Comparative study on the response of marine bivalves to temperature stress

Author: Maziidah Ab Rahman

Ab Rahman, Maziidah, 2020 Comparative study on the response of marine bivalves to temperature stress, Flinders University, College of Science and Engineering

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Suspension-feeding bivalves are the key species in estuaries and are commonly farmed in coastal areas. As the marine environment is experiencing numerous biochemical and physiological perturbations due to the change of temperature, salinity and dissolved oxygen, these changes have significantly impacted bivalve growth, survival and distribution. Among environmental factors, temperature is an important factor that regulates physiological responses in aquatic poikilotherms. As a result of global warming, temperature can increase animal metabolism and activities, resulting in the changes in growth, development and immunological responses of marine bivalves. In most situations, filter-feeding bivalves can clear food particles from the water column and high-water temperature promotes feeding activity. Hence, it is necessary to investigate temperature-dependent responses in grazing rate, growth, metabolism and immunity to improve our understanding on the environmental impact in bivalve aquaculture. This thesis consists of three data chapters (2, 3, and 4).

Chapter 2 addresses the effect of temperature elevation on physical, metabolic and growth of oyster Crassostrea gigas, mussel Mytilus galloprovincialis, and cockle Katelysia rhytiphora. All three bivalve species were separately exposed to three temperatures (15 °C, 20 °C, and 25 °C) in tanks for 30 days with three replicates. Temperature significantly impacted seston clearance, food absorption efficiency, oxygen consumption, excretion, pyruvate kinase enzyme activities, and growth performance. Oysters had the highest clearance rate and food absorption efficiency regardless of temperature when compared to other species. The best growth occurred at 15 oC, whereas the growth rate was negative at 25 oC in all species. At 25 °C, the growth rate of each species became negative due to a significant reduction in food clearance, high metabolism and high excretion. Cockles had the lowest growth rate, ranging from -8.4 to -15.7 J g-1h-1 compared to oysters and mussels, suggesting that cockles have the poorest adaptability to thermal stress.

In Chapter 3, the impact of temperature change on the defence system of bivalves was investigated by measuring immunological parameters and antioxidant enzymes relevant to stress response. Each species was exposed to three temperatures, 15 °C, 20 °C and 25 °C for 14 days. The total haemocyte count, phagocytosis, reactive oxygen species and the activity of antioxidant enzymes such as superoxide dismutase and catalase were used as indicators to measure the response of each species to different temperatures. Water temperature significantly affected immune functions in molluscs and led to oxidative stress and reduction of immunosurveillance in all three species of marine bivalves. Temperature also affected the survival of these molluscan species with highest mortality at 25 °C. Cockles were mostly affected by temperature elevation. This chapter demonstrates that the intertidal species like oysters have a greater tolerance to extreme thermal stress than subtidal species (e.g. mussels) and demersal species buried in sand (e.g. cockles).

As temperature has a significant impact on biology and growth performance in marine bivalves, Chapter 4 compares the seasonal food selectivity and feeding capacity of three bivalve species. Seston abundance varies significantly across spatial and temporal scales due to water transport processes, climate and environmental fluctuations, and has important applications to the success of food filtration of marine bivalves. This study demonstrates that oysters and mussels selectively fed on large food particles (e.g., diatom, dinoflagellate and large picoplankton >2 µm) regardless of season, but mussels could access a wider size spectrum of food particles compared to oysters. Cockles on the other hand, selected for both large and small food particles (e.g., Synechococcus and small picoeukaryotes <2 µm) and fed more efficiently on small particles than both oysters and mussels.

Overall, temperature is an important environmental factor that needs to be taken into account in shellfish aquaculture and management as it significantly alters most functions in the biological process of aquatic organisms, which in turn can impact growth, survival and productivity. The results of this thesis can improve our knowledge on how temperature-dependent stress modulates marine bivalves that have different habitats in nature. These findings would help to choose locations for marine bivalve farming to counteract the possible negative effect of temperature elevation due to global warming in future.

Keywords: Temperature stress, molluscs, physiological, immune response, metabolic

Subject: Aquaculture thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Prof Jian G Qin