Cotylurus flabelliformis

Geographic Range

Cotylurus flabelliformis, the duck fluke, is a common parasite of North American ducks, although it can successfully reproduce inside other definitive avian hosts. The geographic range follows its hosts that are distributed widely across North America. (Campbell, 1973a; Gower, 1938; Olsen, 1974)


Both the definitive and intermediate hosts are generally aquatic species, and stages in between hosts require an aquatic habitat. Eggs are released in the feces of definitive hosts and hatch in water. Miracidia are free-swimming in water. Sporocysts develop in the digestive gland of the intermediate host. Cercariae are free-living and must leave the intermediate host and re-enter the water. Tetracotyles develop in the hermaphroditic gland of the definitive host. Adults reside in the small intestine of the definitive host. (Campbell, 1973b; Olsen, 1974)

  • Aquatic Biomes
  • lakes and ponds
  • temporary pools

Physical Description

Free-swimming miracidia hatch from eggs and possess eye-spots and temporary ciliated epitheliums that will be shed in the next developmental stage. Sporocysts are sack-like, rediae are hollow and possess a pharynx, primitive gut, and excretory system, and cercariae and tetracotyles are characterized by the formation of forked tails. (Olsen, 1974; Rothschild and Clay, 1957)

The adult body of C. flabelliformis is divided into the forebody and the hindbody, generally 0.20-0.28 mm and 0.36-0.57 mm in length, respectively. The entire adult body length is typically around 0.85 mm. Like all flukes, this species has a complex reproductive system, a relatively simple nervous system, and no body cavity. The esophagus and pharynx are almost equally long, and the excretory bladder is lost. (Olsen, 1974; Rothschild and Clay, 1957)

  • Average length
    0.85 mm
    0.03 in


Adults reside in the small intestine of ducks and lay unembryonated eggs, which are released in the duck’s feces. For the life cycle to continue, the eggs must be released in water. After about three weeks the eggs hatch into free-swimming miracidia. These swim around until they encounter their first intermediate hosts, snail species from the family Lymnaeidae. Lymnaea stagnalis and Stagnicola emarginata are common first intermediate hosts. After penetrating the snail, C. flabelliformis' next stage is marked by the miracidium’s ciliated epithelium. Asexual reproduction takes place, and shedding of its epithelium marks its transformation into a sporocyst. The sporocyst multiplies, and the resulting daughter sporocysts migrate to the snail's digestive gland, eventually developing into cercariae. (Olsen, 1974)

Approximately six weeks after the snail initially becomes infected, the free-swimming cercariae leave the snail and re-enter the water. The parasites now seek their second intermediate hosts, not precluding the same snails where the cercariae initially developed (Olsen 1974). Snail species that are good first intermediate host species may not be good second intermediate host species (e.g. L. stagnalis) (Cort and Olivier 1944). Moreover, snails may not simultaneously serve as first and second intermediate hosts for C. flabelliformis; in other words, the presence of sporocysts or developing cercariae prevents most developed cercariae from penetrating the snail (Nolf and Cort 1933, Cort et al. 1945, Anteson 1970). If the cercariae locate second intermediate hosts of preferred snail host species, then it takes six weeks for them to encyst, and they undergo this stage of development in the hermaphroditic gland (Olsen 1974, Cort et al. 1945). Otherwise, the presence of other trematodes in less-than-ideal second intermediate hosts (especially physid and planorbid snails) triggers their continued development—the cercariae become hyperparasites (parasites that parasitize other parasites) and complete their development inside sporocysts or rediae of other trematodes (Olsen 1974, Cort et al. 1945, Cort et al. 1941, Campbell 1973c, Campbell 1997). Development from cercariae to tetracotyle (a type of metacercaria characteristic of the Family Strigeidae) is actually faster if the cercariae become hyperparasites (Cort et al. 1945). (Anteson, 1970; Campbell, 1973a; Campbell, 1997; Cort and Olivier, 1944; Cort, et al., 1945; Cort, et al., 1941; Nolf and Cort, 1933; Olsen, 1974)

Ducks acquire tetracotyles through ingestion. One week later, sexual maturity is reached. Eggs are released a little while later, and the cycle is set to repeat itself. (Campbell, 1973b; Olsen, 1974)

