Key Points
This section will give you the main information you should know to teach the activity.

• A food chain is a series of organisms linked together in order of who eats whom that shows how energy moves through one segment of a community.
• All the energy in a food chain comes from the sun.
• The organisms in a food chain occupy different niches. Plants are producers, because they can make their own food from the energy of the sun. Animals are consumers, because they can only get their food by eating other organisms. The species at the top of a food chain who is not eaten by anything else is known as an apex consumer.
• The energy produced by plants will not transfer in its entirety across the food chain to the apex consumer. Only 10 to 20% of the energy received by one consumer will pass to the next consumer. Some of the energy will be lost in indigestible food parts and some will used by the various organisms in the food chain to perform their life functions (breathing, moving, growing, etc.). The amount of energy contained in the apex consumer is always less than the amount of energy collected from the sun by the producers. For this same reason, there are always less organisms the higher you go up a food chain because there is less energy to sustain them. There will always be less consumers than producers, and less apex consumers than primary consumers.

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Detailed Information
This section gives more in-depth background to increase your own knowledge, in case you want to expand upon the activity or you are asked detailed questions by students.

This activity will focus on one of South Carolina's many aquatic environments- the saltmarsh tidal creek. A South Carolina tidal creek is a creek in a saltmarsh that has both freshwater and saltwater. The freshwater flows down the creek from inland areas and meets ocean water that floods into it during high tide. In tidal creeks the freshwater and saltwater mix together to form brackish water. A tidal creek has a variety of plants and animals, some of which normally live in freshwater, some in saltwater and some that can live in both. For this reason, the tidal creek is a good environment to use for a food chain activity.

There are many different possible food chains in a tidal creek, all of which can be taking place at the same time. This activity will focus on potential food chains consisting of only six organisms. The two food chains in this activity will always start with diatoms and end with river otters. Diatoms are small, single-celled algae that live in water. The diatoms in this food chain rest on the bottom of the tidal creek, however many other types of diatoms float in the water. Diatoms use sunlight to make food through the process of photosynthesis. Since diatoms produce their own food they are called producers. Diatoms are the first stage, or trophic level, of these  food chains. The next link in the chain can be fiddler crabs or marsh snails. They are herbivores, or primary consumers. A herbivore, or primary consumer, is an animal that only eats plants. Fiddler crabs crawl around on the mud when the tide is out. They scoop up the mud and sift out the diatoms. Marsh snails crawl through the marsh and feed on diatoms and other algae. Marsh snails are prey to blue crabs. The next link in the chain is diamondback terrapins because they eat fiddler crabs and marsh snails, or blue crabs because they eat marsh snails. Terrapins and blue crabs are carnivores (they eat other animals) and, in this food chain, are considered to be secondary consumers. Secondary consumers are animals that eat herbivores. Diamondback terrapins are small turtles that swim around in the salt marsh. These turtles and blue crabs are both prey to river otters. River otters are considered to be tertiary consumers. In this food chain, they are also apex consumers. An apex consumer is the final link in a food chain. Some examples of possible food chains are as follows: diatoms- fiddler crabs- diamondback terrapins- river otters. Diatoms- marsh snails- diamondback terrapins- river otters. Diatoms- marsh snails- blue crabs- river otters.

     The terms "food chain" and "food web" are often easily confused. Although both are grounded in similar theory, a food chain is very different from a food web. A food chain is a simplified illustration of the predator-prey relationships between a few organisms within an environment. A food chain depicts the transfer of energy from trophic level (feeding level) to trophic level. A food web, on the other hand, shows the predator/prey relationships of many organisms in an environment. A food web is the elaborate, interconnected feeding relationships of who eats whom in an ecosystem. An illustration of a food web, with lines drawn between predators and prey, creates a web of relationships. In fact, a food web may have many different food chains embedded in it.

If we follow the energy that flows through a food chain, we discover that much of the original energy from the sun is lost from one step in the food chain to the other; thus the shape of an energy pyramid. The loss of energy starts with the plants. Plants are not very efficient at converting the sun’s energy into food. The sun provides solar energy to plants. Plants change this energy through the process of photosynthesis into food (starch). Plants are only able to convert 1-3% of the light that falls on them into food.

