Schistocephalus solidus

Last updated:

Geographic Range

Schistocephalus solidus is isolated in areas of western and eastern North America, including Alaska and provinces of Canada, Europe, and Eurasia. These areas are the ranges of its second intermediate host, Gasterosteus aculeatus (Poulin et al. 2011). This cestode is known to infect over 40 different types of piscivorous birds which are definitive hosts (Nishimura, et al. 2011), so it is likely very widespread on both of these continents. Schistocephalus solidus can also infect mammals, such as hamsters and mice, but this is rarely observed outside of a laboratory (McCaig and Hopkins, 1962). (McCaig and Hopkins, 1962; Nishimura, et al., 2011; Poulin, et al., 2011)

Habitat

Since Schistocephalus solidus is a parasite with a complex life cycle with three separate hosts, its habitat depends on the host's habitat. The adult worm develops in the intestine of its definitive bird host, where it produces eggs (Nishimura et al. 2011). In the environment, coracidia can take anywhere between 22-29 days to emerge from their eggs to infect a host. After infecting their copepod host, they live in the gut and develop, until eaten by the second intermediate host, a fish (Clarke, 1953). This parasite develops in the coelom until finally ingested by the definitive bird host, completing the cycle (Nishimura et al. 2011). Schistocephalus solidus is very adaptive, and can use approximately 40 different aquatic birds as its definitive host (Heins et al. 2002). (Clarke, 1954; Heins, et al., 2002; Nishimura, et al., 2011)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • coastal

Physical Description

As a cestode with a complex life cycle, S. solidus has an anatomy that changes between hosts. After emerging from the egg, it is a free swimming coracidia, with many cilia. In the first intermediate host, S. solidus changes morphology, becoming the procercoid form, becoming elongated, and losing its transparency. This cestode also develops posterior hooks and a posterior ‘bulb’, referred to as a cercomer (Clarke, 1953). In the second intermediate host, it sheds the cercomer, and produces 60-80 proglottids (Smyth, 1946). At this stage it is a plerocercoid. When ingested by the final host, the parasite grows, becoming the reproductive, egg-producing form (Clarke, 1953). (Clarke, 1954; Smyth, 1946)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    1 to 10 mm
    0.04 to 0.39 in

Development

Schistocephalus solidus eggs emerge as a free swimming ciliated coracidia. After being ingested by a copepod, its first intermediate host, it changes to procercoid form, becoming elongated, and losing its transparency. Posterior hooks and a posterior ‘bulb’, referred to as a cercomer, also develop (Clarke, 1953). When ingested by the second intermediate host, S. solidus sheds the cercomer, and develops an excretory system, as well as 60-80 proglottids (Smyth, 1946). This is known as the plerocercoid form. In its final host, the parasite undergoes additional growth, and becomes the reproductive, egg-producing adult(Clarke, 1953). (Clarke, 1954; Smyth, 1946)

Reproduction

Schistocephalus solidus reproduces both sexually and asexually (Clarke, 1953). As a hermaphrodite, it can cross fertilize or self-fertilize. Individuals of S. solidus may prefer to mate with relatively larger individuals. Adults were observed self-fertilizing when a relatively smaller mate was available (Lüscher and Widekind, 2002). (Clarke, 1954; Lüscher and Wedekind, 2002)

Schistocephalus solidus has both sexual and asexual reproduction. Adults can produce over 20,000 eggs in a lifetime (Clarke, 1954). Despite the freedom and convenience of reproducing asexually, S. solidus delays egg production to wait for a mate (Schjørring, 2004). This may be a mechanism of natural selection, as individuals who self-fertilize have lower overall egg production (Schjørring, 2004), as well as lower success in the second intermediate host (Christen and Milinski, 2003). Individuals of S. solidus may prefer to mate with relatively larger individuals, and avoid mating with relatively smaller individuals. In some cases, adults actually self-fertilized even when a relatively smaller mate was available (Lüscher and Widekind, 2002). (Christen and Milinski, 2003; Clarke, 1954; Lüscher and Wedekind, 2002; Schjørring, 2004)

  • Range number of offspring
    20,000 (high)

This species has no parental role after it lays its eggs.

