In a scientist’s world, most scientific knowledge is usually considered temporary. As scientists continue to study a topic, they learn information that either confirms or updates previous knowledge. In this research, for example, the scientists found that knowledge previously held about crab diets was not entirely correct (figure 1).

Are you surprised to learn that understanding crab diets is important? Studying crab diets is important because such knowledge informs how scientists understand the carbon cycle. (You will learn about the carbon cycle in “Thinking About the Environment” below.) Doing research to confirm or update existing knowledge is one of the critical roles of science.

In your own experience, think of something that you thought was true and was later revealed to be false. Being open to new knowledge is important. When you are open to being challenged about what you know, you are thinking like a scientist.

Carbon is an important part of our world. Carbon is an element that is found in water, soil, plants, animals, and the atmosphere. Humans contain carbon too. About 18 percent of your body is carbon!

Carbon is one of the most abundant of Earth’s elements. Humans and other animals get carbon from eating plants and from eating animals that eat plants. A plant contains carbon as long as it lives and until it is eaten, completely decays, or is burned. Plants get carbon by taking in carbon dioxide (CO2). When the plant takes in CO2, it keeps the carbon and releases the oxygen.

In wetlands, much of this carbon is used for belowground root growth in sediments. Little oxygen is found in wetland sediments because all the spaces between sediment particles and roots are filled with water, not air. Microorganisms that would break down these roots have a hard time breathing and as a result, belowground roots decay slowly. Therefore, the carbon from these roots can be stored in wetland sediments for thousands of years. Carbon that is held in wetland sediments is known as blue carbon. Blue carbon ecosystems include salt marshes, sea grasses, and mangrove ecosystems (figure 2).

In this research, the scientists were interested in better understanding the role of decaying mangrove leaves in sesarmid (sǝ sär mid) crab diets. By increasing their understanding, the scientists gained more knowledge of how carbon moves throughout the mangrove ecosystem (figure 3).

Sesarmid crabs are a family of crabs found in tropical coastal mangrove forests worldwide (figure 4). During high tide, these crabs climb onto the mangrove trees or hide in their burrows to escape from predators (figure 5).

Sesarmid crab predators include large fish that swim into the mangroves at high tide. At low tide, when the mangrove forest floor is exposed, the crabs feed on leaves and mud on the forest floor (figure 6).

Sesarmid crabs are an important part of the mangrove forest ecosystem (figure 7). By eating decaying mangrove leaves and microorganisms that live on the sediment’s surface, crabs play a role in the carbon cycle. When the crabs eat the leaves and microorganisms, the carbon is transferred to the crabs (figure 8). When the crabs eat the leaves, other nutrients are also released that can be recycled by mangrove trees and plants. Additionally, when other animals eat the crabs, carbon is transferred from the crabs to the crabs’ predators.

Sesarmid crabs also contribute to the mangrove ecosystem by building complex burrows in the mangrove sediment that covers the forest floor (figure 9).

These burrows function as pathways for the transport of decaying or digested organic matter. When sesarmid crabs build and then enter their burrows, they pull leaves into their burrows and store them (figure 10).

In the burrows, bacteria and fungi begin to grow on the leaves. The bacteria and fungi make the leaves more nutritious and appealing to eat.

The burrows created by sesarmid crabs may also be occupied by other small animals. These burrows provide a constant and safe environment that increases diversity in the mangrove ecosystem.

Many scientists have believed that single-celled microorganisms, such as algae and cyanobacteria, are the largest part of sesarmid crab diets. These microorganisms are found on the surface of the mangrove forest floor’s sediment.

At the beginning of their study, the scientists began to investigate how typhoons, which may also be called hurricanes, impacted the importance of microorganisms in sesarmid crab diets. The scientists discovered, however, that instead of microorganisms, crabs appeared to be mostly eating decayed mangrove leaves.

Therefore, the scientists posed an additional research question for their study. The scientists wanted to discover whether the mangrove leaves were a more important food source than scientists previously thought for this type of sesarmid crab.

The scientists studied sesarmid crabs living in the mangrove forests on the coasts of Yap and Babeldaob (bä bǝl daůb) Islands (figure 11).

To begin the research, the scientists collected samples from mangroves in Yap that had been impacted by a typhoon. The scientists collected individuals of a particular species of sesarmid crab called Parasesarma (pär ǝ sǝ sär mǝ) (figure 12). This crab species was easy to find and capture.

