Endangered giant clams in the Mediterranean Sea are falling victim to a deadly parasite that is spreading rapidly and puzzling scientists who are unsure of where the parasite came from and why it is so deadly.
Ian Hewson is a biological oceanographer working on the impacts of viruses on aquatic biogeochemistry. He says the parasite harming clams in the Mediterranean may have traveled through shipping, aquarium trade or seafood harvest.
“The mass-mortality of fan mussels in the Mediterranean has been associated with the protozoan parasite Haplosporidium pinnae, which belongs to a group of protozoa that also affects other mollusks, including oysters. While researchers have not, as yet, demonstrated pathogenicity (or the disease-causing ability) of the parasite, they make a good case for its association with the disease based on its presence in diseased tissues and absence in healthy individuals.
“The big question is: Why now? The parasite appeared on opposite sides of the Mediterranean (Greece and Spain) at different times. It is unclear whether the parasite exists in other areas in between where healthy clam populations exist - but if it is strongly associated with the disease and absent elsewhere, this may indicate transport by shipping, aquarium trade, or seafood harvest.
“Given its similarity to other known bivalve parasites (Haplosporidium nelsoni caused mass mortality of oysters in the Chesapeake during the 1950s), it is possible this is a parasite introduced from other regions where bivalves exist - and there they may not experience widespread disease. Likewise, there is little information available on other potential etiological agents which may have been at work, for example viruses or bacteria, which often work in concert with protozoa to elicit disease.”
Drew Harvell, professor of ecology and evolutionary biology, researches host-pathogen interactions and the sustainability of marine ecosystems. She says that scientists trying to better understand the parasite Haplosporidium pinnae might learn something from an oyster parasite that has been destructive on the U.S. East Coast.
“It is unusual that the parasite Haplosporidian pinna has now been detected as far east as Greece and is causing high mortality there. The scale of this epidemic and the virulence of the parasite is very bad news for conservation efforts for the clam species Pinna nobilis.
“The pathogen, Haplosporidian pinna is a newly described sporozoan protist. It is a congener of the very destructive oyster parasite on the U.S. East Coast, H nelsoni AKA MSX. However, it is most closely related to a congener that infects juvenile shrimp, Penaeus vannamei, in Central America. Not much is known about where this new species of H pinna came from or what triggered this massive outbreak. Perhaps clues from H nelsoni are useful in forecasting areas where H pinna will cause damage, in that H nelsoni is facilitated by warm temperatures and stopped by low salinity (below 15 parts per thousand). A report for the IUCN also suggests that H pinna also is facilitated by warming conditions.”