Defense System of the Manila Clam Ruditapes philippinarum under High-Temperature and Hydrogen Sulfide Conditions

SIMPLE SUMMARY: During evolution, marine bivalves developed physiological and behavioral strategies to cope with stress. However, the role of behavioral strategies is unclear when the physiological strategies of bivalves contradict behavioral survival and environmental stress. This study presents the effects of high-temperature and hydrogen sulfide conditions on the survival and defensive strategies of the Manila clam Ruditapes philippinarum. The results show that both physiological and behavioral strategies play an important role under stress conditions, but the defense system and response strategy of the Manila clam to cope with H(2)S changed with the temperature. This study aims to achieve an understanding of the relationship between the physiological response, behavioral characteristics, and survival of the Manila clam under stressful conditions, and to provide useful information for the culture of the Manila clam. ABSTRACT: Hydrogen sulfide (H(2)S) acts as an environmental toxin. Despite its toxicity, little is known about the defense strategies of marine bivalves against it. Thus, the tolerance, behavioral characteristics, and physiological response strategies against H(2)S treatment in the sentinel organism Manila clam Ruditapes philippinarum were examined. We monitored the survival and behavioral status of Manila clams exposed to different combinations of temperature and H(2)S. The physiological response strategies were examined by measuring the enzymatic activity of cytochrome C oxidase (CCO), fumarate reductase (FRD), superoxide dismutase (SOD), and catalase enzymes (CAT). Moreover, adverse effects of H(2)S on the tissue and cell structure of Manila clams were also examined under a transmission electron microscope. Manila clams responded to H(2)S stress through behavioral and chemical defenses. With exposure to H(2)S alone, Manila clams primarily enhanced aerobic respiratory metabolic pathways in the beginning stages by opening the shell and increasing the CCO activity to obtain more oxygen; with increasing exposure time, when aerobic respiration was inhibited, the shell was closed, and FRD, CAT, and SOD were activated. At this point, Manila clams responded to H(2)S stress through the anaerobic metabolism and antioxidant defense systems. However, high temperatures (≥28 °C) altered the defense strategy of Manila clams. With co-exposure to high temperatures and high H(2)S concentrations (≥20 μmol/L), the Manila clams immediately closed their shells and changed from aerobic respiration to anaerobic metabolism while immediately activating antioxidant defense systems. Nevertheless, this defense strategy was short lived. In addition to this, apparent damage to tissue and cell structures, including mitochondrial ridge dissolution and many vacuoles, was observed in Manila clams exposed to high temperatures and high H(2)S concentrations. Thus, prolonged exposure to high temperature and H(2)S damages the tissue structure of Manila clams, affecting their behavioral capacity and future survival. In summary, profiling Manila clams’ physiological response strategies to H(2)S exposure provided ecological behavioral support for our current understanding of H(2)S detrimental toxicity on marine bivalves.