World Aquaculture December 2019

46 DECEMBER 2019 • WORLD AQUACULTURE • WWW.WA S.ORG production and the capacity of the organism to produce sufficient antioxidants to neutralize the ROS and to repair oxidative damage, which is when oxidative stress occurs (Fig. 1). The accumulation of ROS in cells can result in damage to proteins, lipids, DNA and other macromolecules, which consequently can affect an organism’s overall performance and survival. Environmental stress may also stimulate the production of heat shock proteins (HSPs). HSPs are a group of molecular chaperones that stabilize damaged proteins and are also present in the cell to help repair natural damage to proteins through other processes such as aging (Feder and Hofmann 1999). Counterintuitively, HSPs are not only produced when organisms are exposed to heat stress, but also upregulated in response to stressors such as UV, salinity and hypoxia (Roberts et al . 2010). There are different types of HSPs that are differentiated and named after their molecular weight, with HSP of molecular weight 70 kDa (HSP70) being one of the most studied forms (Mayer and Bukau 2005). Similar to other HSPs, the up-regulation of HSP70 will be specific to the type and severity of the stress (Valenzuela-Castillo et al . 2019). The New Zealand Greenshell TM Mussel The Greenshell TM mussel Perna canaliculus is New Zealand’s most important aquaculture species, with an export value of more than US$250 million a year (Jeffs et al . 2018, Figs. 2 and 3). The juveniles (spat) of this species, which are the base of New Zealand’s mussel aquaculture industry, are predominantly collected Stress, ROS and HSPs Shellfish, like other organisms, are constantly exposed to a variety of stressors in their natural environment that vary from food limitation to changes in water chemistry or temperature. Exposure to these stressors creates a chain of biochemical and molecular reactions in the organism to cope with the stress. One of these reactions to stress is the generation of reactive oxygen species (ROS) by cells, which are a group of chemically reactive molecules including free radicals and peroxides. In the absence of stress, the cells of shellfish generate a small amount of ROS, representing 1-3 percent of an organism’s consumed oxygen (Lewis and Santos 2016). The low levels of ROS produced as a result of normal metabolism are neutralized by cellular antioxidant defences and any oxidative damage is repaired by mechanisms found naturally in healthy cells (Burritt and Lamare 2016). Enzymatic antioxidants that help neutralize ROS include, among others, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR), whereas non-enzymatic antioxidants include ascorbate, glutathione and α-tocopherol (Lesser 2006). Under normal, non-stressful conditions, there is a natural balance between the amount of ROS generated due to the organism’s metabolism and the antioxidants and repair mechanisms available to neutralize the ROS produced and repair any oxidative damage (Fig. 1). However, under stressful conditions, there is an imbalance between ROS Stress and Health in New Zealand’s Number One Aquaculture Export – The Greenshell TM Mussel Natalí J. Delorme, Norman L. C. Ragg and David J. Burritt FIGURE 1. Under normal, non-stressful conditions, there is a balance between the accumulation of ROS in the cells and natural antioxidant defences. Under stressful situations, this balance becomes compromised by an increase in the production of ROS (oxidative stress). Defence mechanisms are progressively overwhelmed, resulting in oxidative damage. Diagram: Joanna Copedo. FIGURE 2. The Greenshell TM mussel Perna canaliculus : adult and spat. Photo: Cawthron Institute. FIGURE 3. Greenshell TM mussel hatchery-reared spat. Photo: Sanford Ltd.