Feeding habits of Hirudinea
by Andrew Shuster
BIOL/WATER 361, Fall 2013
Despite common beliefs, not all leeches (Hirudinea) will prey on anything; some leeches have specific prey that they will actively seek out to feed upon. In many areas, it is believed that leeches wait to attach and drink the blood of the first thing with a pulse that comes by them. This is true for some species but not all. Some will feed upon a specific group of species, or even only certain life stages of those groups. All leeches cannot be considered generalist feeders. Things are much more complicated and interesting in reality.
Hirudinea, or leeches, is a subclass of the Class Clitellata, which falls in the Annelida phylum. Annelida comprises all of the segmented worms. Hirudinea is unique in their taxon due to their feeding habits. They use modified jaws that form suckers to attach to prey. They then feed upon the blood of the individual they have attached to. Many species secrete various anticoagulants to prevent the blood from clotting, keeping the wound open longer. This allows them to gorge on blood longer. They also will secrete anesthetics to keep the prey from noticing they have attached. This also lengthens the time that they are attached and feeding. It is because of these two adaptations that leeches are used medically by humans. While these are adaptations that many of the species of leeches have, there are certain species that have taken specialization to an even further level.
Although some of the beliefs about leech feeding are true, they typically are more complicated than thought. It is true that some species of leech drink the blood of their prey, but it is often from specific taxa of animals. An example of this is Placobdella burresonae, a species from Oregon. This species has only been seen to be feeding on the blood from amphibians. Since its description in 2006, it has been found to feed primarily on the Oregon spotted frog, Rana pretiosa (Siddall & Bowerman, 2006). In the wild, it has also been found on the Western toad, Bufo boreas. In a laboratory setting they were also seen to be feeding on 5 other amphibian species that can be found in Oregon. This shows that the leeches are not only feeding on any species it can find, but specific a group of species that are taxonomically related.
Another misconception about leeches is that all of them will only suck the blood from organisms. This idea, like the one that all leeches are generalists, is not true. There are species that will only hunt and eat small freshwater invertebrates (Beracko & Kosel, 2011). In a study of the European leech, Dina punctata, the researchers found nothing but these small organisms in the guts of leeches. Of the 527 individuals dissected, none had blood from a larger animal in them. They were found to have primarily the insect larvae of the family Chironomidae and gammarid amphipods. The species Dina punctata is just one example of a leech species that does not feed on the blood of its prey. They and the other species like them show that leeches are not all that people perceive them to be.
There is another species of leech that takes eating only freshwater invertebrates to any even further level of specialization than Dina punctate In Montezuma Well, a 1.8 acre (0.76 hectare) limestone sinkhole located in Montezuma Castle Nation Monument, is the leech Erpobdella montezuma (Blinn et al., 1987). This species is endemic to this one watering hole. It only has one food source, the amphipod Hyalella montezuma. This tiny arthropod is also only found in this sinkhole. In a study of Erpobdella montezuma, Hyalella montezuma dominated the prey that was found in the stomachs of the leeches, despite there being an abundant number of other potential invertebrate species present in the pool. They made up about 90%, the remaining 10% were of multiple different species. This is a strong suggestion that Erpobdella montezuma were heavily choosing Hyalella montezuma over the other prey present in Montezuma Well.
Another remarkable thing about these leeches is the fact they are open water hunters. Every night the leeches will leave their burrows at the bottom of the sinkhole and enter the water column to feed. During the day, they can only be found in the substrate of the hole, never in the water column. The researches hypothesized that this was done because at night and in open water was when the leech’s prey was most abundant. It is believed that these two species have evolved together with little predation or competition for over 10,000 years. This has allowed the leeches to become specialist hunters of the amphipods. Although not everything is known about this species and how it interacts with its prey, it is undoubtedly a unique interaction that is more specific than what people would expect of a leech.
One of the most interesting examples of a leech species that actively seeks out specific prey is Cystobranchus virginicus. This species of leech, native to streams along the east coast of the United States, are the only reported Hirudinea to have been found to feed upon fish eggs (Light et al., 2005). There are other species of leech that have been reported to feed on the eggs of amphibians, but Cystobranchus virginicus is the only one that feed on fish eggs. Not only do they feed on fish eggs but they have only been found to feed on the eggs of four fish species. The leeches will go into the nests made by the fish and gorge upon the eggs. Until recently, it was unknown to what degree the Cystobranchus virginicus was seeking out the eggs.
This is what Light et. al. (2005) were looking at in their study. One of the hypotheses they tested was whether the leeches were feeding upon only the eggs or the adults as well. While doing their field study they observed no adult fish with leeches feeding upon them. In the same area they found full leeches. Upon looking at the stomach contents, they found only undigested egg material. With this evidence they were able to reject the null hypothesis that Cystobranchus virginicus are opportunistically feeding upon eggs and adults. This study supported the idea the leeches were actively searching for eggs of these fish species.
