Leeches are widely known for their reputation as blood-feeding parasites. While a significant portion of the nearly 700 species are hematophagous, meaning they feed exclusively on blood, this diet is not universal. Many leech species have evolved to be predators, scavengers, or specialized fluid feeders. The diversity in their feeding habits is reflected in the specialized anatomy and biochemistry each group uses to secure and process its meals.
The Specialized Diet of Blood-Feeding Leeches
The leeches known for hematophagy have highly specialized tools for consuming blood, which is a nutritionally dense but challenging food source. These parasitic species target a wide array of hosts, including mammals, fish, amphibians, reptiles, and birds.
Once a host is found, the leech anchors itself using its anterior and posterior suckers. The well-known medicinal leech, Hirudo medicinalis, uses three sharp, chitinous jaws to saw a characteristic Y-shaped incision into the host’s skin. Other blood-feeding species use a sharp, protrusible proboscis to pierce the skin instead of jaws.
To ensure a continuous flow of blood, the leech injects compounds into the wound. The most famous of these is hirudin, a powerful anticoagulant that prevents the host’s blood from clotting. This allows the leech to pump blood into its body for up to a couple of hours until it is fully engorged. A single blood meal can be massive, with some species consuming up to five times their own body weight.
Predatory and Scavenging Species
Approximately one-quarter of all known leech species are ambush predators or scavengers. These non-hematophagous leeches play a different role in their ecosystems, predominantly feeding on invertebrates. Their prey includes small animals like insect larvae, aquatic worms, snails, and slugs.
Predatory leeches often use a different feeding mechanism than their blood-feeding relatives. Some species, like members of the family Erpobdellidae, possess a large, toothless mouth that allows them to swallow small prey whole. Other predatory leeches use a protrusible proboscis to spear their prey, sucking out the internal contents or soft tissues.
These predators do not rely on the same infrequent, large meals as blood-feeders, often eating more frequently to sustain themselves. Some leeches are also detritivores, meaning they feed on organic debris and decaying matter. This further diversifies their dietary role in aquatic environments.
How Leeches Consume and Digest Their Meals
Regardless of the meal type, the leech’s digestive system is uniquely adapted for storing and slowly processing large amounts of food. The food is passed into the crop, which is the largest compartment of the digestive tract and functions as a specialized storage chamber. The crop is compartmentalized with paired side pouches, called ceca, which expand significantly to accommodate the meal.
For a blood meal, the leech concentrates the blood by removing water and osmolytes from the crop contents. This concentrated blood can be stored for a long time, with some medicinal leeches able to survive for six months to over a year between feedings. This extended period is possible because leeches lack many typical digestive enzymes, relying instead on symbiotic bacteria.
The crop hosts a community of symbiotic bacteria, notably Aeromonas species, which help break down the blood meal and prevent it from decaying. These microbes play a role in the slow digestion process, which primarily occurs in the intestinum after the blood has been stored. This co-dependent relationship allows the leech to maximize the nutritional value of its infrequent, massive meals.