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Imperiled Clam "Good Species," Deserves ESA Protection

A scientific report released August 7 by the Center for Biological Diversity and Defenders of Wildlife confirms that a clam species found at the mouth of the Colorado River in Baja California occurs nowhere else in the world. The report's conclusions will likely result in Endangered Species Act protection for the clam, whose population hovers at five percent of its historic numbers.

The report's authors are Dr. Karl Flessa, Professor of Geosciences and Ecology and Evolutionary Biology at the University of Arizona, and Miguel Agustin Tellez-Duarte, Professor of Marine Sciences at the Autonomous University of Baja California at Ensenada.

The clam, Mulinia coloradoensis, was first described as a new species early in the 20th century. In the intervening years, however, some taxonomists had questioned the species' validity, suggesting that other Mulinias found in the Sea of Cortez may belong to the same species. "It's a dispute among taxonomists," Flessa told Faultline. "As far as I know, no one disputing the species' validity had an environmental agenda. We examined the type species of both Mulinias in the Smithsonian and the British Museum, and confirmed that M. coloradoensis is a good species."

M. coloradoensis was once the most abundant mollusk in the Colorado River Delta estuary, with billions of individual clams literally making up the terrain. "There are a number of islands and beaches in the delta that arealmost entirely made up of the shells of this species," Flessa said.

Previous studies indicate that the species was restricted to the brackish water of the estuary's "mixing zone," which once extended forty miles south of the current delta. The building of Hoover Dam in the 1930s, and the construction of Glen Canyon Dam thirty years later, severely curtailed freshwater flows into the Lower Colorado, as most of the river's flow was diverted to feed urban taps and irrgation ditches north of the border.

With far less fresh water to adjust the delta's salinity, the species has declined precipitously. Flessa's research team has found just 12 live Mulinia coloradoensis in the Delta since 1992. The loss in abundance of the clam, and a general decline in the health of the estuary, poses a threat to other endangered species in the area, among them the California clapper rail.

Read Environmental Defense's report on Colorado Delta restoration.

The Lower Colorado River MultiSpecies Conservation Program has repeatedly rejected calls from conservationists to consider the effects of USdams and diversions on the river's delta ecosystem in Mexico.

The Centro de Estudios de Almejas Muertas (Center for the Study of Dead Clams) is a good resource for Colorado Delta information.

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published Aug. 10, 2001