Variable coquina clam trail in Florida, USA. (December 2012)

Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

Shown above is a trail in a supratidal sandy marine shoreline environment. Prima facie, it was suspected to be a worm trail, but it was actually made by a variable coquina clam, Donax variabilis (the individual that made the trail is still visible near the upper right corner of the photo). This clam species has a smooth, small, Tellina-like shell that varies considerably in color (hence the species name). It is common in shallow marine sandy environments, ranging from the intertidal zone to moderately shallow subtidal settings. The individual that made this trail was in the supratidal part of a sandy marine shoreline. It likely reached this area during a then-recent storm, which usually results in a temporarily high sea level (storm surge). After the storm, the clam found itself in a non-permanently wet or moist environment and made a trail in search of water before dying. Note that another Donax clam has emerged from its burrow near the top margin of the photo.

Variable coquina clams can be abundant in intertidal zone sandy beach environments of southwestern Florida. When the sand is disturbed (pick up a couple handfuls in the swash zone and place it back down), many Donax are often visible - they immediately burrow back down into the sediments.

Classification: Animalia, Mollusca, Bivalvia, Heterodonta, Veneroida, Donacidae

Locality: supratidal zone of marine beach near the southern end of the western shoreline of Cayo Costa Island, Gulf of Mexico coast of southwestern Florida, USA (vicinity of 26° 36' 51.30" North latitude, 82° 13' 24.39" West longitude)

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