Littorina obtusata, common name the flat periwinkle, is a species of sea snail, a marine gastropod mollusk in the family Littorinidae, the winkles or periwinkles.

Description Littorina obtusata, flat periwinkles, are sea snails. They grow to 1.5 cm in height.n.d. The Wildlife Trusts. https://www.wildlifetrusts.org/wildlife-explorer/marine/sea-snails-and-sea-slugs/flat-periwinkle They can be brown, yellow, olive green, and black.Pizzolla, P.F. 2008. The Marine Life Information Network. April 17. Accessed April 3, 2025. https://marlin.ac.uk/species/detail/1487 They are mostly found in the colors of brown, reddish brown, or yellow, with few being red or white because of the abundance of brown seaweed. L. obtusata get the name flat periwinkle because they have a flattened spire and a large tear-drop-shaped aperture. The last whorl of the shell makes up 90% of their height. The maximum recorded shell length is 13.5 mm.Welch J. J. (2010). "The "Island Rule" and Deep-Sea Gastropods: Re-Examining the Evidence". PLoS ONE 5(1): e8776. . The operculum is yellowish to brown. This species is mainly littoral, but also has gill respiration. This means that they can survive both in water, and extended amounts of time on land.

Distribution This marine species occurs wherever brown seaweeds grow. It is widely distributed: the Baltic Sea, in European waters from Norway down to Southern Spain, in the Mediterranean Sea, in the Northeast Atlantic Ocean along the Gulf of Maine.

Scientific Background The flat periwinkle is not highly studied, so not much is known about them. Despite this, there have been recent developments in research on them, such as the following: It has been suspected that the Littorina obtusata and Littorina falbalis were the same species, then diverged into two species during their evolution. Scientists were still debating when this divergence occurred. L. obtusata tends to be bigger and live longer than L. falbalis. Research suggests that L. obtusata and L. falbalis are starting to hybridize despite the strong differences in their genetics. The name of the hybridized species is L. saxatilis, but there is little information and research on it.

Habitat This species can be found in the littoral and sublittoral zone on rocky shores and piers, usually on brown algae of the genus Fucus. Minimum recorded depth is 0 m. Maximum recorded depth is 110 m. The habitat may define its color. On sheltered shores it has a lighter and uniform color (yellow, brown, orange or olive green). On exposed shores its color is darker and chequered. They are tolerant of low salinities and wide ranges of exposure.

Diet Littorina obtusata are herbivores that primarily eat algae. They scrape algae from surfaces using their radula which contains thousands of tiny teeth.2024. Flat periwinkles: Their diet and feeding behavior. January 26. Accessed April 3, 2025. https://angolatransparency.blog/en/what-do-flat-periwinkles-eat/ They are commonly associated with, and feed upon, the dominant intertidal brown algae, Ascophyllum nodosum.Wilbur, Andrew K., and Robert S. Steneck. 1999. "Polychromatic Patterns of Littorina obtusata on Ascophyllum nodosum: Are Snails." Eagle Hill Institute 189-198. https://www.jstor.org/stable/3858593

Threats Littorina obtusata are very good at adapting to new areas and predators. Their largest predator is the Carcinus maenas, the green crab. As the threat of these crabs raised, Littorina obtusata's shells have thickened to protect themselves.Velasco, Schuyler. 2025. "Shells of their former selves: How sea snails have adapted to invasive predators." Northeastern University. doi:https://phys.org/news/2025-03-shells-sea-snails-invasive-predators.html Wader birds also threaten Littorina obtusata's population. They forage in intertidal zones, preferring the low-water mudflats, but when they are submerged, they use the Ascophyllum/Fucus belts.

Reproduction Female Littorina obtusata deposit spawn masses on Ascophyllum nodosum, Fucus serratus, Fucus vesiculosus, and sometimes on rock surfaces. Their eggs are whitish and oval, or kidney shaped. They are encased in a mass of transparent jelly. Littorina obtusata eggs take up to four weeks to fully develop and for the snail to emerge.Pizzolla, P.F. 2008. The Marine Life Information Network. April 17. Accessed April 3, 2025. https://marlin.ac.uk/species/detail/1487. Sexual maturity is reached at about two years. They continue to reproduce all year round with their maximum from spring to fall

References Notes

Bibliography Backeljau, T. (1986). Lijst van de recente mariene mollusken van België [List of the recent marine molluscs of Belgium]. Koninklijk Belgisch Instituut voor Natuurwetenschappen: Brussels, Belgium. 106 pp BODC (2009). Species list from the British Oceanographic Data Centre Pizzolla, P.F. 2008. The Marine Life Information Network. April 17. Accessed April 3, 2025. https://marlin.ac.uk/species/detail/1487. 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 Schmidt, Paul S., Megan Phifer-Rixey, Graeme M. Taylor, and John Christner. 2007. "Genetic heterogeneity among intertidal habitats in the flat periwinkle, littorina obtusata." Molecular Ecology 16 (11): 2393-2404. doi:10.1111/j.1365-294x.2007.03323.x. Sotelo, Graciela, Mårten Duvetorp, Diana Costa, Marina Panova, Kerstin Johannesson, and Rui Faria. 2020. "Phylogeographic history of flat periwinkles, Littorina Fabalis and L. obtusata." BMC Evolutionary Biology 20 (1). doi:10.1186/s12862-019-1561-6. n.d. The Wildlife Trusts. https://www.wildlifetrusts.org/wildlife-explorer/marine/sea-snails-and-sea-slugs/flat-periwinkle. Velasco, Schuyler. 2025. "Shells of their former selves: How sea snails have adapted to invasive predators." Northeastern University. doi:https://phys.org/news/2025-03-shells-sea-snails-invasive-predators.html. Wilbur, Andrew K., and Robert S. Steneck. 1999. "Polychromatic Patterns of Littorina obtusata on Ascophyllum nodosum: Are Snails." Eagle Hill Institute 189-198. https://www.jstor.org/stable/3858593. 2024. Flat periwinkles: Their diet and feeding behavior. January 26. Accessed April 3, 2025. https://angolatransparency.blog/en/what-do-flat-periwinkles-eat/.

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