Development varies with the intensity of the initial infection, suggesting that competition among individuals in the free-living stages of this parasite negatively impacts their development. (Campbell 1973b). In addition, the development of C. flabelliformis is affected by its specific definitive avian host (Campbell 1973a). Traits most impacted by host specificity are the total body size, distribution of the vitellaria (a group of glands that produce yolk around the eggs of invertebrates whose eggs do not contain yolk), and the size and position of the gonads (Campbell 1973a). Development is further impacted by temperature—cooler temperatures tend to slow down metabolic processes (Campbell 1973b). (Campbell, 1973b; Campbell, 1973c)


Adults are hermaphrodites and self-fertilize. Other stages in the life-cycle are asexual. (Olsen, 1974; Rothschild and Clay, 1957)

After Cotylurus flabelliformis penetrates a snail as a miricidium, asexual reproduction takes place and it becomes a sporocyst. A sporocyst multiplies into daughter sporocysts. Sexual reproduction takes place after the tetracotyle stage develops into an egg laying hermaphroditic adult, which can be within 48 hours of its penetration into a mallard duck. The development of this parasite depends on temperature and migratory patterns of their avian hosts, and rates of infection follow a bimodal distribution, so their reproductive cycle is likely dependent on these factors. (Campbell, 1973b; Olsen, 1974)

There is no parental investment in this species.

  • Parental Investment
  • no parental involvement


Each stage of C. flabelliformis' life cycle is short-lived. In experimental conditions, tetracotyles in mallard ducks developed to egg-laying stages in forty-eight hours. The small size of the fluke is thought to be part of the reason it matures so quickly in its avian hosts. Adults were passed in the feces within 7-10 days. (Campbell, 1973b)


Cotylurus flabelliformis parasitizes snails (intermediate hosts) and numerous avian species (definitive hosts). Tetracotyles can also be hyperparasites of other trematode species already present inside the snail host. (Campbell, 1973c; Campbell, 1997; Cort, et al., 1945; Cort, et al., 1941; Olsen, 1974)

C. flabelliformis becomes dormant when temperatures fall far enough, so their activity, and, subsequently, their rates of infection, follow a bimodal distribution that also follow the hibernation of their snail intermediate hosts and the migratory patterns of their avian definitive hosts. (Campbell, 1973c)

Cercariae are free-living and very fast swimmers, typically finding snail hosts within five hours. After this time, their activity decreases significantly, and they eventually die (Campbell 1973a). Cercariae that are 0-3 hours old tend to swim longest and farthest (Campbell 1997). If cercariae have not yet detected snail intermediate hosts in their environment, they tend to float near the surface of the water and are pushed along by wind currents (Campbell 1997). If they approach snails that are already carrying developing cercariae, they swim along the snail’s surface with a “looping movement” before turning around and swimming away (Cort et al. 1945). (Campbell, 1973b; Campbell, 1997; Cort, et al., 1945)

Communication and Perception

The current literature on communication and perception addresses the cercariae. C. flabelliformis cercariae are very good at locating their snail intermediate hosts. In fact, they have been shown to locate hosts at distances up to 1.2 m away (Campbell 1997). In the same experiment, cercariae moved little in the absence of snail hosts, and they had little response to the presence of inanimate objects. However, they responded immediately, and rapidly approached, their preferred intermediate hosts, L. stagnalis (Campbell 1997). This suggests that C. flabelliformis is capable of sensing its environment by chemotaxis, but the mechanism behind this has not been resolved (Campbell 1997). Moreover, snails likely swim in the vicinity of potential hosts by accident before they respond to their presence (Campbell 1997). If a snail host is already infected with cercariae, it is possible for the chemotactic factor in the snail to be blocked in such a way that other cercariae are not attracted to these snails and do not try to penetrate them (Anteson 1970). (Anteson, 1970; Campbell, 1997)

Cotylurus flabelliformis has demonstrated sensitivity to light (Campbell 1973c). Artificial light can induce cercarial development, but darkness prevents it. Importantly, temperature still trumps light, and lower temperatures will tend to decrease emergence even when enough light is present (Campbell 1997). (Campbell, 1973a; Campbell, 1997)

Food Habits

Since a digestive system is absent in C. flabelliformis, it feeds by absorbing blood, lymph and possibly even cells of the mucous membrane lining of the small intestine (Rothschild and Clay 1957). The position of the tetracotyles in the hermaphroditic gland allows them to feed on these tissues also (Cort et al. 1945). If the tetracotyles are acting as hyperparasites, then they steal nutrients from their parasite hosts (Cort et al. 1945). (Cort, et al., 1945; Rothschild and Clay, 1957)

  • Animal Foods
  • blood
  • body fluids


There is no information available on the predation of C. flabelliformis during any of its developmental stages.

