''Haplosporidium nelsoni'' is a

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...

s that originally caused oyster populations to experience high mortality rates in the 1950s, and still is quite prevalent today. The disease caused by ''H. nelsoni'' is also known as MSX (multinucleated unknown or multinuclear sphere X). MSX is thought to have been introduced by experimental transfers of the

Pacific oyster The Pacific oyster, Japanese oyster, or Miyagi oyster (''Magallana gigas''), is an oyster native to the Pacific coast of Asia. It has become an introduced species in North America, Australia, Europe, and New Zealand. Etymology The genus ''Mag ...

(''Crassostrea gigas''), which is resistant to this disease. MSX was first described in 1957, when it caused serious mortalities in Delaware Bay. Through extensive studies, an estimated 50% of

eastern oyster The eastern oyster (''Crassostrea virginica'')—also called the Atlantic oyster, American oyster, or East Coast oyster—is a species of true oyster native to eastern North and South America. Other names in local or culinary use include the We ...

(''C. virginica'') mortalities in lower Delaware Bay since the 1960s could be attributed to MSX infections, a devastating loss to the area. Mortalities are usually highest in the summer, and also increase in higher-salinity waters. MSX reduces the feeding rates of infected oysters, leading to a reduced amount of stored carbohydrates. The reduction in stored

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...

s inhibits normal

gametogenesis Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic di ...

in the spring, with a reduction in

fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...

Chesapeake Bay

In the spring of 1959, the MSX appeared in

Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...

. Within 3 years, more than 90% of the oysters in the lower bay were affected. Salinities of at least 15 ppt are considered necessary for infection with MSX, and salinities of about 20 ppt usually are necessary to elicit high oyster mortalities. The presence of ''H. nelsoni'' in the Chesapeake Bay has hindered attempts to return eastern oyster harvests to historic levels. Particularly severe has been the effect on oyster aquaculture, which was not practiced on a large scale in Chesapeake Bay for 25 years following the introduction of the parasite, and currently is confined primarily to the low-salinity areas of the bay.

Resistance

After more than 50 years of selection pressure, some populations of ''C. virginica'' have become resistant to the MSX. They are able to tolerate the infection for longer before dying. Artificial selection is able to achieve a similar result.

References

Citations

Other sources

*''Who Killed Crassostrea virginica? The Fall and Rise of Chesapeake Bay Oysters'' (2011), Maryland Sea Grant College (60 min. film)

External links