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Seashells of Southern Florida, Volume 1:
Living Marine Mollusks of the Florida Keys and Adjacent Regions: Bivalves
Paula M. Mikkelsen & Rüdiger Bieler

Book Description | Endorsements | Table of Contents

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Florida Keys

Natural History

The Florida Keys form a chain of islands that separate Florida Bay from the Florida Straits at the southern tip of peninsular Florida, extending about 380 km from Soldier Key (15 km south of Miami) to Key West and westward to the Dry Tortugas archipelago (about 24°20′ to 25°21′N and 80° to 83°W). Numerous passes through the ridge of the Florida Keys permit exchange of bay waters with waters of the outer shelf to the south and east. The elevated rocky Keys, many of which nowadays are connected by a roadway leading from Key Largo to Key West, are made of limestone. Of these, the slender arc of Upper and Middle Keys (extending westward to Bahia Honda) are an emergent crest of Key Largo Limestone, the remnants of a Pleistocene coral reef. The Lower Florida Keys from Big Pine Key to Key West, with their dramatically different comblike arrangement, are made of oolitic Miami Limestone. They represent a reef-and-bar/channel complex that also developed about 125,000 years ago when the sea level was 6–8 m higher than today. This formation consists of ooids (a particular kind of near-spherical calcareous sand grain that precipitates in areas of strong reversing tidal currents), which were cemented together when the sea level fell at least 120 m during the advance of glaciers in the northern latitudes and the sand was exposed to slightly acidic rain. In contrast to both of these formations, the islands in Florida Bay and those west of Key West (including the Marquesas Keys and the Dry Tortugas archipelago) are mostly accumulations of modern sediment, carbonate mud, sand, and mangrove peat.

The Florida Reef Tract, a series of offshore bank reefs, marks the southern edge of the Floridian Plateau, 7–13 km from shore. Between the reef tract and the island chain lies Hawk Channel, a wide, V-shaped basin of 5–12 m depth that contains various shoals and patch reefs. South of the reef tract begins the Florida Straits, which is dominated by the fast-flowing Florida Current, a subsystem of the Gulf Stream. Its western portion, responsible for the mild winter temperatures permitting extensive coral reef development in the Keys, is composed of water from the Gulf of Mexico. Tidal range is less than 1 m throughout the Keys. Although the reef tract is exposed to normal salinities (24–37 parts per thousand), enclosed bays and ponds often experience widely fluctuating conditions resulting from heavy rainfall or evaporation.

Few marine environments in the United States can compete with the Florida Keys in terms of natural beauty and natural resources. The region is of particular note for its extensive offshore coral reefs. However, this subtropical region also sustains many other interdependent habitats including fringing mangroves, hypersaline ponds, seagrass meadows, mud banks and tidal channels, sandbars, deep sand plains, and hard pavement below a thin veneer of sand. Florida Bay and its islands serve as nesting, nursery, and/or feeding grounds for numerous marine animals including American crocodiles, West Indian manatees, loggerhead turtles, bottlenose dolphins, and a variety of bird species. Florida Bay serves as nursery grounds for many economically important fish species, pink shrimp, and the Caribbean spiny lobster or crayfish. Taken as a whole, the complex marine ecosystem of the Keys supports one of the most unique and diverse assemblages of plants and animals in North America and is the foundation for the commercial fishing and tourism-based economies that are vital to South Florida.

Need for Protection

About three million annual visitors join the fewer than 100,000 permanent residents of the Keys, most of them attracted by various water-based activities, including offshore fishing and scuba diving. The heavy use of marine resources over the decades has contributed to environmental problems, particularly for the coral reefs that are rapidly declining. Major issues include water-quality degradation, increase in coral diseases and coral bleaching (which have devastated once-magnificent stands of elkhorn and staghorn corals), as well as invasion of macroalgae into seagrass beds and coral reefs. The stressors on the reef are numerous and those identified range from overfishing, physical damage from ship groundings and individual bottom/coral contact by anchoring boaters, snorkelers, and divers, to pollution and nutrient overload from cesspools and other land-based sources, artificially modified water-flow regimen out of the Everglades, as well as thermal stress caused by climate change. Whatever the full range of causes, the marine habitats of the Keys are under heavy stress and deserve all the protection and nurturing they can get. Formal protection of local marine habitats began in 1960 with the establishment of the world’s first underwater park, the John Pennekamp Coral Reef State Park off Key Largo. Today the Keys have numerous preservation areas that have been established for conservation and management of local ecological (and some historical) resources, all now encompassed by the largest, the Florida Keys National Marine Sanctuary (FKNMS), established by the U.S. government in 1990. This vast preserve covers about 10,000 km2, including the waters surrounding the island chain out to the 91-m (300-foot or 50-fathom) isobath, including Dry Tortugas National Park at the western end and bordering on the Everglades National Park and Biscayne National Park in the east. The FKNMS employs a system of sanctuary-wide regulations combined with special marine zones (about 6% of the sanctuary is set aside as fully protected zones known as ecological reserves, sanctuary preservation areas, and special use areas). In addition, there are 27 wildlife management areas protecting especially sensitive habitats within the FKNMS, as well as numerous areas managed by other agencies. These include a variety of national parks, national wildlife refuges, state parks, and aquatic preserves.