Ecosystem Roles

Although C. flabelliformis primarily parasitizes ducks, it is able to infect a large number of avian hosts. Intermediate hosts include snails in the family Lymnaeidae. Snails in the family Planorbidae may also serve as intermediate hosts (although see above discussion for viability). Individuals in the free-living stages compete with one another to find hosts. This reduces the ability of some individuals to find hosts.

It is unknown whether any groups specifically prey on any of this parasite's life stages. (Campbell, 1973c; Campbell, 1973a)

Economic Importance for Humans: Positive

Their importance to humans is neutral.

Economic Importance for Humans: Negative

Their importance to humans is neutral.

Conservation Status

Currently, no conservation efforts are underway.

Other Comments

Larger snails generally carry the most infections, but size matters with respect to the size of the snails ducks can actually ingest; the life cycle is more likely to reach a dead-end if the snails are too large for the ducks to ingest. (Campbell, 1973b; Campbell, 1973c)


Rodica Kocur (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in landscapes dominated by human agriculture.


reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate


a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body


marshes are wetland areas often dominated by grasses and reeds.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

polarized light

light waves that are oriented in particular direction. For example, light reflected off of water has waves vibrating horizontally. Some animals, such as bees, can detect which way light is polarized and use that information. People cannot, unless they use special equipment.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


living in residential areas on the outskirts of large cities or towns.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Anteson, R. 1970. On resistance of the snail, Lymnaea catascopium pallida (Adams), to concurrent infection with sporocysts of the strigeid trematodes, Cotylurus flabelliformis (Faust) and Diplostomum flexicaudum (Cort and Brooks). Annals of Tropical Medicine and Parasitology, 64: 101-107.

Bortone, S. 2005. Estuarine indicators. Boca Raton, Florida: CRC Press.

Campbell, R. 1997. Host-finding behavior of Cotylurus flabelliformis (Trematoda: Strigeidae) cercariae for snail hosts. Folia Parasitologica, 44: 199-204.

Campbell, R. 1973. Influence of temperature, host, and host size on metacercarial development of Cotylurus flabelliformis (Trematoda: Strigeidae). Transactions of the American Microscopial Society, 92: 662-665.

Campbell, R. 1973. Studies on biology of the life cycle of Cotylurus flabelliformis (Trematoda: Strigeidae). Transactions of the American Microscopial Society, 4: 629-640.

Campbell, R. 1973. Studies on host-specificity and develpoment of adult strigeid trematode Cotylurus flabelliformis. Transactions of the American Microscopial Society, 92: 256-265.

Cort, W., S. Brackett, L. Olivier. 1945. Influence of larval trematode infections in snails on their second intermediate host relations to the strigeid trematode, Cotylurus flabelliformis (Faust 1917). Journal of Parasitology, 31: 61-78.

Cort, W., L. Olivier. 1944. Lymnaeid snails as second intermediate hosts of the strigeid trematode, Cotylurus flabelliformis (Faust 1917). Journal of Parasitology, 30: 309-321.

Cort, W., L. Olivier, S. Brackett. 1941. The relation of physid and planorbid snails to the life cycle of the strigeid trematode, Cotylurus flabelliformis (Faust, 1917). Journal of Parasitology, 27: 437-448.

Gower, W. 1938. Studies on the trematode parasites of ducks in Michigan with special reference to the mallard. Michigan State Agricultural Experimental Station Memoir, 3: 1-94.

Nolf, L., W. Cort. 1933. On immunity reactions of snails to the penetration of the cercariae of the strigeid trematode, Cotylurus flabelliformis (Faust). Journal of Parasitology, 20: 38-48.

Olsen, O. 1974. Animal Parasites: Their Life Cycles and Ecology. Baltimore, MD: University Park Press.

Rothschild, M., T. Clay. 1957. Fleas, Flukes, and Cuckoos: A Study of Bird Parasites. NY: The Macmillan Co.

Thomas, F., F. Renaud, J. Guegan. 2005. Parasitism and Ecosystems. NY: Oxford University Press Inc.

Ulmer, M. 1957. Notes on the development of Cotylurus flabelliformis tetracotyles in the second intermediate host (Trematoda: Strigeidae). Transactions of the American Microscopial Society, 76: 321-327.