University of FloridaSolutions for Your Life

Download PDF
Publication #EENY-724

Marsh Rams-Horn, Marsh Ramshorn Helisoma (Planorbella) trivolvis (Say) (Gastropoda: Planorbidae)1

Carrie Suen and Jennifer L. Gillett-Kaufam2


The marsh rams-horn, Helisoma (Planorbella) trivolvis (Say), is a species of air-breathing freshwater aquatic snail that is native to Florida. It belongs to the family Planorbidae, commonly known as the ramshorn snail, which comes from the shape of its shell.

Figure 1. 

Top view of a marsh rams-horn, Helisoma trivolvis (Say), next to a strand of hydrilla. Specimen approximately 2 cm.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]


Planorbella trivolvis intertexta (Sowerby 1878)

Planorbella trivolvis (Say 1817)

Planorbis trivolvis (Say 1817)


The marsh rams-horn is native to North America, with its habitat ranging from northern arctic Canada and Alaska to Florida. This species has also been discovered in parts of Mexico, Peru and Ecuador. This species of snail is often seen breathing air or grazing on plants on or near the surface of the water (Baker 1945). The marsh rams-horn can be also be found in the bottom of swamps, lakes, rivers, and ponds.

Figure 2. 

Distribution of the marsh rams-horn, Helisoma trivolvis (Say).


Distribution data used with permission from The IUCN Red List of Threatened Species

[Click thumbnail to enlarge.]


The marsh rams-horn is characterized by its sinistral (left-turning) flat coil shell. The color of the shell ranges from light to dark brown with the body of the snail similar in color. The visible portion of the body of the snail consists of a head, a foot and a pair of tentacles. The snail uses its singular foot for movement via muscle contraction. This foot also allows the snail to walk underneath the surface of the water via surface tension and viscous drag.

The head of the snail contains the mouth and a pair of tentacles with eyes at their base. The mouth is a triangular structure located at the bottom of the head. Inside the mouth is the radula which allows the snail to graze on food via a ribbon structure of tiny teeth (Baker 1945). The snail uses its tentacles to gather chemical cues about its surroundings. Adults reach a maximum size of approximately 15–18 mm in shell diameter, with the body being a similar length. The young of this species hatch as crawl-away juveniles, with a strikingly flat-topped shell bearing little resemblance to that of the adult.

Figure 3. 

Front view (left) and back view (right) of the sinistral shell of a marsh rams-horn, Helisoma trivolvis (Say). The snail has a planispiral (flat coiling) shell.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]

Life History

Members of this species are hermaphrodites, containing both male and female reproductive organs. This snail is capable of both sexual and asexual reproduction (Burch 1989). The eggs of the marsh rams-horn are contained in a hard, gel-like egg sack that is less than 5 mm. The egg sacks are initially a clear light orange and darken as the eggs mature. The egg sacks can be found stuck on solid surfaces such as aquatic vegetation (Hung et al. 2018). The primary author has observed the marsh rams-horn on bulltongue arrowhead (Sagittaria lancifolia) and pickerelweed (Pontederia cordata). However, the snail is not limited to these two species.

Figure 4. 

Empty egg sack of Helisoma (Planorbella) sp., the egg sack is approximately 3 mm in length.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]

Figure 5. 

Egg sack of Helisoma (Planorbella) sp., with five snail eggs in various stages of development. It takes approximately two weeks for eggs to hatch depending on the conditions. This egg sack is approximately 3 mm in length. An egg sack can contain anywhere from five to 20 snails.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]

Figure 6. 

Egg sack of Planorbella sp. with six developing snail eggs on a piece of Lomariopsis sp. The eyes of the developing snails are visible via the two black dots on the right uppermost three and lower right developing eggs.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]


Snails belonging to the family Planorbidae have been known to serve as intermediate hosts for a variety of parasitic trematodes such as Ribeiroia and Schistosoma, both commonly known as flukes. The marsh rams-horn has been observed to be hosts for the larval stage of Echinostoma trivolvis, which are trematodes that can infect humans, fish, and other animals (Fried et al. 1996).

The marsh rams-horn is also a host to another type parasitic trematode, Ribeiroia ondatrae. This trematode is thought to be responsible for limb malformation in amphibians. This can include either deformed, additional, or missing limbs. Warm temperatures (26°C) induce the eggs of Ribeiroia to develop up to four times faster. Warming global temperatures can potentially have a major effect on the abundance of this parasite (Paull and Johnson 2011).

