INFLUENCE OF HYPOXIA AND ANOXIA ON THE PHYSIOLOGICAL-RESPONSES OF THECLAM RUDITAPES DECUSSATUS FROM SOUTHERN PORTUGAL

The infaunal bivalve Ruditapes decussatus L. was collected from Ria Fo rmosa, Fare, southern Portugal, and subjected to a range of hypoxic co nditions and anoxia. Physiological measurements, clearance rates, resp iration rates and absorption efficiency were undertaken at slightly di fferent oxygen partial pressures (11, 6, 3 and 1.2 kPa for clearance r ates and absorption efficiency and 12, 7, 5, 1.9 and 0.9 kPa for respi ration rates). Metabolic rates under hypoxia were measured as oxygen c onsumption and anoxic metabolism was measured using direct calorimetry . Increasing hypoxia resulted in lower clearance rates, leading to low er ingestion rates and reduced faeces production. Clearance and ingest ion rates declined below similar to 6 kPa, reflecting decreasing venti lation and feeding activity, although complete cessation was not obser ved even at 1.2 kPa. Under extreme hypoxia (< 2 kPa) clams showed an i rregular behaviour, with valves either closed or only slightly open, a nd siphons compressed or retracted. Clearance rate was 12% and respira tion rate was 35% of normoxic rates. R decussatus responded to increas ing hypoxia by lowering its metabolic rate. Regulation of respiration was absent through moderate hypoxia (similar to 7 kPa), but was observ ed in the lower hypoxia range (7 to 0.9 kPa). Under anoxia, rates of h eat dissipation were 3.6% of normoxic rates. The low anoxic metabolic rate is indicative of a reduced energy expenditure, and this energy-sa ving mechanism is common in bivalves. Scope for growth was always posi tive, and even at low oxygen levels clams did not have to utilize thei r energy reserves. The ability to reduce metabolic costs but still mee t the maintenance costs by aerobic catabolism enables R. decussatus to tolerate hypoxia. Such conditions can occur, particularly in the summ er, in southern Portugal.