Species Description: The marsh periwinkle, Littorina irrorata, is an abundant snail in the salt marshes of the western Atlantic. The shell is elongate conic in shape, longer than it is wide (Andrews 1994). Coloration of the shell is dull grayish white with tiny dashes of reddish brown on the ridges of the spiral. Eight to ten gradually increasing flat whorls comprise the shell, with the body whorl measuring about half of the total height. The aperture is oval with a sharp outer tip and regular grooves on the inside edge.
Potentially Misidentified Species: Many of the species of littorinids common to the western Atlantic are found in the IRL, including: the mangrove periwinkle, Littorina angulifera; slender periwinkle, L. angustior; lineolate periwinkle, L. lineolata; white-spot periwinkle, L. meleagris; and the zebra periwinkle, L. ziczac. All of these species share a similar shell shape and an intertidal distribution.
The mangrove periwinkle (Littorina angulifera) attains a shell length of about 2.5 to 3.0 cm, and varies in background color from bluish white, orange to dull yellow, reddish brown to grayish brown (Andrews 1994). The shell is comprised of 6 whorls, with the body whorl about half of the total height of the snail. Darker dashes on the ribs of the shell are often fused to form stripes on the body whorl. The early whorls around the base bear regularly-spaced vertical white spots below the channeled sutures.
The slender periwinkle (Littorina anguistor) is relatively small, reaching a length of about 0.8 cm (Abbott 1974). The upper whorls of the shell are marked with 6-9 spiral lines, the sides of the foot are mottled black and gray, and the operculum is mostly round in shape.
The lineolate periwinkle (Littorina lineolata) reaches a length of about 1.2 to 2.5 cm, has a gray background color on the shell with oblique zigzag lines of dark brown, and an apex of reddish brown (Andrews 1994). The shell is composed of 6-8 gradually increasing whorls, with the body whorl spanning more than half of the total length, and the suture between whorls is well marked. The pear-shaped aperture has a sharp, thin outer lip meeting the body whorl at an acute angle. Shells of males are smaller and more strongly sutured then females.
The white-spot periwinkle (Littorina meleagris) is also small like the slender periwinkle, measuring about 0.8 cm in length (Abbott 1974). The shell has a pointed spire with a thin periostracum or organic covering. The aperture is reddish brown and the exterior of the shell is brown with large, irregular white spots, often arranged in spiral roles.
The zebra periwinkle (Littorina ziczac) has a shell length of about 1.3 cm, and is whitish with dark brown or black wavy stripes (Andrews 1994). The aperture is small and oval, and the operculum is chitinous. This species is often confused with L. lineolata, but has a lighter colored shell with a narrower apical angle than the lineolate periwinkle.
Regional Occurrence & Habitat Preference: The marsh periwinkle ranges from New York to Texas (Abbott & Morris 1995, Bequaert 1943), and shares a similar distribution with the salt marsh cordgrass, Spartina alterniflora (Hamilton 1978). Individuals are found above the water line on and around vegetation throughout salt marsh areas (Hutchens & Walters 2006), often on dead, upright leaves of Spartina alterniflora (Crist & Banta 1983). Occasionally, populations are found on jetty rocks and seawalls (Gosner 1978).
IRL Distribution: The distribution of L. irrorata is mainly limited to salt marsh habitats of the IRL, mostly confined to the northern areas of the lagoon. In south Florida, including the southern areas of the IRL, L. irrorata is largely replaced by the mangrove periwinkle, L. angulifera (Kaplan 1988).
Age, Size and Lifespan: The maximum age of L. irrorata is unknown, and the lifespan can vary with food availability and environmental factors. The maximum reported shell length for the marsh periwinkle is 3.2 cm (Kaplan 1988), but most individuals are around 2.5 cm (Andrews 1994).
Abundance: The marsh periwinkle can reach high densities in the salt marshes of the east coast of North America. In many areas, populations range between 15 and 66 individuals m-2 (Hutchens & Walters 2006, Schindler et al. 1994). However, when left unchecked be predators, they may approach densities over 2,600 snails m-2 (Hutchens & Walters 2006).
Reproduction: Reproductive strategies are quite diverse within the Littorina genus. Some species release egg masses from with larvae hatch, others attach egg masses to hard substrata, and some brood their young until giving birth to larvae or juvenile snails (Ruppert & Barnes 1994).
Temperature: The large range and distribution of L. irrorata throughout temperate to tropical latitudes suggests the species has a wide thermal tolerance. Laboratory experiments on the marsh periwinkle successfully exposed the snail to a temperature range of 5 to 45°C, although depressed respiration rates suggested thermal stress at the extremities of this range (Shirley et al. 1978). The ability of the marsh periwinkle to attach to a surface via a mucous holdfast and withdraw completely into its shell is likely a means of escaping extreme temperatures and desiccation (Andrews 1994, Shirley et al. 1978).
Salinity: The salinity tolerances of L. irrorata are poorly documented, but most populations are found in low salinity waters (Andrews 1994).
Trophic Mode: The marsh periwinkle is herbivorous, grazing on algae, fungi and the marsh cordgrass, Spartina alterniflora (Andrews 1994, Bärlocher & Newell 1994, Graça et al. 2000, Gustafson et al. 2006, Hutchens & Walters 2006, Silliman et al. 2004, Warren 1985).