Figure 7. 

Life cycle of the parasitic trematode Echinostoma sp.


Image used with permission from the Centers for Disease Control and Prevention

[Click thumbnail to enlarge.]

Ecological Significance and Management

Gastropods play a crucial role in nutrient cycling through the grazing of algae and decaying material. The marsh rams-horn is primarily a detrivore, grazing on decaying plants, dead organisms, and algae. In some circumstances they may consume living macrophytic tissues. They also play a role in the aquatic food web by serving as prey for other species such as fish, turtles, birds, and young alligators (Johnson et al. 2018). The marsh rams-horn has been used by researchers as an indicator species for wetland permanence in exurban areas (Urban and Roehm 2018). Exurban areas are the developing rural areas outside of urban areas.

Figure 8. 

Ventral (underside) view of marsh rams-horn, Helisoma trivolvis (Say), showing foot, head and tentacles.


Lyle J. Buss, UF/IFAS

[Click thumbnail to enlarge.]

In some areas marsh rams-horn populations build to levels that have the potential to disrupt fisheries operations because parasitic trematodes from the snails may infect the fish. In these areas management with perimeter shoreline treatment and aquatic applications have be used to decrease not just marsh rams-horn, but also trematode populations. Treatment options are available for these snails and parasites (Mitchell 2002; Meepagala et al. 2004).

Selected References

Baker FC. 1945. The Molluscan Family Planorbidae. Urbana, Illinois: University of Illinois Press. pp. 10–11, 17.

Burch JB. 1989. North American Freshwater Snails. Hamburg, Michigan: Malacological Publications. pp 41–44.

Fried B, Nanni TJ, Reddy A, Fujino T. 1996. "Maintenance of the life cycle of Echinostoma trivolvis (Trematoda) in dexamethasone-treated ICR mice and laboratory-raised Helisoma trivolvis (Gastropoda)." Parasitology Research 83: 16–19.

Hung T, Stevenson T, Sandford M, Ghebremariam T. 2018. "Temperature, density and ammonia effects on growth and fecundity of the ramshorn snail (Helisoma anceps)." Aquaculture Research 49: 1072–1079.

Johnson PD, Bogan AE, Brown KM, Burkhead NM, Cordeiro JR, Garner JT, Hartfield PD, Lepitzki DA, Mackie GL, Pip E, Tarpley TA, Tiemann JS, Whelan NV, Strong EE. 2013. "Conservation status of freshwater gastropods of Canada and the United States." Fisheries 38: 247–282.

Meepagala KM, Sturtz G, Mischke CC, Wise D, Duke SO. 2004. "Molluscicidal activity of vulgarone B against ram's horn snail (Planorbella trivolvis)." Pest Management Science 60: 479–482.

Mitchell AJ. 2002. "A copper sulfate-citric acid pond shoreline treatment to control the rams-horn snail Planorbella trivolvis." North American Journal of Aquaculture 64: 182–187.

Norton CG, Johnson AF, Nelson BM. 2018. "The genetic basis of albinism in the hermaphroditic freshwater snail Planorbella trivolvis." American Malacological Bulletin 36(1): 153–157.

Paull SH, Johnson PT J. 2011. "High temperature enhances host pathology in a snail-trematode system: Possible consequences of climate change for the emergence of disease." Freshwater Biology 56(4): 767–778.

Sturm CF, Pearce TA, and Valdes A. 2006. The Mollusks: A Guide to Their Study, Collection, and Preservation. Boca Raton, FL: Universal. 257 p.

Urban MC, Roehm R. 2018. "The road to higher permanence and biodiversity in exurban wetlands." Oecologia 186(1): 291–302.

Sturm CF, Pearce TA, and Valdes A. 2006. The Mollusks: A Guide to Their Study, Collection, and Preservation. Boca Raton, FL: Universal. 257 p.



This document is EENY-724, one of a series of the Department of Entomology and Nematology, UF/IFAS Extension. Original publication date January 2019. Visit the EDIS website at for the currently supported version of this publication. This document is also available on the Featured Creatures website at


Carrie Suen; and Jennifer L. Gillett-Kaufam, associate Extension scientist; Department of Entomology and Nematology, UF/IFAS Extension, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.