Animals are not very efficient either. They are not able to digest 100% of the energy stored in the foods they eat. A lot of the energy is lost via indigestible pieces and in the production of heat. In fact, the amount of energy that moves from one step in the food chain to another is only about 10-20%. Eventually the energy runs out. The food chain cannot go on forever. Some food chains can support up to five trophic levels, while others can only support two or three.

Because there is less energy available at each feeding level (trophic level), there are also fewer individual organisms at each level, as well. Therefore, there are fewer secondary consumers than primary consumers, and fewer primary consumers than autotrophs. Scientists often depict the flow of energy within a community or ecosystem as a pyramid of energy. The triangular shape represents two things:

1. As you move up the food chain, more of the original energy from the sun is lost
2. As you move up the food chain, there are fewer organisms at each feeding level (trophic level).

It takes a lot of energy and a lot of organisms at the bottom of a food chain to support a few organisms at the top.

South Carolina Aquarium Spotlight
Diatoms (class Bacillariophyceae)
Diatoms are very small, single-celled algae. Algae are aquatic plants that use the sun’s energy in the process of photosynthesis to create sugars for food energy and oxygen. Diatoms have an external skeleton made of silica that is divided in two pieces. The pieces can split and form two new diatoms, which is one way that they can reproduce. The shells of diatoms come in a variety of sizes and shapes and are quite beautiful in their designs. A microscope is needed, though, to see them.     

Fiddler Crab (Uca pugilator)
Fiddler crabs are small crabs, usually less than two inches in size, that are found predominately in the intertidal zone of the salt marsh. Fiddler crabs are easily recognizable, because the males have one very enlarged pincer, or front claw. They live along the sandy edges of salt marshes. They dig burrows up to two feet deep to escape predators and to protect them when  the high tide covers their burrows. They eat bacteria, diatoms (algae), and detritus. Their predators include blue crabs, terrapins, fish, raccoons, clapper rails and other marsh birds.  

Marsh Periwinkle (Littorina irrorata)
Marsh periwinkle snails are gray to white in coloration and grow to be 1 ¼ in long. They live in salt marshes on blades of Spartina grass. They eat detritus and algae, including diatoms, that is deposited on the grass by the tidal water. Their radula, which is similar to a tongue, can have up to 300 rows of teeth on it. They use the teeth to scrape algae and detritus off of the marsh grasses. Their predators include blue crabs and shore birds. Periwinkles are also eaten by humans as escargot. 

Blue Crab (Callinectes sapidus)
Callinectes means "beautiful swimmer" and sapidus means "tasty", so their scientific name means tasty beautiful swimmers. An adult blue crab can have a shell nine inches wide. Blue crabs have ten legs with the back two modified into paddle shaped swimming legs. Their claws are bright blue and their shell color is olive-green. Blue crabs live in brackish estuaries and salt marshes, but may wander into freshwater. They eat fiddler crabs, marsh periwinkles, shrimp, small fish, and animals that have died. Blue crabs are eaten by octopus, fish, gulls, herons, and humans. These crabs are a very important commercial species, the annual catch ranging from 20-80 million pounds.  

Diamondback Terrapin (Malaclemy terrapin centrata)
Diamondback terrapins are turtles that grow to be four to nine inches long and have shells with bold patterns of dark rings on them. They swim in the waters of salt marshes, and are the only turtles regularly found in brackish water. They can often be found sunbathing on a log or the banks of a marsh. These turtles eat crabs, snails, insects, fish and sometimes worms. Their predators include raccoons, crows and humans.  

River Otter (Lontra Canadensis)
River otters are mammals with an elongate body, webbed toes with claws and a thick coat of brown fur. They can grow to a length of three to four feet and a weight of 11-23 pounds. River otters live in aquatic habitats from coastal estuaries and lower river systems to mountain streams, where they eat fish, crayfish, crabs, turtles, amphibians and even bird eggs. Young otters are prey to bobcats and alligators. River otters can swim up to 12 mph and can hold their breath for four to eight minutes.