The scientists also collected decaying leaf samples from the Rhizophora (rīz ǝ fȯr ǝ) species of mangrove tree (figure 13). The scientists had observed the Parasesarma crabs eating the leaves of this mangrove species. The decaying leaves were collected from the forest floor (figure 14). Sediment samples containing single-celled algae and other microorganisms were collected by scraping and removing the top 1 centimeter (cm) of sediment. Algae were also collected by gently using filtered seawater to rinse downed wood. Downed wood is wood that has fallen to the forest floor.

The scientists took tissue samples from the crabs. They used an isotope ratio mass spectrometer (spek trä mə tər) for their next step. This equipment enabled the scientists to determine the ratio of carbon-13 to carbon-12 isotopes. Isotopes are different forms of the same element. A ratio analysis was done on the crab tissues, the decaying leaves, and the microorganisms. (Read more about carbon isotopes below.)

After evaluating the samples from Yap, the scientists collected samples on Babeldaob. They captured Parasesarma crabs and collected decaying mangrove leaves from the forest floor. Unfortunately, the scientists were unable to collect enough microorganisms from the sediment on Babeldaob.

The crabs and leaves were taken to a laboratory in Palau. The crabs were placed in plastic containers and allowed to sit for 2 days so that their stomachs were empty. Then, a decaying mangrove leaf was added to each container and replaced every 3–4 days (figure 15). The crabs were kept in the containers for 44 days. On days 0, 1, 7, 17, 21, and 44, crab tissues were collected from the crabs.

Every living and once-living animal and plant contains carbon. The scientists used special equipment to measure and compare the amount of carbon in the crabs and the mangrove leaves. When scientists compare the amount of carbon-13 (13^C) with the amount of carbon-12 (12^C) in tissues, they are calculating a value called delta carbon-13, or ∆13^C. The “∆” symbol is the Greek letter for delta. In science, this symbol implies that something has changed.

When an animal eats a plant or another animal, they can gain a little bit more 13^C than was present in the plant or animal that they ate. Scientists call this enrichment. Understanding the amount of enrichment that occurs when an animal eats something helps scientists to determine the importance of the food in the animal’s diet.

In food chain studies, scientists have considered the amount of enrichment that occurs between an animal and its food source to be the same for a variety of species and ecosystems. In past studies of sesarmid crabs, scientists used enrichment values determined by other studies. The scientists in this study wanted to determine the enrichment value for sesarmid crabs in the Yap and Babeldaob ecosystems instead of using values from other studies. The scientists thought that using a value from other studies might produce incorrect information about crab diets in these mangrove ecosystems. By calculating a ∆13^C value for sesarmid crabs in Yap and Babeldaob, the scientists were able to accurately calculate the enrichment value for sesarmid crab diets.

The scientists entered different enrichment values into a computer program. This program estimated what percentage of the sesarmid crabs’ diet was decaying mangrove leaves. The scientists ran the computer program using enrichment values from other studies and their current study. Running the program with different enrichment values enabled the scientists to compare their results with results from other studies.

The scientists discovered that the 13^C enrichment values for crabs eating mangrove leaves in the laboratory were higher than the values used in the earliest studies (table 1).

Recall that the scientists used a computer program to determine the relative importance of mangrove leaves in the crabs’ diet. The scientists used enrichment values from their study first. Then, the scientists ran the program using enrichment values from other studies that were not for sesarmid crabs but that also have been used in previous sesarmid crab food chain studies (table 2).

The scientists discovered that previously published 13^C enrichment values might underestimate the value of decaying mangrove leaves in sesarmid crab diets. This finding is important for two main reasons.

First, this finding suggests that scientists should determine 13^C enrichment values for unique species and ecosystems when conducting food chain studies. At the least, available enrichment values from the same type of animal (or related animal) should be used. Scientists should not simply use the 13^C enrichment values determined in other studies regardless of the animal being studied. Using previously determined enrichment values from studies of different animal species may result in an incorrect assessment of the importance of food sources in an ecosystem’s food chain.

Second, this research suggests that decaying mangrove leaves are more important in sesarmid crab diets than previously thought. This finding helps ecologists to better understand how carbon is transferred in a mangrove ecosystem. This finding also helps to clarify the role of mangrove trees within the mangrove ecosystem.