As you would probably expect, you stand a far higher chance of coming face
to face with a leech if you hike a tropical rain forest or tow Humphrey Bogart's African
Queen than if you explore our Southwest desert basins and mountain ranges.
You may be surprised to discover, however, that you can find a respectable community
of leeches, including blood sucking species, in locations such as our quiet river
basin wetlands and high country ponds.
For example, in Montezuma Well, a pond in a geologically distinctive setting
in the Sonoran Desert north of Phoenix, "Blood sucking leeches are…abundant
in the aquatic vegetation," according to the National Park Service in its
Internet site. Other species are even more abundant.
Meet the Leech
Worldwide, somewhere between 700 and 1000 species populate freshwater, saltwater,
estuarine and moist land ecosystems, said leading authorities Fredric R. Govedich
and Bonnie A. Bain in the All About Leeches Internet site. About 10 percent of
the species occur in the United States.
Related to earthworms, leeches tend to have a colorful, tear-drop-shaped,
flattened and segmented body, ranging in length from less than an inch to several
inches in length. They have suckers on the underside of each end of their body—an
oral sucker at the forward (narrower) end and a second sucker at the rear end.
They have, on the head and body, sensory organs that enable them to detect light,
odors, vibrations and temperature variations.
In water, leeches swim with an undulating motion, something like eels. On
solid surfaces, they move "by alternately attaching and detaching [their]
suckers, crawling about in a looping inchworm like motion," said Govedich
and Bain.
The leeches, all with their mouth parts centered in their forward suckers,
comprise three kinds of carnivores. One kind – the "engulfer," which
has no teeth or jaws – feeds on small invertebrates, swallowing prey whole.
A second kind – equipped with a hollow needle-like proboscis – spears
the flesh of worms and snails, secretes an anticoagulant into the tissue to assure
the free flow of blood, and sucks out the juices of prey. The third kind – the
parasitical blood sucker, which has jaws and razor-sharp teeth – bites
into prey, including amphibians, reptiles, fish, waterfowl, mammals and – given
the opportunity – human beings. Using both the forward and rear suckers
to remain attached to its host, the toothed blood sucker excretes a mucous that,
along with suction, fixes it in place while it feeds. It releases an anesthetic
to numb the wound site. It secretes an anticoagulant into the wound. If left
undisturbed, it will ingest several times its weight in blood. Engorged, it releases
its hold and drops off, seeking out a quiet dark refuge to digest its meal, a
process that may last for several months. While all leeches are predators, they
also serve as prey for fish, waterfowl, reptiles, large aquatic insects and even
other leeches.
Historically, possibly since the Stone Age, leeches have played medicinal
roles. According to the BBC Internet site, leeches served Egyptians, Persians,
Greeks, Chinese and other early civilizations in bloodletting ceremonies with
purposes such as sucking out evil spirits from one's body.
After falling out of favor for most of the 20th century, leeches have returned
to the medical scene, providing anticoagulants and anesthetics for micro- and
reconstructive surgery and, possibly, in the future, heart attack and stroke
treatments.
Biology of the Leech
Any two leeches of the same species can mate because all have both male and
female sex organs. That is, they are "hermaphrodites," like earthworms.
In mating, two leeches entwine their bodies and deliver sperm into each other. "The
sperm," said Govedich and Bain, "are then transported to the eggs where
fertilization occurs."
The leech deposits its eggs in a gelatinous cocoon. While there is considerable
variability among species, a typical leech may attach the cocoon to submerged
plant material or to a stone surface. The cocoon is so tough that it will survive
passage through the digestive track of a waterfowl. After several weeks or months,
the eggs hatch, and the young emerge, looking like diminutive adult leeches.
Surprisingly, many leech species make excellent parents, according to Govedich
and Bain. A leech may care for its young "in a manner that resembles the
care shown by birds or even mammals." It may build a nest for its brood.
It may carry its brood attached to its under side or quartered in an internal,
marsupial-like pouch. It may even capture and kill prey for its babies until
they can provide for themselves. The leech dies after one or two reproduction
cycles.
Attack of the Blood Suckers
According to the Field Guide to Venomous and Medically
Important Invertebrates Affecting Military Operations, "When a person
enters leech-infested habitats, the leeches quickly swim toward the source of
the water disturbance." A
leech will quickly attach to the skin with one of its suckers. It explores
the body in its inchworm style until it finds its choice site for a meal, for
instance, at the toes or along the shin. It quickly perforates the skin and begins
to feed. After it releases its hold, engorged, the wound may continue to bleed,
a result of the anticoagulant, for an hour or more.
According to a horror story on the BBC Internet site, leeches in unfiltered
and untreated water drunk by Napoleons' soldiers operating in the Syrian Desert
in 1799 attached themselves to the men's noses, mouths and throats. As they filled
with blood, they caused some soldiers to die of suffocation and others of blood
loss.
Fortunately, leeches raise comparatively small risk in the Southwest. (After
all, we filter and treat our water.) While repulsive, the bites tend to be small
and inconsequential, especially if you remove the leech and treat the wound properly.
(See Removing Leeches and Treating Leech
Bites for
more information.)
