English: Atrina pen shell bivalves on a marine beach in Florida, USA (December 2012).

Sanibel Island is one of dozens of barrier islands in the West-Central Florida Barrier Chain. This 191-mile long, slightly sinuous stretch of islands is located along the Gulf of Mexico coast of southern Florida. The southern-most island in the chain is Cape Romano Island. The northern-most islands are the Anclote Keys.

Sanibel Island is located between Captiva Island and mainland Florida, just offshore from the towns of Fort Myers and Cape Coral, Florida. Much of Sanibel Island is developed, but significant tracts have been allowed to become wilderness - especially Ding Darling National Wildlife Refuge.

Sanibel is on the southern side of Pine Island Sound, a large lagoon just south of Charlotte Harbor. A tidal inlet occurs on the island's western side - Blind Pass (a man-made construct). To the east of Sanibel is a moderately broad waterway - San Carlos Pass.

Sanibel Island and nearby Captiva Island, North Captiva Island, and Cayo Costa Island are Holocene barriers that rim the southern and western sides of a Late Miocene depression that is now Pine Island Sound. Middle Miocene limestone bedrock was subject to significant dissolution and karst/cave development. The Pine Island Sound area was a large karst depression in the Late Miocene. It is now filled with sediments - most of modern Pine Island Sound is significantly shallow.

Sanibel is famous for its shell-rich marine beaches. The Sanibel Island area has the 3rd-richest seashell beaches on Earth and the # 1 richest shell beaches in the Western Hemisphere. Shell collecting is best after a storm. The beach shown above is Algiers Beach, which is part of Gulfside City Park on the southern shoreline of Sanibel Island. About 99% of the shells on Sanibel Island beaches are bivalves (clams). Relatively few snails (gastropods) are present, but they are more common and conspicuous after storm events. Other marine remains observed on these beaches include sea urchins (echinoids), starfish (asteroids), crabs (decapods), horseshoe crabs (xiphosurans), sponges (poriferans), stony corals (anthozoan cnidarians), sea squirts (tunicates), sea hares (anaspidean opisthobranch gastropods), worm tubes, fish & fish skeletons, and stingray barbs.

The dark-colored shells shown above are Atrina pen shells, which are distinctive bivalves having thin, brittle, brownish-colored shells.

Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Pinnidae

Locality: Algiers Beach, southern shore of Sanibel Island, Gulf of Mexico coast of southwestern Florida, USA (vicinity of 26° 25' 27.64" North latitude, 82° 03' 56.17" West longitude)

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West-Central Florida Barrier Chain geologic info. synthesized from:

Evans et al. (1985) - Bedrock controls on barrier island development: west-central Florida coast. Marine Geology 63: 263-283.

Davis (1989) - Morphodynamics of the West-Central Florida barrier system: the delicate balance between wave- and tide-domination. Proceedings, Koninklijk Nederlands Geologisch Mijnbouwkundig Genootschap Symposium, 'Coastal Lowlands, Geology and Geotechnology', 1987: 225-235.

Evans et al. (1989) - Quaternary stratigraphy of the Charlotte Harbor estuarine-lagoon system, southwest Florida: implications of the carbonate-siliciclastic transition. Marine Geology 88: 319-348.