Lifespan/Longevity

The lifespan of S. solidus depends on the time it takes to complete each stage of the cycle. Eggs can take anywhere between 22-29 days to hatch. A coracidium has 24-48 hours to find a suitable host. Once in the first intermediate host, it develops over 3-4 weeks before it infects its second intermediate host (Clarke, 1953). In the second intermediate host, the plerocercoid form grows significantly over approximately 17 days (Sharsack et al. 2007). After being consumed by the definitive host, the adult form needs 3-4 days before producing its eggs and being excreted (Smyth, 1946). The longevity of the adult form depends on the host. In a laboratory setting, S. solidus survived 6 days in rats, 10 days in pigeons, 14 days in ducks, and 18 days in hamsters (McCaig and Hopkins, 1962). (Clarke, 1954; McCaig and Hopkins, 1962; Sharsack, et al., 2007; Smyth, 1946)

Behavior

In a natural environment, infection intensity seems to be controlled by a ‘crowding effect’; no more than four Schistocephalus solidus plerocercoids have been found in a single host (Lobue and Bell, 1993). However, the prevalence of infection can be highly variable. There have been reports with 85%-95% prevalence in a given population, to as low as 1 per 3730 individuals in another (Lobue and Bell, 1993).

Schistocephalus solidus manipulates the behavior of its fish host, making it more vulnerable to predation to piscivorous birds. This makes the parasite more likely to reach its next host (Franz and Kurtz, 2002; LoBue and Bell, 1993). (Franz and Kurtz, 2002; Lobue and Bell, 1993; Sharsack, et al., 2007)

Communication and Perception

Cestodes in general have a nerve ring in the scolex that has ganglion (nerve cells). There are tactile receptors associated with attachment (Brusca and Brusca, 2003).

Multiple Schistocephalus solidus individuals are capable of infecting a single host. In the case of a multiple infection, S. solidus individuals grow in proportion to total intraspecific density, resulting in smaller overall size for all individuals than if it were a single infection (Michaud et al. 2002). (Brusca and Brusca, 2003; Michaud, et al., 2002)

Food Habits

As cestodes that infect the intestinal tract of their hosts, S. solidus shares the diet of the organism where it resides (Clarke, 1953). Since it resides in the gut, it requires at least semi-anaerobic conditions for respiration (Barrett, 1984). Schistocephalus solidus is known to metabolize glycogen within its host (Hopkins, 1950). (Barrett, 1984; Clarke, 1954; Hopkins, 1950)

  • Animal Foods
  • body fluids

Predation

There are no known predators of Schistocephalus solidus. This species is only in the open environment during the egg and coracidium phase, in which it is waiting to be consumed by a member of the Cyclops genera (Clarke, 1954). (Clarke, 1954)

Ecosystem Roles

Schistocephalus solidus manipulates the behavior of its host. In "host manipulation", a parasite produces either behavioral or phenotypic changes in the host to increase the likelihood of its transmission. Infected copepods increased overall feeding activity, but a decrease in ability to escape predation; studies examined other variables and found no significant decrease of predation, suggesting host manipulation (Franz and Kurtz, 2002). In the common second host, the three-spined stickleback Gasterosterus aculeatus, the immune system shows no upregulation during an infection. The system showed brief recognition during two periods of growth, which is evidence for an anti-inflammatory response, or possibly antigenic variation (Sharsack et al. 2007). In another case, S. solidus induced multiple phenotypic variations (white body color, black eyes), and an increased likelihood to swim near the surface. Both changes greatly increase the sticklebacks likelihood of being consumed by a piscivorous bird (LoBue and Bell, 1993).

Recent work has suggested an ecosystem rich with parasites is generally a healthy one. Parasites successfully transported through the different stages of the life-cycle, suggests the ecosystem is healthy on multiple trophic levels (Nishimura et al. 2011). For example, if a definitive or intermediate host were to disappear completely from an ecosystem, a parasite would be unable to complete its life cycle. This would liberate any species in the ecosystem that were previously parasitized (Hudson, et al. 2006). If G. aculeatus was not parasitized, there would be lower rates of demelanization and behaviors leading to increased predation. This may lead to piscivorous birds having a harder time finding prey. (Franz and Kurtz, 2002; Hudson, et al., 2006; Lobue and Bell, 1993; Nishimura, et al., 2011; Sharsack, et al., 2007)

Species Used as Host

Economic Importance for Humans: Positive

There is no known economic importance of this species for humans.

