by Amy Lyn Edwards
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The coast of Georgia consists of a series of barrier islands that enclose several small shallow bays. The outer coast line is made up of beaches that provide mostly a sand habitat, while the bays have a variety of habitats: mud flats, salt marshes, pilings and oyster reefs. The water above the substrates provides an additional habitat for floating and swimming animals.
Animals that live on the sandy beaches encounter surf, currents and shifting sand. Beach inhabitants that cannot swim or crawl quickly must be able to burrow into the substrate to keep from being swept out to sea or up onto the shore. Almost all the animals that inhabit the beach live in under-sand communities. Some animals build permanent burrows with strengthened walls to withstand the forces of the tide. The sand left behind by the retreating tide still holds more than enough water to keep the animals that live in the intertidal area from drying out. This layer of sand and water also acts as a buffer to rapid temperature and salinity changes.
Many intertidal animals migrate on and off the beach with the tides, instead of making permanent burrows. These animals are able to quickly burrow back into the safety of the sand after they are uncovered by the waves. As the tide retreats down or advances up the beach, masses of small bivalves known as Coquinas (Donax variabilis) are washed out of the sand and begin to reburrow frantically back to safety. These small clams are the regular diet of a very beautiful and common snail, the Lettered Olive (Oliva sayana) , and the very common large round Moon Snail (Polinices duplicatus). Illustrated on the right is a Lettered Olive.
At two locations, the north end of Tybee Island and the south end of Cumberland Island, man has erected jetties and breakwaters. These rocky structures were built to secure the river channels and land masses next to them, but they also provide a different habitat. Animals that live on rocky coasts elsewhere in the United States are found here and nowhere else along the coast of Georgia.
These two rocky jetties in Georgia are habitats for animals that can feed while holding tightly to the rocks. Some of these mollusks filter plankton from the passing water, others graze on the algae or diatoms that grow on these rocks, or scavenge for food washed up by the tide and left on the rocks. Limpets and chitons are common grazers on the jetties. The Florida Rock Snail (Thais haemastoma) can be found on the southern jetty where it hunts barnacles and young oysters. The Tinted Cantharus or Purple Snail (Pisania tincta) is common in the sand near jetties. Illustrated on the right are two Keyhole Limpets.
The marine and estuarine habitats within each bay have different substrates, wave exposures, food resources and other important environmental aspects. The animals that live outside of the barrier islands receive more abuse from waves, but experience fewer seasonal changes in temperature and salinity than the animals that live in the bay. The barrier islands protect animals within the bays from heavy wave action and offshore currents. The waters in the bay are slower, allowing the finer particles carried downriver to be deposited in the bay. This produces the muddy substrate found in the bays. The extensive, stable soft bottom in the bays provides an ideal habitat for burrowing animals.
However the animals that live in the bays must be able to tolerate other changes. The rivers that flow into the bays can cause abrupt changes in salinity, turbidity and oxygen content of the bays' waters. Within the shallow bays the water is easily heated by the sun or cooled by the wind. Due to the lack of turbulent mixing in the bays the anoxic (meaning it lacks oxygen) layer, the black mud that smells of sulfur dioxide (an area uninhabitable for many of the bays' animals) forms closer to the surface. During low tide much of the shallow bays are exposed to the air and non-burrowing animals can be left helpless on the surface of the mudflat, suffering from heat, water loss and exposure to predators.
Within the bays a variety of submerged hard substrates are available to other animals. Years of oysters cementing their shells on top of one another creates oyster shell bars. Oysters always choose an oyster-shell-like substrate. Oyster bars grow to massive sizes where the currents can deliver large amounts of food. These bars are partly exposed during low tide, but create protected nooks and crannies for animals to hide in until the tide returns.
Oyster bars create large habitats for many other bay animals. Pilings, floating docks and boats create other hard substrate on which sessile animals can attach and live. The Common Oyster (Crassostrea virginica) builds the oyster bars found in the bays of Georgia. They continue to grow yearly as new oysters attach themselves to the bar. Many other bivalves and snails find these bars to be the perfect habitat. Hooked Mussels (Ischadium recurvum) attach themselves to the oysters by byssal threads. Three oyster drills (Thais haemastoma, Eupleura caudata and Urosalpinx cinerea) all feed on oysters in the bays. Illustrated on the right is a Hooked Mussels.
Areas of the bays that are not covered by salt marshes but only by a few inches of water at low tide are filled with a variety of burrowing bivalves. In the sandier areas Razor clams (Tagelus plebeius and Ensis directus) are common. Once alarmed, these clams can disappear into the mud more quickly than a person can dig. In the large mud flats within the bay a common resident is the Hardshelled Clam (Mercenaria mercenaria) and its predator the Knobbed Whelk (Busycon carica).
Another whelk that is frequently found in the bays, the Lightning Whelk (B. sinistrum), is also a predator of bivalves. Closer to the mouth of the bays the clayey mud is the home of thousands of Dwarf Surf Clams (Mulinia lateralis). These tiny clams can survive the wide ranges of temperatures and salinity in this area. Two snails are also common here, the Common Mud Snail (Nassarius vibex) and the Common Atlantic Auger (Terebra dislocata). Illustrated on the right is a Common Atlantic Auger.
In areas near the mouth of the bays a combination of plant material and clay forms a stiff mud, called a peat bog. This type of substrate is inhabited by a group of bivalves called Angel Wings (Cyrtopleura costata, Barnea truncata and Petericola pholadiformis). These animals cannot close their shells completely and as a consequence their shells are often found empty in peat bogs above the low tide mark. As the peat deposit increased and the tide no longer covered them the animals died.
The salt marsh creates another habitat for animals. Some animals hide among the reeds when the tide is in and feed on the substrate when it retreats, others come in with the tide to feed in the salt marshes. Still other animals live buried in the mud at the base of the plants among the roots. There are three very common snails found in the Georgia salt marshes, the Marsh Periwinkle (Littorina irrorata), the Mud Snail (Ilynassa obsoleta) and the air-breathing Cofee-bean Snail (Melampus bidentatus). Illustrated on the right is a Cofee-bean Snail.
These snails crawl up the spartina reeds to escape the incoming tide and decend the stalks to feed on detritus, algae and diatoms left on the mud when the tide is out. At the base of the spartina lining the river channels, among the plants roots, the common Ribbed Mussel (Geukensia demissa) is found in abundance. These mussels bury themselves up to the top of their shells, leaving just enough shell above the substrate so that they can open their shells to feed on small particles in the passing water. They attach themselves to the plant roots by fibrous threads (byssus) spun by the mussel's foot.
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