Predators: Documented predators of L. irrorata include: the squareback marsh crab, Armases cinereum; Atlantic mud crab, Panopeus herbstii; blue crab, Callinectes sapidus; and the crown conch, Melongena corona. (Buck et al. 2003, Schindler et al. 1994, Silliman et al. 2004, Warren 1985). Additional predators of the marsh periwinkle likely include large fishes, birds and mammals.
Associated Species: No known obligate associations exist for L. irrorata. However, marsh periwinkles are associated with several organisms common to salt marshes and other intertidal areas. For extensive lists of other species found in the habitats in which L. irrorata occurs, please refer to the 'Habitats of the IRL' area of the inventory.
Ecological Importance: High densities of marsh periwinkles have the potential to drastically reduce coverage of their main food source, the cordgrass, Spartina alterniflora. As a top predator of L. irrorata, the Atlantic mud crab, Panopeus herbstii, helps to keep snail populations in check that could otherwise decimate salt marsh vegetation (Silliman et al. 2004).
Abbott, RT. 1974. American seashells: the marine Mollusca of the Atlantic and Pacific coasts of North America. Van Nostrand Reinhold Co. New York, NY. USA.
Abbott, RT & PA Morris. 1995. A field guide to shells: Atlantic and Gulf coasts and the West Indies, 4th edition. Houghton Mifflin Co. Boston, MA. USA.
Andrews, J. 1994. A field guide to shells of the Florida coast. Gulf Publishing Co. Houston, Texas. USA. 182 pp.
Bärlocher, F & SY Newell. 1994. Growth of the salt marsh periwinkle Littoraria irrorata on fungal and cordgrass diets. Mar. Biol. 118: 109-114.
Bequaert, JC. 1943. The genus Littorina in the western Atlantic. Johnsonia. 1: 1-27.
Crist, RW & WC Banta. 1983. Distribution of the marsh periwinkle Littorina irrorata (Say) in a Virginia salt marsh. Gulf Res. Rep. 7: 225-235.
Gosner, KL. 1978. A field guide to the Atlantic seashore: Invertebrates and seaweeds of the Atlantic coast from the Bay of Fundy to Cape Hatteras. Houghton Mifflin Co. Boston, MA. USA. 329 pp.
Graça, MA, Newell, SY & RT Kneib. 2000. Grazing rates of organic matter and living fungal biomass of decaying Spartina alterniflora by three species of salt-marsh invertebrates. Mar. Biol. 136: 281-289.
Gustafson, DJ, Kilheffer, J & BR Silliman. 2006. Relative effects of Littoraria irrorata and Prokelisia marginata on Spartina alterniflora. Estuar. Coasts. 29: 639-644.
Hamilton, PV. 1978. Intertidal distribution and long-term movements of Littorina irrorata (Mollusca: Gastropoda). Mar. Biol. 46: 49-58.
Hutchens, JJ, Jr. & K Walters. 2006. Gastropod abundance and biomass relationships with salt marsh vegetation within ocean-dominated South Carolina, USA estuaries. J. Shellfish. Res. 25: 947-953.
Kaplan, EH. 1988. A field guide to southeastern and Caribbean seashores: Cape Hatteras to the Gulf coast, Florida, and the Caribbean. Houghton Mifflin Co. Boston, MA. USA. 425 pp.
Newell, SY & F Bärlocher. Removal of fungal and total organic matter from decaying cordgrass leaves by shredder snails. J. Exp. Mar. Biol. Ecol. 171: 39-49.
Ruppert, EE & RD Barnes. Invertebrate zoology, 6th edition. Saunders College Publishing. Orlando, FL. USA. 1056 pp.
Ruppert, EE. & RS Fox. 1988. Seashore animals of the Southeast: A guide to common shallow-water invertebrates of the southeastern Atlantic coast. University of SC Press. Columbia, SC. USA. 429 pp.
Schindler, DE, Johnson, BM, MacKay, NA, Bouwes, N & JF Kitchell. 1994. Crab: snail size-structured interactions and salt marsh predation gradients. Oecologia. 97: 49-61.
Shirley, TC, Denoux, GJ & WB Stickle. 1978. Seasonal respiration in the marsh periwinkle, Littorina irrorata. Biol. Bull. 154: 322-334.
Silliman, BR & MD Bertness. 2002. A trophic cascade regulates salt marsh primary production. Proc. Nat. Acad. Sci. 99: 10500-10505.
Silliman, BR, Layman, CA, Geyer, K & JC Zieman. 2004. Predation by the black-clawed mud crab, Panopeus herbstii, in Mid-Atlantic salt marshes: further evidence for top-down control of marsh grass production. Estuaries. 27: 188-196.
Silliman, BR, van de Koppel, Bertness, MD, Stanton, LE & LA Mendelssohn. 2005. Drought, snails, and large-scale die-off of southern US salt marshes. Science. 310: 1803-1806.
Warren, JH. 1985. Climbing as an avoidance behavior in the salt marsh periwinkle, Littorina irrorata (Say). J. Exp. Mar. Biol. Ecol. 89: 11-28.