Florida Keys Bivalves


Bivalves occur in nearly all of the highly diverse aquatic habitats of the Florida Keys. Marine bivalve species are present from the intertidal to great depths and often are characteristic faunal elements of particular habitats, as components of the coral reef and octocoral communities (e.g., Chama, Pteria, and Dendostrea), the mangrove fringe (Isognomon), seagrass beds (Pinctada and Argopecten), sand flats and beaches (Tellinidae), hard ground (Glycymeris), sandy and muddy bottom (Varicorbula), or intertidal rocks (Barbatia and Arcopsis). As a result of massive development activities over the past 100 years, man-made structures are providing ubiquitous settlement opportunities on hard substrata otherwise rarely available in the region and have shifted relative abundance in favor of cementing or byssally attaching forms such as oysters on bridges, causeways, docks, pilings, and lobster traps. Deliberately scuttled vessels that provide artificial reefs for the scuba diving industry are being settled by large-shelled species of Spondylus and Hyotissa, which otherwise are rare (or even unknown) in the region. Species tolerant of low-salinity, brackish-water conditions (Mytilopsis, Anomalocardia) occur in those parts of Florida Bay that receive substantial freshwater runoff from the Everglades, and others (Polymesoda) that can cope with low-saline and extremely hypersaline conditions have found a home in occasionally flooding ponds with conditions hostile to most other marine animals. Assemblages of larger empty shells often provide a three-dimensional substratum for other animals (a reason given by management authorities for the ban on shell collecting in some of the Keys’ protected areas), and bivalve shell parts, together with those of other mollusks, calcareous algae, foraminiferans, and coral debris, form the vast majority of the “sand” in the Keys. Permanent surface freshwater is limited to a few ponds on the larger rocky islands and the few freshwater bivalves of the region are not treated in this volume.

History of Exploration and the Florida Keys Molluscan Diversity Project

The relative ease of access to the Keys has made it a popular shell-collecting site and Florida Keys specimens have become widely distributed in private and museum collections. Many scientific and informal publications on mollusks, including the many popular shell books by R. Tucker Abbott, have included Keys taxa. But despite its rich history of popular and professional mollusk collecting, formal scientific inventories have been few. William Stimpson of Chicago’s Academy of Sciences made the first comprehensive attempt in the late 1860s. He accumulated all available records and specimens on loan from various institutional collections. His study material included the extensive holdings of the Smithsonian Institution in Washington, D.C., and original collections from the Florida Straits and the Pourtales Terrace obtained by Louis Francois de Pourtales (of the Museum of Comparative Zoology at Harvard University) during the U.S. Coast Survey expeditions of the 1860s. Tragically, all the specimens amassed by Stimpson were lost in the Great Chicago Fire of 1871. Stimpson also lost his nearly completed manuscript and never attempted to recreate the research. In 1883, William Healey Dall renewed the inventory effort, beginning by discussing the results of collecting by amateur conchologist Henry Hemphill and by analyzing the works of British collector James C. Melvill (1881; who reported on material obtained mainly in Key West during 1871–1872) and of amateur conchologist William W. Calkins (1878; who collected in the Keys during the late 1870s). Botanist/conchologist Charles Torrey Simpson (1887–1889) then produced the first effective Florida Keys checklist by including a separate column in his tabulation of Florida mollusks. This included 98 bivalve species names, of which 86 are currently recognized as valid taxa. Concurrently, Dall published the results of renewed dredging efforts off southern Florida by the U.S. Coast Survey (Dall, 1886, 1889b), culminating in a preliminary species catalog (Dall, 1889a, revised in 1903) that tabulated 225 species from the Florida Keys (plus 15 species now regarded as synonyms and 34 species out of the range of this survey). The next major collecting effort was made by amateur conchologist John B. Henderson, Jr., who sampled the molluscan fauna of the Florida Keys with his private yacht Eolis from 1910 to 1916. These annual cruises resulted in massive collections that were donated to the National Museum of Natural History. No comprehensive taxonomic treatment of the Eolis expeditions was ever published, although numerous Eolis specimens have been cited in scattered scientific papers (an annotated station list was published by Bieler and Mikkelsen in 2003). In 1936, a privately issued checklist by Norman Wallace Lermond, one of New England’s foremost naturalists who also explored Florida’s mollusks, reported 247 nominal bivalve species for the Keys, 214 of which are here considered valid. No other comprehensive attempt at summarizing the Keys fauna was made until the inception of the Florida Keys National Marine Sanctuary (FKNMS) in the 1990s, when the FKNMS Draft Management Plan provided a listing of 163 Keys bivalve species compiled by William Lyons and James Quinn (1995).

