this connection is the obvious segmentation of the body (both externally and internally). Leeches differ from earthworms in having two suckers on the body. The anterior one surrounds the mouth and the posterior one is used for attaching to a host and for locomotion.
In size they can range from less than an inch to over a foot when stretched out, courtesy of their highly elastic, flexible bodies. Colors can also vary. Background colors are usually brown, black, or greenish. Some species may be marked with colorful blotches on their backs.
Leeches are, of course, notorious for feeding on blood (remember poor Charlie Allnut). They may parasitize fish, amphibians, waterfowl, and mammals. I have also seen them on turtles and alligators. However, some leeches are predatory and eat other organisms. They extract their prey’s body fluids and excrete the harder parts. Items on the diet of such leeches could include insect larvae, aquatic worms, snails, and other leeches.
Leeches can be found in a variety of aquatic habitats including marshes, slow-moving streams, and the shallow areas of ponds. An intermittent stream that flows near my house harbors leeches. These sometimes venture into our yard, especially in the spring when the ground is very moist. Otherwise, they remain unobtrusively hidden among the rocks and debris in the stream.
The species of leeches which act as external parasites, by taking blood from a host, can increase their size greatly while feeding. Much like a tick, leeches can increase their body bulk by 4X or 5X. When full, they drop off and may not feed again for many months. One authority stated that in certain species this can be as long as two years. Leech bites often bleed after the animal is removed because their saliva contains a powerful anticoagulant called hirudin. The presence of this peptide ensures that blood flow continues while the animal is feeding.
Like earthworms, leeches obtain their oxygen and release carbon dioxide by diffusion via their skin surface. Leeches reproduce sexually and, like earthworms, are hermaphroditic (have both male and female reproductive organs). Most often they mate with another leech however. This is advantageous because it provides for greater genetic diversity and thus evolutionary adaptability. Eggs may be deposited in soft, muddy substrate or carried in a sac on the ventral side of the body.
During the winter, or prolonged dry spells, leeches become inactive and often burrow into the substrate. Leeches may be beneficial in an environment because they can serve as prey themselves. They are eaten by fish, salamanders, wading birds, and snakes for example. They may also be used as fishing bait. On the negative side, they may attach to people who are swimming or wading. Usually this causes little harm, but it is an experience most people do not relish.
I lived in the Southeast Asian country of Malaysia for a few years. While there, I was surprised to learn that not only did they have aquatic leeches, but terrestrial ones as well. Even though one might be watchful while hiking in the rainforest, attracting leeches was guaranteed. In fact, my experience was that one just couldn’t keep them off one’s legs. Their movements were stealthy and their bite painless. Typically, I would stop to take a breather and find that several had, unbeknown to me, already left a bloody calling card, where they had fed. Quite often a couple of their colleagues would still be attached and feeding – as in the accompanying photo!
Interestingly, leeches have a long history of being used for medicinal purposes. Many years ago, a common procedure for treating disease was so-called bloodletting. It was believed that removing blood from a patient would also remove what it was that was causing the disease. The practice goes far back in history, to the time of the early Greeks and ancient Egyptians. It was also used by the Romans and spread to other parts of the world as well.
The practice was so common that doctors in medieval Europe were sometimes referred to as leeches. Although bloodletting has long since passed from the pharmacopeia of most physicians, there is a fascinating use for medicinal leeches even today.
Certain procedures, such as reattachment of severed limbs or digits can result in significant swelling at the surgical site. This could impair blood flow through the tiny vessels which have been sutured back in place. Lack of sufficient blood flow may cause the procedure to fail. By attaching leeches to the injury site, swelling and vascular compromise are reduced as the leeches efficiently feed on blood and tissue fluid from the edematous area. As noted, their saliva will also prevent clotting at the injury site.
All in all, the leeches represent one more group of animals that may be seldom seen or even given thought. But, like myriad other species, they contribute to the complex web of life that is the Goose Pond FWA ecosystem.
Photo Credits:
Leech by Robyn Jay at commonswikimedia.org
Group of leeches courtesy North American BioPharma, Inc.
Leech attack by the author
Physician applying leech to patient at:
https://commons.wikimedia.org/wiki/File:A_physician_administers_leeches_to_a_patient._Colour_reprodu_Wellcome_V0011719.jpg