GENOMIC ADAPTATION OF SHELLFISH LARVAL DEVELOPMENT AND HOST DEFENSE TO STRESS OF OCEAN ACIDIFICATION  

Wei Xu
Aquaculture Research Station
Louisiana State University, Agricultural Center
Baton Rouge, LA 70820
wxu@agcenter.lsu.edu

Early-life history stages of shellfish species, including gametes, embryos and larvae are vulnerable to environmental stressors. In most cases, less than 10% of larvae will be successfully recruited into the adult population. Many factors attribute to the large mortalities in early life stages of shellfish including over-consumption of energies carried in embryos and environmental stresses which are significantly affected by climate change and anthropogenic activities. As an outcome of human activities in energy consumption, the global atmospheric concentration of CO2 has been dramatically increased since the industrial revolution and has grown more rapidly in past decades. As a result, the pH value of the ocean surface has been significantly drop due to increased absorbance of CO2 from the atmosphere.

Although a few studies revealed the potential impact of ocean acidification on larval stages of oysters, efforts in understanding the mechanism of this impact are very limited. Several critical developmental steps, along with these significant morphological changes, are involved in the oyster life cycle following gamete fertilization. The first key event starts within 24 hours post fertilization when the shell begins to form. Formation of the shell provides physical support and protection for oysters as well as a sealed gill-chamber for filter feeding. Additionally, the shell provides carbonate to buffer the pH value when environmental pH decreases. Another critical event involved in oyster larval development is settlement, which happens two to three weeks post fertilization (Fig. 1). Bivalve larvae must undergo settlement before they can successfully develop into the adults. Because of the great energy consumption needed by shell formation and larval settlement, major mortality can be observed after this process. Very limited work has been done to study the effect of ocean acidification on those critical morphological changes during bivalve larval development. Earlier studies demonstrated the disturbance of bivalve shell calcification and larval calcification with stress of acidification which is caused by increase of dissolved CO2. However, genomic responses of larval bivalves to this stress remains unclear.

Among a large number of bivalve species, oysters (Crassostrea spp) have been widely studied as valuable species in worldwide aquaculture. Recent investigations in genomics of oysters generated a huge number of genomic data, which are of great importance in basic research of bivalve development as well as selective breeding of oyster strains in aquaculture. In our study, the Eastern Oyster (Crassostrea virginica) was used as a model because of its economic importance in the United States. Historically the Eastern Oyster accounts for 86% of total oyster harvest in U.S. Leading the nation in oyster production. The acidification of environment was simulated in laboratory conditions with input of CO2 (pH 8.2 with ~200 atm pCO2, pH 7.7 with ~1000 atm pCO2, and pH 7.2 with ~2800 atm pCO2).  Fertilized oyster gametes were kept in these pH conditions and the larval development was monitored. Genomic changes during larval development were captured by next generation sequencing (NGS).  Identification of specific gene regulation pathway related to shell formation and settlement under the pressure of acidification will be performed to reveal the molecular mechanism of delay of oyster larval development caused by ocean acidification. Specific genes related to larval response to acidification stress will also be used as molecular markers to evaluate the growing conditions of larval oysters and to optimize environmental factors for oyster production.