He told The Associated Press that officials have estimated that hundreds of thousands, possibly even millions, of mussels have died in the Clinch River.The mass die-off in the river of one kind of mussel, called the pheasantshell, has been especially worrisome.Officials say the population of pheasantshells dropped from 94,000 in 2016 to less than 14,000 in 2019.They measured the decrease along a 200-meter stretch of the Clinch River.Similar die-offs have been reported on at least five U.S. rivers and another in Spain.Richard has studied reports of similar die-offs over the years in rivers around the world.But so far he has not come up with many answers about why they happened.Speaking about the Clinch, Richard said the river could even be compared to the Amazon because it contains so many different kinds of life.He spoke to the AP while examining the river's mussel population in a rural community in Tennessee.Mussel populations around the world have dropped sharply over the past 100 years.Scientists blame this on pollution, habitat loss and climate change. But they suspect the most recent decreases have another cause.Richard and his team believe an infectious disease could be responsible.The team is comparing healthy pheasantshell mussels to dying ones in an attempt to narrow down a list of possible diseases."All living things are chock-full of microorganisms, and we don't have any sort of map for what is healthy inside a mussel," Richard said.University of Wisconsin disease specialist Tony Goldberg is helping with the investigation.He specializes in wildlife illnesses of unknown cause.Goldberg told the AP that often disease is the final thing to kill a species that has already been harmed by other environmental conditions.However, he says he is hopeful that the freshwater mussel team will be able to discover the cause of the massive die-offs and find a way to prevent them.The team includes scientists from the U.S. Geological Survey and a nonprofit conservation group."I see it as a race against time, not an impossible task," Goldberg said, adding "...if we lose these mussels, the rivers we all love are never going to be the same."I'm Bryan Lynn.The Associated Press reported on this story.During 2019, we covered some big developments involving computer and device technology.We also reported on growing opposition to facial recognition systems and warnings by the inventor of the World Wide Web about the internet's future.Google announced it had completed successful experiments that led to a major development in quantum computing technology.In one test, the team said its quantum computer processor was able to complete a complex mathematical problem in just 200 seconds.The scientists said the same problem would have taken the world's fastest supercomputer 10,000 years to finish.The researchers described the new progress as "quantum supremacy."This term describes a point at which a quantum computer can perform a calculation that a traditional computer could never complete within its lifetime.Israeli researchers reported they had built the world's first 3D-printed heart using a patient's own cells.