Study analyzes toxic metals found in crabs

Research conducted on mangrove crustaceans along the Brazilian coast reveals details about contamination and paves the way for new environmental control actions.

By Fabio Reynol

Agência FAPESP – A research study conducted at the Biosciences Institute of the University of São Paulo (IB/USP) has collected important data about ambient pollution in studies of the mangrove crab (Ucides cordatus), a mangrove crustacean found along the coast of São Paulo State.

Coordinated by researcher Flavia Pinheiro Zanotto of the Department of Physiology at the Biosciences Institute of USP, the project, called “Characterization of cadmium transport and its interaction with calcium on epithelial cells of Ucides cordatus, a mangrove crab,” received a Regular Research Grant from FAPESP and has revealed important information about ambient pollution and the contamination of these animals.

The study focused on analysis of the traces of cadmium found in the crabs. “The animals generally do not use this metal for any biological function; that’s why we would like to know how it gets into the cell in the absence of a specific transporter to introduce it,” Zanotto said.

The answer, she explained, is that cadmium is used by calcium transporters to penetrate the cell structure. Cadmium and calcium have similar divalent ionic radii, which makes them competitors for certain transporters when they are together; this activity may be extrapolated to situations in which they are present in the environment.

The researchers found that when the crabs were simultaneously exposed to calcium and cadmium, the metals did not compete to enter the cell, but when the animals were administered a calcium inhibitor, cadmium absorption decreased.

Zanotto said, “This paves the way for new environmental control actions such as introducing into the environment harmless ions that have transport priority over cadmium, causing them to be absorbed in place of the toxic metal.” She explained that in calcium-rich environments, for example, animals will be less susceptible to the absorption of metals such as copper, zinc and cadmium.

Calcium is an essential element for crabs. Cadmium, however, is a potentially toxic metal found in batteries and is an ambient pollutant. In previous studies carried out in mangroves along the coast of São Paulo, Zanotto detected the presence of toxic metals in animals as well as in the plants that served as their food.

“The amount detected was within the limits of Brazilian law, with the exception of chromium,” Zanotto said, on the basis of the findings of another study conducted in partnership with researcher Marcelo Antônio Amaro Pinheiro from the São Vicente campus of the Universidade Estadual Paulista (Unesp).

More cadmium in the cells

The previous study entailed collecting animals from mangroves along the southern coast of São Paulo to analyze the so-called epithelial exchange cells such as those of the gills, hepatopancreas and antennal gland, a structure analogous to the human kidney. These sites regulate the passage of ions from the external medium and are therefore of particular interest to researchers.

A fluorescent marker for cadmium was used to measure the quantity of the metal. The marker penetrates the cell and binds to the metal, generating fluorescence. If the cell takes up more cadmium, the marker increases in fluorescence, the precise amount of which is detected through the use of a fluorescence reader.

When comparing the animals collected in different regions, the researchers discovered that crabs from Itanhaém had an easier time acquiring cadmium because Itanhaém is a region with high rates of metal contamination, which rendered the animals more susceptible than animals collected in the region of Jureia, where there are lower concentrations of this contaminant.

“This means that the more cadmium is found in its cells, the less the animal is able to handle the metal and the more susceptible it will be to the contaminant. In other words, more cadmium is absorbed by animals that live in a region that is already contaminated,” said Zanotto.

Despite these findings, the U. cordatus crab did not appear to be a good biomarker of ambient pollution. Zanotto explains that because this animal has a fairly long life (10 years on average), it could be expected to carry a history of ambient contamination.

“But it managed to have relative success in eliminating the metal. A good part is concentrated in the shell, which is shed periodically and thus does not reliably reflect ambient pollution,” she said. This characteristic is more common in filter feeding animals such as oysters and mussels that accumulate what is present in the environment.

The research resulted in the chapter “Cellular Cadmium Transport in Gills and Hepatopancreas of Ucides cordatus, a Mangrove Crab,” published in the book Crabs: Anatomy, Habitat and Ecological Significance (edited by Kumiko Saruwatari and Miharu Nishimura. Hauppauge, NY: Nova Science Publishers, pp. 107-122).

The study also provided the basis for the master’s thesis entitled “Cadmium transport in exchange cells of the Ucides cordatus mangrove crab,” by Priscila Ortega, presented at the IB/USP in 2012.

Zanotto said, “Among the major contributions of this project is information about the protective role of calcium in preventing the absorption of toxic metals and the cumulative effects of the cadmium that are accentuated to the extent that the animal is exposed to the contaminant.”

“Since these crabs are consumed by humans, it is extremely important for us to know the level of contamination found in them,” she added.

Recently, the researcher also published the article “Characterization of copper transport in gill cells of a mangrove crab Ucides cordatus” in the journal Aquatic Toxicology.

The article presents the findings of another study, which showed how another toxic metal, copper, penetrates the gills of crabs in the mangroves of São Paulo. Like cadmium, copper interacts with calcium in the cells of the gills. The article may be accessed at: www.sciencedirect.com/science/article/pii/S0166445X13002877.