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

ABSTRACT

Hydrogen sulfide (H2S) 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 H2S 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 H2S. 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 H2S on the tissue and cell structure of Manila clams were also examined under a transmission electron microscope. Manila clams responded to H2S stress through behavioral and chemical defenses. With exposure to H2S 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 H2S 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 H2S 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 H2S concentrations. Thus, prolonged exposure to high temperature and H2S damages the tissue structure of Manila clams, affecting their behavioral capacity and future survival. In summary, profiling Manila clams' physiological response strategies to H2S exposure provided ecological behavioral support for our current understanding of H2S detrimental toxicity on marine bivalves.