In 1994, the present authors began the Florida Keys Molluscan Diversity Project, an effort to explore the identity, distribution, and biology of all molluscan species in the region. This research involves new field collections as well as critical study of existing literature and museum collections, with the latter two resources allowing the reconstruction of past distribution of species in the Keys. The previously variously delineated “Florida Keys” region was explicitly defined for this project as the waters surrounding the entire island chain from Broad Creek (about 25°21′N, 80°15′W) at the northern end of Key Largo (including Card and Barnes Sounds but not Biscayne Bay, southwest of but not including Old Rhodes Key) to west of the Dry Tortugas (at 83°30′W). The southern half of Florida Bay is included (with a northern border at the levels of, from east to west, the northern end of the Nest Keys, Russell Key, and the northern limit of Rabbit Key Basin), eliminating that part of Florida Bay that is more properly considered the southern extent of the Florida Everglades. Two annotated species lists were published, listing 325 (Mikkelsen & Bieler, 2000) and, after additional research, 389 bivalve species (Bieler & Mikkelsen, 2004b). The latter publication also set a formal depth limit at the 300-m (= 164-fathom or 984-foot) isobath, which eliminated some exclusively deepwater species included in earlier listings. This book treats and illustrates 377 species, reflecting a slight decrease from our earlier census, based on the elimination of, for example, those only living in freshwater and those for which the only records were based on undocumented species lists (for which no supporting specimens were located).

Using This Book

This book focuses on the bivalves of the Florida Keys, and specimen photographs have been drawn from there whenever possible. The most frequent sources of this material were the American Museum of Natural History (AMNH) and Field Museum of Natural History (FMNH) mollusk collections, including many specimens specifically collected for this project. A “featured species” was selected for each family, and details of hinge, anatomy, and living appearance are provided specifically for that species whenever possible; in some cases, where these details were not available for the featured species, they are based on another species in the family. Following the featured species, additional species found in the Florida Keys are presented. In a few cases, for which we have credible data attesting to a Florida Keys record, but no specimens readily available, an image from the literature has been reproduced. Image data and photo credits in the back of the book provide detailed information for each set of photographs.

Family descriptions are constructed to be as complete and comparable as possible, especially with regard to soft-body morphology. Some taxa discussed in the family descriptions are not found in the Florida Keys; in the case of species, their geographical location is provided. For some families, scanning electron micrographs are included to better emphasize features not readily seen in light photographs (often with that feature artificially highlighted in green). Technical terms (in small capital font) used in the descriptions are defined in the Glossary (pp. 415-443).

Each set of specimen images includes an external view of the right valve and an internal view of the left valve, unless otherwise noted by RV (right valve) or LV (left valve) designations. Hinge drawings and “transparent clam” anatomical drawings are each presented with the anterior end of the animal to the right. In species where external coloration is demonstrably different on both valves (e.g., Pectinidae) an additional external view of the left valve is provided. Inequivalve species are accompanied by a view of the articulated valves with the smaller valve toward the reader. Other special cases (e.g., Pholadidae and Teredinidae) include extra images of accessory shell plates and other “hard parts.” Each set of images is accompanied by a diagnosis that should be sufficient to distinguish the species from similar forms, its known western Atlantic distribution, and size given as approximate anteroposterior length (= “width”), unless otherwise noted, in millimeters of the photographed specimen. If the maximum recorded size from the literature is substantially larger than the photographed specimen, this is indicated by the phrase “to xx mm.” Many species that look similar differ substantially in adult size; the reader is encouraged to attend to this information when attempting to identify specimens from this book. Synonyms and former names given here are restricted to those commonly found in popular and regional literature. Comparative statements are provided for species pairs that have been frequently confused in collections and the literature.

Each species account is also accompanied by three icons, reflecting its abundance, depth, and most typical habitat in the Florida Keys:

Abundance: abundant (>50 records); common (11–50 records); rare (<11 records). These categories are based on the records of each species’ occurrence in our geographically arranged database of Florida Keys species. A “rare” indication does not necessarily mean that this is an overall rare species, it merely shows that this species in rarely encountered in the Keys (it could be very common elsewhere).

Depth: shallow (known from depths not greater than sampled by conventional scuba diving, i.e., maximally 35 m or 100 feet); deep (greater than conventional scuba depth).

Habitat: sand, mud, or gravel; seagrass, algae, or salt marsh (usually overlying sand or mud); mangrove; seafan (octocorals); rock; wood. These designations point to a typical habitat situation of the species; it is not necessarily exclusive (e.g., a species that usually lives on mangrove roots can alternatively be found on other hard substrata such as rock surfaces).

Geographic distribution includes: (1) Greenland or Iceland; (2) the eastern North American coastline, beginning with eastern Canada, expressed as northern limit to Florida (e.g, North Carolina to Florida); (3) Bermuda; (4) the Bahama Islands; (5) West Indies (Greater and Lesser Antilles and Windward Islands); (6) Gulf of Mexico, including from western Florida to Texas; (7) Caribbean Central America, including from Mexico (including Yucatan) to Panama; (8) South America (“to” southern limit of range, or individual countries); and (9) extralimital localities (e.g., eastern Pacific). The southernmost extensions here given are in particular need of revisionary work. Ongoing work by South American malacologists indicates that many species deemed to have such wide ranges might in fact be misidentified, unrecognized, cryptic species in South America.

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File created: 12/11/2007

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