Dark Banquet (17 page)

Leeches belong to the phylum Annelida (the segmented worms). The group contains around twelve thousand species and has a worldwide distribution. In addition to leeches (which belong to the class Hirudinea), annelids also include the earthworms (class Oligochaeta) and their freshwater relatives, and as well as marine wigglers like sandworms and bloodworms (class Polychaeta).

Numbering around 650 species,
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leeches can be found in cool freshwater environments or stagnant tropical mud wallows. About 20 percent of leech species are marine and their habitats range from shallow coastal waters to thermal vents located twenty-five hundred meters below the surface. Other leeches are completely terrestrial and they too exist in a diverse array of habitats: from tropical rain forests to the sub-Himalayan hills of northern India. There's even an unpigmented species that inhabits a single cave in New Guinea where it feeds on the blood of bats. Although leeches are most famous (or infamous) for their bloodsucking ability, many species are predatory rather than parasitic. A few species even provide some benefit to their rather oblivious hosts.

Annelids range in size from less than a millimeter in length to over three meters long, in the case of a giant Australian earthworm. They're called segmented worms because their bodies are composed of ring-shaped segments (annuli) stacked one upon another (kind of like the Michelin Man—but with no arms or legs and a bit more slime).

From an evolutionary perspective, the adaptive advantage of segmentation (also referred to as metamerism) appears to be related to the fact that annelid bodies are literally divided into a series of more or less independent sections (walled off from each other by thin septa). In the distant past, these annuli may have served as an early framework for regional specialization of the animal body into a head, midsection, and tail end.

Apparently, segmentation also allowed annelids to move more efficiently than their nonsegmented ancestors. This is because along with a metameric body plan, another adaptation evolved in this group—a body cavity known as a coelom. This fluid-filled chamber is part of the worm's hydrostatic skeleton, so that when muscles that encircle each body segment contract, the compressed fluid within the coelom is forced toward the head end, projecting the front of the body forward. You can demonstrate this type of locomotion for yourself by squeezing one end of an elongated, water-filled balloon around the middle. Your hand represents the annelid's circular body muscles, while the water and the expandable balloon interior act as the coelomic fluid and the coelom, respectively. In earthworms, as the body extends forward, the worm secures itself to the substrate with ventrally located pairs of microscopic, toothlike setae (or chaetae). The back end of the body is then pulled forward as longitudinal muscles that run down the length of its body are contracted. The vermiform crawling that results from this anatomical arrangement is quite a bit more efficient than the wild whipping and thrashing movements that characterize other worms like nematodes (commonly known as roundworms).

Leeches, however, are capable of employing an alternative form of locomotion, and in these instances they resemble inchworms (which are insect larvae, not real worms). This type of locomotion is possible because of the presence of a pair of sucker disks—one located near the head end (the anterior sucker) and the other near the tip of the tail (the caudal sucker).
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During inchworm crawling, the leech attaches its caudal sucker to the substrate (which may be oriented horizontally or vertically). Next, the muscles encircling the body contract, extending the anterior end forward (as in vermiform locomotion). The anterior sucker then takes hold of the substrate and the caudal sucker releases its grip. Finally, the tail end of the body swings forward, planting the caudal sucker directly behind the anterior one. Inchworm crawling can be employed underwater or as aquatic leeches leave the water to lay their eggs. It provides an additional advantage by allowing the leech to move efficiently across vertical or smooth wet surfaces. Terrestrial leeches also use inchworm crawling to zero in on their warm-blooded targets. Potential meals can be tracked from a distance of about two meters away. This is done primarily through detection of vibrations produced as the prey (or host) moves through its environment as well as the carbon dioxide it exhales. Vision is also employed as photoreceptors provide the leech with information on changes in light intensity (e.g., from shadows passing nearby).

Many other leeches are quite adept at swimming, but unlike fish, which move by undulating from side to side, leech bodies bend with an up-and-down motion that is reminiscent of dolphins and whales. Leonardo Da Vinci (1452–1519) may have been the first to study leech locomotion and he actually spent quite a bit of time getting the intricacies of their dorsoventral undulations just right.