Another interesting case of leeches displaying prey selection is the species Helobdella conifera. Its main food source is snails. Again this is not an uncommon source of food across the subclass, but it is the selection of prey that fits into distict age classes that makes it go against the common belief. Helobdella conifera will only feed on young, small snails (Davies & McLoughlin.1997). Once the snails get to a certain size and age, the leeches will stop feeding on them. In an experiment done by Davies & Mcloughlin (1997), they found that the leeches would not feed on the larger snails even if they had not been fed for 60 days. Several of their test subjects even died of starvation while the large snails were present for them to feed on. They also found that the leeches were willing to attach and suck the blood of freshly dead snails of all sizes, but only if they had not been feed. The researchers found that the Helobdella conifera preferred to prey on the young and were willing to scavenge dead snails if need be, but would completely ignore the large ones. This selection of age groups demonstrates that these leeches have a preferred and specific prey that they will consume.
While many species have specific prey that they will consume, even specific populations can have unique feeding habits. In a commercial medical leech, Hirudo medicinalis, farm in Germany there have been reportes that its leeches cannibalize each other (Kutschera et al., 2005). Certain individuals will feed from the blood supplied to them. Then seven or eight other leeches would wait and then feed off of those who were full of blood. The victim leeches would be killed in the process. This is the only case of cannibalism in the Hirudinea and even the phylum Annelida. Even within the species Hirudo medicinalis, this intraspecies interaction has not been widely reported. It is only known to happen in a few commercial leech farms. The researchers were not sure if the cannibalism was due to the fact the leeches were in captivity or if it occurs in the wild. This interaction demonstrates that not all leech populations are the same. Even groups of the same species can have very different feeding habits.
Another nail in the coffin of leeches being passive predators was the research done by Simon & Barnes (1996). They looked at a species of leech, Harmopis marmorata, to see if it uses olfaction to search for its prey or not. After conducting their experiments, they found that they had the evidence to support the hypothesis that the leeches do indeed use smell to find earthworms. This discovery is completely against the common idea about leech being purely ambush hunters. This species has evolved a way to find its preferred food source. The use of using olfaction to locate prey supports the idea that some species of leech are active predators.
One of the main reasons for the ignorance of leech ecology is the lack of education and research. Humans have an unfounded fear of the creepy and crawly things. As a whole, Hirudinea’s connotation has never moved beyond blood sucking leeches attaching to a swimmer’s leg. It is this fundamental fear that has caused people to ignore the fascinating interactions. Fear is not the only reason for the poor connotation. The species of leech that has been widely researched is the medicinal leech. This species fits the stereotypical leech description. It is a generalist blood sucker, which will feed on whatever it finds. There are very few papers looking into other, more interesting species as in depth. With more research, the idea of the blood sucking leech being in every freshwater, will disappear.
Hirudinea is truly a misunderstood taxon. There are many examples of species that go against the stereotype. They are have become pigeonholed into the role of blood sucking worm things. In reality, they are much more than that. They are an incredibly diverse and interesting group. With more research, more examples of leech species that break the mold will be found. We just need to get past the fear and misunderstandings. When this is done, leeches will be appreciated for their diversity and ability to hunt.
- Beracko, P. & V Kosel. 2011. Life cycle and feeding habits of Dina punctata Johansson, 1927 (Erpobdellidae, Hirudinea) in a small carpathian stream. Internation Review of Hydrobiology 96:39-47.
- Blinn, D.W., R.W. Davies & B. Dehdashti. 1987. Specialized pelagic feeding by Erpobdella montezuma (Hirudinea). Holarctic Ecology 10:235-240.
- Davies, R.W. & N.J. McLoughlin.1997. The life-cycle and feeding of the African freshwater leech Helobdella conifera (Glossiphoniidae). South African Journal of Zoology 32:1-4.
- Kutschera, U. & M. Roth. 2005. Cannibalism in a Population of the Medicinal Leech (Hirudo medicinali L.). Biology Bulletin 32:626-628.
- Light, J.E., A.C. Fiumera & B.A. Porter. 2005. Egg-feeding in the freshwater piscicolid leech Cystobranchus virginicus (Annelida, Hirudinea). Invertebrate Biology 1:50-56.
- Siddall, M.E. & J. Bowerman. 2006. A new species of glossiphonid leech from Rana pretiosa (Amphibia: Ranidae) in Oregon. Journal of Parasitology 92:855-857.
- Simon, T.W. & K. Barnes. 1996. Olfaction and prey search in the carnivorous leech Haemopis marmorata. The Journal of Expairimental Biology. 199:2041-2051.