Purpose:
Florida represents the southernmost limit of the northern hard clam,
Mercenaria mercenaria, where
subtropical temperatures allow for a long growing season. However, growers across Florida have experienced chronic losses of market-size clams when summer water temperatures exceed 90oF. Climate change will certainly have an effect on worldwide agriculture; crops that are currently near climate thresholds, such as clams, are likely to suffer. There is a need for a heat-tolerant clam strain if the Florida industry is to reduce current summer mortalities and adapt to future climate change. In previous breeding studies (triploidy, hybridization), the project team demonstrated that thermal tolerance in clams is under genetic control. In both projects, we produced families from single-parent crosses, with parents selected at random from available broodstock. Some families consistently performed better than others. Our work suggests that heat shock proteins may be the mechanism whereby different strains of clams tolerate high temperatures. Heat-shock proteins (Hsp) are involved in the formation, transportation, and degradation of proteins. Some Hsp increase when cells are exposed to elevated temperatures, or other stressors that damage proteins, and are referred to as inducible. Other forms of Hsp are synthesized under non-stressful conditions for cellular housekeeping and are referred to as cognates. In one project, we found that a hard clam family having approximately twice as much cognate Hsp compared to two other families, also had significantly greater survival (93% compared to 28% and 39%) after heat challenge. In addition, other studies indicate that changes in metabolic rate in response to thermal stress may play a role in survival and could also be heritable. Together, these data suggest that using biomarkers of thermo-tolerance, such as Hsp, we can target particular genetically distinct groups for selective breeding, thus reducing the time and resources needed for strain development.
Approach:
The project goal is to increase summer survival and productivity of cultured hard clams in Florida and the Southeastern U.S. This will be achieved by identifying biomarkers of heritable thermal tolerance in hard clams for use in implementing selective breeding programs for heat-tolerance. The project objectives are to:
- Examine levels of three putative biomarkers of thermal tolerance in hard clam broodstock, offspring, and among offspring families;
- Measure putative biomarkers of thermal tolerance in hard clam families at different life stages; and,
- Determine survival, production, shelf life, and laboratory thermo-tolerance of hard clam families.
Adult hard clams from natural assemblages (“wild”) and aquaculture facilities are to be collected and individuals will be non-destructively sampled for cognate Hsp70 concentration. Three families will be produced from high-expressing parental stock and three families will be produced from low-expressing parental stock (single pair matings for each family). Thermo-tolerance (survival rates) of seed clams and growout clams from each of the six hard clam families will be determined in laboratory-based heat challenges (75 and 92ºF). Cognate Hsp70 concentrations will also be determined for seed, growout, and market-size clams. Oxygen consumption rates will be used as a proxy for standard metabolic rate. Oxygen consumption rates of broodstock and growout clams in each of the six families will be determined at temperatures ranging from 68 to 92ºF. Data will be used to determine the aerobic temperature threshold, the upper temperature at which the molecular mechanisms underlying oxygen consumption become disturbed. Finally, each clam family will be harvested for measurement of production characteristics (survival, shell length, shell width, total live weight, dry meat weight) and shelf life. We will statistically examine the effects of biomarker levels and family on clam survival and production characteristics.
Results to Date:
Adult hard clams were collected between October and December 2010. Potential broodstock (n = 550 total from 11 groups) were sampled for measurements of cognate Hsp70 concentration during January through April of 2011. We found that relative levels of cognate Hsp70 varied significantly among individual adult clams within the examined geographic or cultured groups and could be categorized as high, medium, and low (Figure). Therefore, we were able to select high- and low-cognate Hsp70 expressing broodstock for breeding; on average, the putative high Hsp70 broodstock had four times as much relative Hsp70 (based on absorbance value) compared to the low Hsp70 parents. Three families were produced from single-parent crosses of high-expressing parental stock and three families were produced from single-parent crosses of low-expressing parental stock in May 2011. Egg numbers ranged from 0.96-2.2 million and survival of larvae from day 2 to day 7 ranged from 43-84% for the low-expressing group and 78-98% for the high-expressing group. In September 2011, seed were transferred to field nursery systems on a commercial lease in the Indian River Lagoon located near Sebastian; triplicate bags of 8,000 seed were planted for each of the six families. Production of these families occurred too late to conduct the field nursery and subsequent growout and harvest during the natural heat stress of summer. Therefore, a second production spawn of putative high- and low-Hsp70 families was conducted in December 2011 and field nursed on an experimental lease in the Gulf of Mexico near Cedar Key during the summer of 2012. Final harvest of both replicate sets of hard clam families will occur in 2013. If Hsp levels in progeny are correlated with parental Hsp levels and high-Hsp families exhibit higher survival in the field and laboratory challenges, Hsp may be considered a biomarker for selective breeding of heat-tolerant hard clams.
A presentation on this project was made at an industry meeting and can be viewed below in PDF format.
Investigators:
- Shirley Baker, University of Florida IFAS Fisheries and Aquatic Sciences Program
- John Scarpa, Harbor Branch Oceanographic Institute at Florida Atlantic University
- Leslie Sturmer, University of Florida IFAS Shellfish Extension Program
Funding:
- USDA NIFA Special Research Grants Program, 2010 – 2013
- National Sea Grant Aquaculture Research Program, 2010 – 2013
