Sensitivity of molluscs to temperature, osmotic shock, and infection by protozoa: Implications for temperate and polar bivalves

Though buffered by water, marine organisms contend with many of the same stressors that affect terrestrial life, including desiccation, thermal stress, and infection by parasites. These stressors are particularly important along the coastal margins of the oceans. This dissertation was directed at understanding the effects of stress on marine bivalves by investigation of biotic and abiotic stressors in the oyster Crassostrea virginica, the clam M. mercenaria, and the Antarctic clam Laternula elliptica .; To determine the risk of disease to shellfish stocks in Delaware's Inland Bays, prevalence of the bivalve pathogens Perkinsus marinus, Haplosporidium nelsoni, and QPX was monitored in oysters and clams sampled throughout the bays. P. marinus infection levels were measured with a novel real-time, quantitative PCR protocol that enables high throughput detection of as few as 31 copies of a ribosomal, nontranscribed spacer region in 500 ng oyster DNA. Though QPX and H. nelsoni infections were absent or at very low prevalence, P. marinus infections of oysters were present in 8.7% (6 infections, n = 58) of the oysters. All six infections occurred in established oyster populations (35% incidence, n = 17). The absence of P. marinus epizootics in the Inland Bays suggests that oyster densities are not yet high enough to initiate a lethal cycle.; The known range of the oyster pathogen H. nelsoni (MSX) extends from Nova Scotia to Florida. This study demonstrates that H. nelsoni is also present throughout the Gulf of Mexico. Thirty (73.2%) of 41 oysters sampled from western Florida to as far south as Venezuela were positive for MSX by PCR amplification of the ribosomal rRNA gene complex. Ribosomal DNA cloned from infected oysters was ≥99% identical to H. nelsoni and clearly divergent from known haplosporidian congeners. The absence of MSX epizootics in the Gulf of Mexico despite the wide distribution of H. nelsoni infections raises questions about its pathogenicity and the host-parasite relationship in subtropical latitudes.; Few data describe the sensitivity of Antarctic biota to cell volume stress. The impact of osmolality on the Antarctic clam Laternula elliptica respiration was assessed at three levels: mitochondrion, tissue, and organism. A microplate respirometry technique was adapted to simultaneously measure mitochondrial oxygen consumption and membrane potential in high throughput with a single fluorometer. Low osmolality medium stimulated respiration of gill and hepatopancreas mitochondria 1.5 to 3-fold. In contrast to euryhaline bivalves, non phosphorylating (state 4) mitochondrial respiration was more sensitive to cell volume stress than phosphorylating (state 3) respiration, depressing the respiratory control ratio (state 3/state 4), a measure of mitochondrial coupling, of gill mitochondria significantly (p = 0.025). Additionally, the cost of ADP phosphorylation in gill mitochondria increased under low osmolality, as indicated in reduction of the ADP:O ratio from 5.62 to 1.99 (p = 0.0012). While respiration of isolated gill mitochondria increases in response to hypoosmotic conditions, gill respiration dropped three-fold under lower salinity (26 ppt, p = 0.025). This suggests that L. elliptica gill exerts strong top-down control of mitochondrial respiration.; Mitochondrial physiology of intertidal bivalves must function when the organism is exposed to rapid, high magnitude shifts in temperature. To test whether Mercenaria mercenaria mitochondria are compromised by heat stress, mitochondrial physiology was investigated at 15, 18, and 21°C. Though phosphorylating and resting respiration rates were unaffected over this temperature range, the RCR decreased significantly above 18°C. The reduction in RCR, however, was not associated with a drop in the efficiency of phosphorylation (ADP:O). Effects of heat on mitochondria were also assessed by analysis of mitochondrial protein phosphorylation with SDS-PAGE. Three protein...