One of the most mysterious creatures of Chesapeake Bay is a microscopic parasite that infects oysters, called Perkinsus marinus.
Perkinsus is a tiny, single-celled animal – a protozoan -- that swims about the bay, propelled by a pair-of whip-like flagella.
When oysters suck in water and accidentally ingest one of these critters, the parasite burrows in and hijacks its host. Perkinsus uses the oyster’s body to multiply its own offspring, leaving the oyster pale, emaciated, and shriveled.
This chronic wasting disease, called Dermo, has been slowly killing between 10 and 20 percent of the Bay’s oysters in recent years. Fifteen years ago, it was much worse – with Dermo killing more than half of the Bay’s oysters.
While some parasites in the Chesapeake Bay are invasive species – transported here inside the ballast tanks of ships from Asia and elsewhere – scientists believe Perkinsus has always been in the Chesapeake.
So, if the parasite has always been here, why has Dermo only been wreaking havoc on the Bay’s oysters only in recent decades?
Denise Breitburg, senior scientist at the Smithsonian Environmental Research Center, recently published a study in the scientific journal PLOS ONE that shed some light on this mystery.
Breitburg and colleagues concluded that nitrogen and phosphorus pollution from farm and lawn fertilizer and other sources feed algal blooms that cause wild swings in oxygen levels in shallow parts of the Bay between the day and night. These night-time low-oxygen “dead zones” – which scientists call hypoxia -- do something to weaken the oysters immune systems, and made the oysters more susceptible to infection with Dermo.
“We definitely do have a link,” Breitburg said, “between hypoxia – which is strongly influenced and worsened by the amount of nutrient we are dumping into the bay – and the amount of oyster disease.”
Now, for many years, scientists have known about these low-oxygen dead zones on the bottom of deep parts of the Bay. But oysters don’t grow in the deep parts of the Bay. What Breitburg documented was a new kind of “dead zone”that appears only at night in shallow inlets, where oysters do grow.
“Basically, during the daytime, phytoplankton – plants in the water – are producing oxygen through photosynthesis,” Breitburg said. “But throughout the day, everything in the water is respiring, they are breathing like you and I would. And so they are consuming the oxygen – they are using the oxygen up – and they are producing carbon dioxide also. And so at night, when oxygen is not being produced, this respiration is what’s dominating the signal. And oxygen concentrations go down as things respire.”
These low levels of oxygen at night are associated with higher Dermo infection rates.
Mike Naylor, Director of the shellfish management program at the Maryland Department of Natural Resources, said there is another factor associated with Dermo infection and death rates in oysters – and that is the salinity of the water. The salt-water-dwelling parasite thrives in years when there is little rain, and therefore less fresh water in the Bay and more salt water from the ocean.
Naylor said that many fewer oysters are dying from the parasite in the last decade, compared to the more than 50 percent killed off between 1999 and 2002. But that lower death rate might not be because of less pollution.
“We are losing fewer oysters,” Naylor said. “However, it is important for everyone to remember that Dermo disease become very lethal when we have droughts – particularly prolonged droughts. And we have not had a significant drought in the Chesapeake Bay area in 11 years, which is exactly the amount off time since the last very significant Dermo mortality event.”
But several mysteries remain. Historically, did doughts cause die-offs among oysters in the Chesapeake Bay? Some researchers speculate that the Perkinsus parasite might have mutated and suddenly became more virulent in the 1980s and 1990s. In response to this change, are oysters in Maryland themselves evolving and developing resistance to the parasite?
If so, this might explain why oyster populations in the Bay – still extremely low, at perhaps one percent of historic levels -- have doubled over the last five years.
Solving these riddles will require more scientific detective work.
