Ecophysiological Assessment of Climate Change Resilience in Commercial Polyploid Pacific Oysters (Magallana gigas)

Abstract

Global increases in extreme, transient temperature events are an imminent threat within our oceans. For coastally on-grown Pacific oysters (Magallana gigas) high thermal stress can perturb physiological processes, leading to sub-lethal if not lethal consequences; posing a significant concern for New Zealand Aquaculture, economically reliant on M. gigas produce. Since the 1980’s, polyploid stocks (i.e., chromosomally manipulated individuals) have gained popularity, given strong evidence of superior growth, improved organoleptic properties, and the opportunity for year-round harvest compared to their diploid (2n; i.e., most naturally occurring) conspecifics. However, a sound understanding of triploid (3n) oyster environmental resilience remains unclear; exacerbating uncertainty among commercial partners, eager to accelerate investments amidst compounding challenges of climate change on farmed populations. To this end, this research applied an ecophysiological approach to the characterisation of upper thermal limits and subalongside non-lethal heat stress responses of four commercial polyploid stocks (i.e., 2n, 3nCi, 4N1, and 4N2) in New Zealand. LT50 (i.e., temperature in which 50% of the population undergo mortality) revealed modest differences between adult oyster lineages, with triploid 4N2 and 4N1 emerging as most thermotolerant (LT50: 41.8 oC and 41.4 oC, respectively) against diploids (LT50: 41.1 oC). Survival two weeks post-heat stress substantiated LT50 discrepancies, particularly where diploid (compared to 4N2) populations demonstrated a 39% higher incidence of mortality following acute shock at 41 oC. Specific anaerobic enzyme activity of strombine dehydrogenase (SDH) suggested among 2n, energy demands of efficient repair and recovery post-challenge are potentially unmet, contributing to decreased survival. Through scope for growth (SFG) assessment under challenge (30 oC) and recovery (20 oC) conditions, insights into Pacific oyster bioenergetics also broadly aligned with the notion diploid resource constraints may supress upper thermotolerance. Clearance rates (CR) under challenge were increased for 2n, compared to 4N1 and 4N2 exhibiting distinct declines. A pattern then reversed by recovery. Additional inconsistencies in organic ingestion rates (OIR) under recovery, suggested potential gill damage, degradation of food quality, or a combination of both may also be implicated in observed ploidy differences. I, therefore, recommend further investigation into refining whether diploids (2n) compared to triploids (3nCi, 4N1, and 4N2) here, uphold opposing metabolic strategies for coping with thermal stress post-challenge. With the former electing for conservation; the latter compensation. In doing so, this project offers foundational parameters for understanding non- and sub-lethal thermal stress responses, alongside lethal thermal limits in commercially relevant Pacific oyster stock lines. Hopefully, then feeding into more complex multi-stressor studies, purposed to assist industry in the sustainable development of husbandry practices heading into an uncertain future.