Economic Importance for Humans: Negative

There is no known negative economic impact of this species for humans.

Conservation Status

This species has no conservation status.

Contributors

Drew Dietrich (author), Radford University, Renee Mulcrone (editor), Special Projects.

Glossary

Nearctic

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

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

asexual

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

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

heterothermic

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

metamorphosis

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.

motile

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.

oviparous

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

parasite

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

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

semelparous

offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.

sexual

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

tactile

uses touch to communicate

year-round breeding

breeding takes place throughout the year

References

Barrett, J. 1984. The anaerobic end products of helminths. Parasitology, 88: 179-198.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Christen, M., M. Milinski. 2003. The consequences of self-fertilization and outcrossing of the cestode Schistocephalus solidus in its second intermediate host. Parasitology, 126: 369-378.

Clarke, A. 1954. Studies on the life cycle of the pseudophyllidean cestode Schistocephalus solidus. Journal of Zoology, 124/2: 257-302.

Franz, K., J. Kurtz. 2002. Altered host behaviour: manipulation or energy depletion in tapeworm-infected copepods. Parasitology, 125: 187-196.

Heins, D., J. Baker, H. Martin. 2002. The “crowding effect” in the cestode Schistocephalus solidus: Density-dependent effects on plerocercoid size and infectivity. Journal of Parasitology, 88/2: 302-307.

Hopkins, C. 1950. Studies on cestode metabolism. I. Glycogen metabolism in Schistocephalus solidus in vivo. The Journal of Parasitology, 36/4: 384-390.

Hudson, P., A. Dobson, K. Lafferty. 2006. Is a healthy system one that is rich in parasites?. Trends in Ecology & Evolution, 21/7: 381-385.

Lobue, C., M. Bell. 1993. Phenotypic manipulation by the cestode parasite Schistocephalus solidus of its intermediate host, Gasterosteus aculeatus, the threespine stickleback. The American Naturalist, 142/4: 725-735.

Lüscher, A., C. Wedekind. 2002. Size-dependent discrimination of mating partners in the simultaneous hermaphroditic cestode Schistocephalus solidus. Behavioral Ecology, 13/2: 254-259.

McCaig, L., C. Hopkins. 1962. Studies on Schistocephalus solidus. II. Establishment and longevity in the definitive host. Experimental Parasitology, 13/3: 273-283.

Michaud, M., M. Milinski, G. Parker, J. Chubb. 2002. Competitive growth strategies in intermediate hosts: Experimental tests of a parasite life-history model using the cestode, Schistocephalus solidus. Evolutionary Ecology, 20/1: 39-57.

Nishimura, N., D. Heins, R. Anderson, I. Barber, W. Cresko. 2011. Distinct lineages of Schistocephalus parasites in threespine and ninespine stickleback hosts revealed by DNA sequence analysis. PLos ONE, 6/7: NA.

Poulin, R., C. Blanar, D. Thieltges, D. Marcogliese. 2011. The biogeography of parasitism in sticklebacks: distance, habitat differences and the similarity in parasite occurrence and abundance. Ecography, 34: 540-551.

Schjørring, S. 2004. Delayed selfing in relation to the availability of a mating partner in the cestode Schistocephalus solidus. Evolution, 58/11: 2591-2596.

Sharsack, J., K. Koch, K. Hammerschmidt. 2007. Who is in control of the stickleback immune system: Interactions between Schistocephalus solidus and its specific vertebrate host. Proceedings: Biological Sciences, 115/1629: 3151-3158.

Smyth, J. 1946. Studies on Tapeworm Physiology. Journal of Experimental Biology, 23/1: 47-69.

Widekind, C. 1997. The infectivity, growth, and virulence of the cestode Schistocephalus solidus in its first intermediate host, the copepod Macrocyclops albidu. Parasitology, 115/3: 317-324.