Haplosporidium nelsoni
Encyclopedia
Haplosporidium nelsoni is a pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

 of oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

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 (multi-nucleated unknown). MSX is thought to have been introduced by experimental transfers of the Pacific oyster
Pacific oyster
The Pacific oyster, Japanese oyster or Miyagi oyster , 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 :...

 (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 oyster 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 months, 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
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s inhibits normal gametogenesis
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 division of diploid gametocytes into various gametes,...

 in the spring, with a reduction in fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

.

Chesapeake Bay

In the spring of 1959, the MSX appeared in Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

. Within three 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.

Other sources

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