Seasonal variation of physiological parameters in the eastern oyster Crassostrea virginica from a tropical region of the Gulf of Mexico

Journal of Shellfisheries Research, August, 2009 by M. Gullian, L. Aguirre-Macedo

ABSTRACT Natural variation in the physiological variables of the eastern oyster Crassostrea virginica from a tropical climate have until now been poorly documented. This study was conducted to characterize the influence of hydrological variables (salinity, temperature, nitrate, nitrite, ammonium, soluble reactive phosphorus [SRP], and soluble reactive silica [SRSi]) on oyster physiological variables (total hemocyte cells (THC), reactive oxygen intermediates (ROIs), agglutination, lysozyme, and protein concentration). Oysters were collected during 3 different months (April, August, and December) from seven sites in Terminos Lagoon SW-Mexico. The association of physiological variables with oyster condition index (CI) was also studied. The physiological variability explained by environmental variables was high (30.7%) when compared with the variability explained by the condition index (8.9%). The independent effect of salinity explained 69.7% of the total variability in the physiological variables (F-ratio = 3.4; P = 0.002, Monte Carlo test with 499 permutations). Protein concentration was significantly correlated with spatial salinity variation ([r.sup.2] = 0.89; H = 27.01, P < 0.0001). Site effects were salinity-protein discriminated (F-ratio = 3.406; P = 0.002). Temperature was not a significant variable in the oyster physiological variability. The oysters from marine freshwater transition zones were less affected in their CI and physiological parameters than oysters from permanent high (>28 psu) or low salinity conditions (<10 psu). This study represents the first step toward integrating specific C. virginica physiological information from tropical areas.

KEY WORDS: oyster, Crassostrea virginica, physiological variables, salinity, protein, condition index

INTRODUCTION

Some of the physiological functions of Crassostrea virginica (Gmelin, 1791) include cellular and humoral hemolymph parameters. Hemocyte density and reactive oxygen intermediate species (ROIs) are related to phagocytosis and the respiratory burst (Anderson et al. 1992). Humoral parameters as serum agglutinins and lysozyme play a key role in binding and hydrolyzing nonself particles and microbes (Fisher et al. 1996). Although tolerant of a broad range of natural variables, C. virginica is susceptible to various forms of physical and chemical disturbances (Dekshenieks et al. 2000, Kimmel & Newell 2007), which cause extreme changes in their metabolism, physiology, and defense related-functions (Greenway & Storey 1999, Chu et al. 2002, Pernet et al. 2007). Historically, it has been emphasized that temperature is the most important environmental factor controlling oyster physiology (Galtsoff 1964). In temperate climate, seasonal variation of temperature drives the oyster reproductive cycle, affecting condition index, protein, lysozyme concentration, agglutination, and hemocyte density (Fisher et al. 1996, Mercado-Silva 2005, Wilson et al. 2005, Pernet et al. 2007). In tropical coastal climate the spatial and temporal variation of physicochemical parameters steeply changes between dry and rainy seasons. The seasonal change of nutrient concentrations (e.g., nitrate, ammonium, silica, phosphorus) is an important stress factor, which greatly influences the physiological condition and temporal abundance of nektonic and planktonic species (Yanez-Arancibia & Day 1988, Herrera-Silveira 1996). On this note, the study of oyster physiological adaptation from different climates in response to stressors, provides valuable information regarding inherent physiological capabilities (Chu 1999).

Although oyster fishery in the coastal lagoons of the southern Gulf of Mexico represents an industry of substantial size, ranking about sixth in the world (MacKenzie & Wakida-Kusunoki 1997), information linking environmental pressures on oyster physiological variables has been poorly documented. In Terminos Lagoon, the largest coastal lagoon in Mexico, several studies have characterized environment, focusing on geological origins, primarily productivity and nutrient variations (Yanez-Arancibia & Day 1988, David & Kjerfve 1998, Ramos-Miranda et al. 2005, 2006). Three high-flow rivers (Candelaria, Chumpan, and Palizada) provide most of the freshwater input to the lagoon with weak circulation (Yanez-Arancibia et al. 1983). Freshwater input changes seasonally from 59 [m.sup.3] [s.sup.-1] in the dry season to 401 [m.sup.3] [s.sup.-1] in the rainy period (Oczkowski, 2005), receiving high nutrient concentrations from the low lands of Tabasco, the Yucatan Peninsula, and the high lands of Chiapas and Guatemala (Bach et al. 2005). In this lagoon; salinity, nitrite, phosphorus, silica, and ammonium concentrations have been the main parameters associated to short-term seasonal changes influenced mainly by the river runoff (Ramos-Miranda et al. 2006). Many studies have addressed the themes of biological effects of pollutants, the reproduction cycle, and the parasite status of C. virginica (Rogers & Garcia-Cubas 1981, Martinez et al. 1995, Vazquez et al. 1995, Gold-Bouchot et al. 1995, Norena-Barroso et al. 1999, Zamora et al. 2003, Baqueiro-Cardenas & Aldana-Aranda 2007, Aguirre-Macedo et al. 2007) but no study has been made that focus on their physiological condition.

In this context, the aim of this study was to: (1) to identify the influence of interseasonal environmental changes on C. virginica physiological variables from a tropical Mexican lagoon and (2) to determine the physiological variability associated with the oyster condition index. Simultaneously, a multivariate analysis was used combining the hydrologic and physiological variables to test possible relationships between them.