Infection of the hard clam M. mercenaria by the protist QPX (Quahog Parasite Unknown, Fig. 4) has been known since this parasite emerged in 1989 in Prince Edward Island, Canada, and in 1991 in Massachusetts.
The parasite was probably present along the Atlantic coast much earlier. Its southward distribution extends to Virginia, where it has caused occasional mortality.
QPX infections are usually localized at the mantle and gills, where various cell forms 2 to 48 µm in diameter, typically surrounded by clear halos, can be observed disrupting connective tissues.
Infections can become systemic and sometimes result in serious mortality. Eighty percent of the clams in some affected beds have been lost.
It is not certain how QPX infections arise. Labyrinthulid protists, the group to which QPX belongs, are ubiquitous saprophytes, meaning that they feed on dead or dying material.
It is possible that QPX is a typical saprophytic labyrinthulid that, under certain conditions, also functions as a pathogen. That is, it may be capable of living externally in the environment or internally as a pathogen in clams.
While we do not completely understand the cause of QPX outbreaks, clam culturists in Virginia avoid exceptionally high densities, a potential source of stress on the clams, and prevent excessive fouling of nets because the fouling algae are a potential environmental source of the parasite.
It is possible, but not certain, that warm water temperatures limit the southward distribution of QPX. It does appear that estuarine salinities are inhospitable to the parasite, as QPX has rarely been observed in Chesapeake Bay. As QPX is not broadly distributed within the region, local clam stocks south of Virginia or in waters below 20 ppt may never have been exposed to QPX.
Indeed, clam stocks originating south of Virginia are very susceptible to QPX when grown in northern waters. For this reason, clams from QPX-enzootic waters, which presumably are resistant to QPX disease, should be used in areas where QPX occurs, even if seed is to be produced in more southern clam hatcheries.
Introduction of exotic diseases:
The arrival of exotic molluscan pathogens is impossible to predict. The likelihood of a pathogen introduction may have decreased somewhat in recent decades because the deliberate transplantation of exotic molluscan hosts has slowed and surveillance for disease has improved.
Yet the risk of an accidental introduction via shipping traffic (hull fouling, ballast water, etc.) persists, and other vectors have become apparent, such as the import of exotic molluscs for the ornamental trade.
The parasite Perkinsus olseni was recently detected in Florida in giant clams imported from Vietnam for ornamental use. This raises concerns that the release of unwanted aquarium species by hobbyists could bring dangerous pathogens to the region.
Careful disease management by the ornamental fish and shellfish industry is imperative. With the rejection of efforts to introduce the Suminoe oyster Crassostrea ariakensis to Virginia waters, it is unlikely that an exotic mollusc will again be cultured in open waters of the Southeast anytime soon.
Nonetheless, hatchery work with non-native species may continue or expand for the international sale of larvae and seed. Imported, exotic broodstocks should be carefully quarantined to prevent the introduction of exotic pathogens, including herpesviruses.
Aquaculture health management:
While some aspects of aquaculture health management are specific to certain disease problems, others are universal. At the farm level, health management begins with creating conditions that minimize stress on cultured animals and with attention to site selection, farm organization, planting densities, and routine husbandry to ensure good nutrition and growth.
It is important to use genetic strains of oysters and clams that exhibit good growth and survival under local conditions. Cultured molluscs should be monitored for unusual reductions in growth, condition or survival. If any of these situations occurs, pathological examinations should be performed to identify problems such as emerging disease at the earliest stage.
At a broader level, the diversity and distribution of pathogens should be determined so that disease-enzootic regions can be better defined. Stocks destined for transplantation should be screened to ensure that diseases are not spread to new areas.
The International Council for the Exploration of the Sea (ICES) Code of Practice on the Introductions and Transfers of Marine Organisms describes procedures for safe transplantation. Where there is a risk of introducing a disease, culturists are advised to plant only certified disease-free progeny produced in quarantine from introduced broodstock.
New fast-growing, disease-resistant oyster and clam stocks should be developed for regional use, and the natural diversity of aquaculture species should be conserved. It is the diverse wild populations that are capable of evolutionary response to new disease and environmental challenges, and from which new aquaculture lines may continually be developed.
Finally, the diversification of the aquaculture industry to include new local species should continue. An industry with a broader base of cultured species should be better able to withstand the effect of disease on any one.
Author:
Ryan B. Carnegie
