Rough periwinkle
A shell of Littorina saxatilis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Caenogastropoda
Order:	Littorinimorpha
Family:	Littorinidae
Genus:	Littorina
Species:	L. saxatilis
Binomial name
Littorina saxatilis (Olivi, 1792)[1]

Littorina saxatilis, common name the rough periwinkle, is a species of small sea snail, a marine gastropod mollusc in the family Littorinidae, the winkles or periwinkles. First identified in the 1700s, it has been misidentified as a new species 112 times.^[2]

This species is native to the shores of the North Atlantic Ocean, including Hudson Bay, Baffin Island, Greenland, and the Barents Sea, south along the American East Coast to Chesapeake Bay, and along the European coast to the Straits of Gibraltar.

This species has also been introduced to San Francisco Bay, on the West Coast of the United States, where it was first observed in 1992.

The shell in life often appears green with algae, but the shell itself can be white, red, or brown, sometimes with checkered lines. The shell has 4–5 whorls. Maximum recorded shell length is 19 mm.^[3][4]

• 
  Apertural view
• 
  Abapertural view
• 
  A series of shells of Littorina saxatilis

This species frequently lives in salt marshes. it can also be found in crevices of intertidal bedrock, in empty barnacle shells, and under rocks. Like many other periwinkles, this species can survive long exposures out of the water.^{[citation needed]}

The species has been recorded alive from depth range 0 – 46 m^[3] or up to 183 m (for shells only).^[4]

In the exposed Galician coast in the Northern Spain, two well differentiated ecotypes are adapted to different shore levels and habitats.^[5] The RB ecotype (Ridged and Banded) lives on barnacles in the upper shore.^[5] This ecotype displays a larger and more robust shell to resist the attack from predators such as crabs, and a smaller shell aperture in order to reduce the desiccation due to high sunshine exposure.^[5] The SU ecotype (Smooth and Unbanded) is found at the lower shore living on mussels.^[5] This ecotype shows a smaller and thinner shell with a wider shell aperture to allocate a relatively larger muscular foot providing a higher ability to avoid the dislodgment caused by the heavy wave action.^[5] Both ecotypes coexist in an intermediate habitat at the middle shore.^[5]

Littorina saxatilis has been shown to be an excellent model system for speciation genetics.^[6] In the seminal 2001 paper, Wilding et al demonstrated, using amplified fragment length polymorphism, that the low shore M form of the species were divergent from the high shore H form at number of loci despite gene flow between the forms.^[6]

This snail is a herbivore which grazes on the surface of rocks and mud flats.

The marine snail Littorina saxatilis has separate sexes, internal fertilization, and a brood pouch with non-planktonic shelled embryos.^[5]

• Common periwinkle

This article incorporates CC-BY-2.0 text from the reference ^[5][6]

• Carvajal-Rodríguez, A.; Conde-Padín, P. & Rolán-Alvarez, E. (2005). "Decomposing shell form into size and shape by geometric morphometric methods in two sympatric ecotypes of Littorina saxatilis. Journal of Molluscan Studies 71: 313–318.
• Galindo, J.; Morán, P. & Rolán-Alvarez, E. (2009). "Comparing geographical genetic differentiation between candidate and noncandidate loci for adaptation strengthens support for parallel ecological divergence in the marine snail Littorina saxatilis". Molecular Ecology 18: 919–930.
• Hayward, P. J.; Ryland, J. S. (Ed.) (1990). The marine fauna of the British Isles and North-West Europe: 1. Introduction and protozoans to arthropods. Clarendon Press: Oxford, UK. ISBN 0-19-857356-1. 627 pp.
• Johannesson, B. (1986). "Shell morphology of Littorina saxatilis Olivi the relative importance of physical factors and predation". Journal of Experimental Marine Biology and Ecology 102: 183–195.
• Johannesson, K.; Johannesson, B. & Rolán-Alvarez, E. (1993). "Morphological differentiation and genetic cohesiveness over a micro-environmental gradient in the marine snail Littorina saxatilis". Evolution 47: 1770–1787.
• Reid, D. G. (1989a) "The comparative morphology, phylogeny and evolution of the gastropod family Littorinidae". Philosophical Transactions of the Royal Society of London, Series B 324: 1–110
• Reid, D. G. (1996). Systematics and evolution of Littorina. The Ray Society 463p.
• Rolán-Alvarez, E. (2007). "Sympatric speciation as a by-product of ecological adaptation in the Galicia Littorina saxatilis hybrid zone". Journal of Molluscan Studies 73: 1–10.
• Rolán-Alvarez, E.; Johannesson, K. & Erlandsson, J. (1997). "The maintenance of a cline in the marine snail Littorina saxatilis: the role of home site advantage and hybrid fitness in ecotype formation". Evolution 51: 1838–1847.

Wikimedia Commons has media related to Littorina saxatilis.

• In the British Isles
• As an invasive species in San Francisco Bay