Thurlow Nelson was seeing jello-like blobs, dozens of them, each one stuffed with dark, round dots.

Sitting at his microscope, he was squinting at oyster tissue riddled with some kind of unnamed parasite. The blobs, roundish and irregular, seemed to be single-cell plasmodia, and the dots seemed to be nuclei, but Nelson could not identify the mystery killer, and neither could Harold Haskin, his protégé and now the director of the New Jersey oyster laboratories.

The mystery only deepened when they called in other experts. Leslie Stauber from Rutgers was a leading authority on parasites, and John Mackin from Texas A&M had looked at more oyster slides than anyone else in the world. Both pored over the new slides and paged through the research literature, pulling out photographs and drawings, hunting for a match. Both searched their memories, and in the end both made the same report: they had never seen this organism before.

What they could see was the route of infection. The parasites invade through the gills, sucked in as the oyster feeds. Once in the gills, they divide and multiply rapidly, eventually breaking through to the circulatory system and moving easily throughout the body. With its tissues overwhelmed and its nutrients absorbed by the parasite, the oyster soon shrinks and dies.

Even Walt Canzonier, the grad student, got his turn at the scope. "The plasmodium was spherical, and it was full of nuclei," says Canzonier." We were sitting around looking at this parasite. And we didn't know what it was."

Since the new parasite needed a name, lab director Haskin gave it one. "Well, it's obvious," he told the scientists gathered around the microscope.

"This is multi-nucleated sphere X. Or MSX." The X stood for origin unknown, and the name stuck because the mystery stuck.

Gene Burreson swings a small oyster dredge over the side of the research boat and splashes it down towards the bottom of the James River. The research boat guns forward, the line starts vibrating, and the dredge digs into the unseen oyster reef below.

The biologist and his crew are now working the Horsehead oyster grounds just downstream from the Ghost Fleet. Scientists like Burreson have been coming back to these James River reefs every year for nearly half a century now, hoping to find evidence the dying is over.

Burreson is one of a second generation of oyster scientists to tackle the mysteries of MSX, and his assistants will be part of a third generation. Early researchers at the Rutgers shellfish laboratories were able to classify the parasite as Haplosporidium nelsoni, a protozoan with at least two life history stages: spores and plasmodia. Over the decades they would document how warm winters, drought years, and high salinities helped spread MSX and how cold winters, rainy years, and low salinities helped slow it down. By selecting and breeding MSX survivors, scientists even made good progress in developing disease-tolerant oysters in the lab.

Certain tough questions, however, remain unanswered, passed down to Burreson's generation. What are the other life stages and where are they? Scientists think there is more to MSX than spores and plasmodia, the two life stages they can see in oysters. Those other life stages probably occur in another organism, an alternate host that carries MSX around and releases it where it can infect oysters. Could it be carried by copepods floating in the plankton, worms in the sediment, even fish or crabs? There are dozens of possibilities and no answers to the question (see The Missing Link).

While researchers have wrestled with these questions over the last half century, MSX has been depopulating the oyster grounds of Chesapeake Bay, especially during drought years. In the last decade and a half, a second disease called Dermo — caused by another protozoan parasite, Perkinsus marinus — also exploded through the Bay's dwindling oyster populations (see The Culture of Disease, below). The Chesapeake lost bottom reefs, a major habitat for biodiversity, as well as millions of filter-feeding oysters, a major force for water clarity. By the early 21st century, 99 percent of the oysters were gone and the ecology of the Bay had changed dramatically.

As the small dredge rises, dripping, out of the river, Burreson and a young, red-haired lab assistant swing it on board and dump its contents. Like rocks hitting a boardwalk, dozens of oysters thud across the culling board. Burreson and his crew tug on gloves and start digging through the pile with knives and hammers.

Cracking apart a clump of oysters, Burreson holds open two interior shells, each covered with a brown, scum-like biofilm. He points to several barnacles growing inside the shell. "This is an oyster that's probably been dead for six months," he says, flipping it overboard. "Maybe a year or more."

He taps an oyster with his knife. "It looks perfectly healthy," he says, but the hollow-sounding knock tells him otherwise. Cranking it open, he finds no meat and no biofilm, just two shiny, pearl-white shells, "which means this is a real recent mortality, probably within the last week or so. It's an indication that there is a mortality going on up here at Horsehead," he says. "The salinity is just high enough that it could be from MSX." Dead oysters have a tale to tell, and the moral of this tale is no surprise: after nearly 50 years MSX is still killing oysters in Chesapeake Bay.

As Burreson scrabbles through his piles of oysters, he keeps looking for live adults. Gapers and boxes and small oysters go back overboard, but his keepers go into a small bucket for the boat ride back to the lab. Adults usually carry heavy loads of parasites, and Burreson has been pioneering a technique for analyzing the MSX samples, a technique never dreamt of back in 1957 when MSX first showed up.

Live oysters, it turns out, also have some tales to tell — and one of the tales comes with a surprise ending, an answer to the enduring mystery: where did this killer parasite come from?

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