Unlike the parasitic torpedo attack launched against Bogart's character in the 1951 classic
The African Queen
(those “leeches” were actually made out of rubber), there are aquatic leeches that gain access to their hosts via different routes. Some hop aboard as their victims dip their heads for a drink. Entering through the nostrils, the leeches attach themselves to the mucous membranes that line their host's nasal cavity. There, in the warm, humid chamber, they feed and mature, safe from detection—at least for a while.

One famous story recounts how this type of leech attack afflicted a group of Napoleonic soldiers, crossing from Egypt to Syria in 1799. As with any army in a foreign land, obtaining water was always a major concern and things were even dicier in the days before purification techniques (like boiling or adding iodine or chlorine) could make most water at least semisafe to drink. Apparently, some of these men drank water from a lake infested with tiny larval leeches. Unbeknownst to their hosts, the creatures quickly attached themselves and began to feed. Days later the men began to take ill and medical personnel were horrified to find their patients' noses, mouths, and throats carpeted by blood-engorged leeches. The doctors tore frantically at the vampires and it's not hard to envision a gory scene made all the more horrible by the cries of fearful, frantic men.

As in other similarly large groups of animals, there is a wide degree of variation in leech diets. About three-fourths of all known species are bloodsucking parasites, feeding primarily on the blood of vertebrates (including sharks, bony fishes, frogs, turtles, snakes, crocodiles, birds, and mammals). Parasitic leeches do not generally specialize on any one particular prey. For example, the medicinal leech
Hirudo medicinalis
usually feeds on frogs, but it will readily take human blood. And while we're on the topic, it should also be noted that dietary relationships between leeches and other organisms aren't all stacked in the leech's favor. Leeches are commonly fed upon by fish, birds, salamanders, snakes, and even other leeches.

According to Dr. Mark Siddall, a leech expert and curator at the American Museum of Natural History, the first leeches were freshwater blood feeders, but alternative feeding modes have evolved at least six times in various leech groups. Several of these lineages have become predators, feeding solely on invertebrates (like their earthworm cousins, snails, or even other leeches). Unlike parasitic leeches, which can survive for extended periods of time without feeding, predaceous leeches feed frequently (generally, every one to three days). Another difference is that the predators digest their food rapidly, while their parasitic relatives are able to hold blood within their guts in an undigested state for up to several months. Predatory species are also equipped with a highly mobile, hoselike proboscis that is first employed to probe the potential prey. Once this tactile reconnaissance is complete, the leech inserts the proboscis into the flesh of its soon-to-be meal and proceeds to suction feed for up to several hours—vacuuming out virtually all of the prey's soft-bodied interior. Often, the predator is joined by others of its kind, and the scene comes to resemble a Chinese buffet table attacked by senior citizens. In each instance, the ravenous feeding frenzy leaves behind only carnage.

Some leeches are neither parasites nor predators. Among these are several members of the family Branchiobdellidae. These species are remarkable for the manner in which they partition their highly specific habitat—the shells of the freshwater crayfish. Although the hard outer covering of these minilobsters might seem an unlikely place for leeches to hang out, there can be up to seven species living on a single individual. In what stands as an extreme example of microhabitat partitioning, different leech species inhabit different regions of the crayfish's body. For example, one type of leech lives attached to the crayfish's antennae, while another lives out its life adhering to the pincer-tipped legs (the chelipeds). Unlike parasitic leeches that feed on the blood of their hosts, the behavior of these branchiobdellids is more comparable to the previously mentioned endosymbiotic bacteria living in the guts of mammals like cows. Although not as important as bacterial endosymbionts are to their hosts, these
ecto
symbionts provide a service to the crayfish by grazing on the micofauna (algae, diatoms, and bacteria) that attach to its body.

This leech/crayfish association is not always positive for the host since some branchiobdellids don't provide the aforementioned cleaning services. A few species cause no harm—they're merely opportunists, consuming scraps of organic matter scattered about by the messy crayfish as it shreds its food into bitesized bits. Another branchiobdellid, however,
is
parasitic. It lives within the crayfish's gill chamber where it feeds on the gill filaments and blood of the same freshwater crayfish whose bodies are so precisely partitioned by its nonparasitic cousins. Even worse, however, are the instances in which crayfish are infested by lethally high numbers of leeches, which can sometimes cover their bodies like living carpets, ultimately killing them.
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