Physiological and biochemical responses of the intertidal crabs Hemigrapsus crenulatus and H. sexdentatus to hypoxia and salinity.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Many marine habitats, intertidal zones in particular, are changing due to the global temperature shifts of climate change. These areas are characterized by daily, seasonal and irregular changes in dissolved oxygen and salinity. This forces species to adapt and often regulate in order to survive. Nutrient enrichment is becoming one of the foremost environmental issues impacting species distribution and diversity worldwide. Hypoxia is a decline in water oxygen levels that is increasing globally through anthropogenic aquatic nutrient loading. The New Zealand crabs H. crenulatus and H. sexdentatus are commonly found in intertidal zones, such as estuaries and rocky shores, which are subject to daily variations in dissolved oxygen and salinity. Consequently, these species are faced with homeostatic challenges involving their physiological and biochemical regulatory systems. The current study investigated the physiological and biochemical responses of the crabs to decreasing dissolved oxygens (including hypoxia) and salinity variations.
The crabs were tolerant of hypoxia maintaining metabolic rates (MO₂) down to a critical oxygen tension (PCRIT) of 52.5 mmHg for H. crenulatus and 47.8 mmHg for H. sexdentatus. These critical oxygen tensions were unaffected by salinity for both species. At higher oxygen levels (above each species PCRIT) salinity effected the metabolic rate of both species. A bradycardic heart rate response (decrease in heart rate) was also seen for both species in low dissolved oxygen levels (10 mmHg). Salinity had no effect on the heart rates of H. crenulatus, whereas H. sexdentatus had an increased heart rate when exposed to hypo- and hyper-salinities. The biochemical studies investigated various haemolymph parameters (osmolarity, chloride, potassium, sodium, glucose, lactate, and haemocyanin). The dissolved oxygen treatments had little effect on any of the haemolymph parameters tested for either species which is indicative of a lack of stress in these animals. Salinity, however, affected the haematological composition of both crabs. For H. crenulatus osmolarity, chloride, and potassium were all hyper-regulated at low salinities and hypo-regulated at high salinities. In contrast, H. sexdentatus conformed haemolymph osmolarity and chloride in the high salinities. Haemolymph glucose showed no apparent trends in relation to dissolved oxygen or salinity for either species which is again suggestive of a lack in stress response.
The physiological and biochemical abilities of both New Zealand crab species are indicative of a strong regulatory response in both varying dissolved oxygens and salinities. These findings may have important environmental implications in the future as global warming intensifies. These species could be used as biological indicators of ecological stress and could therefore be implemented in risk assessment analyses.