The Effects of Heatwave-Related Stressors on Gametogenesis and Pathogenesis in Two Model Molluscs From Aotearoa, New Zealand

Abstract

Elevated temperature challenges associated to changing climate are a risk to many marine molluscs. As average sea temperatures rise, marine heatwaves (MHW) are also likely to increase in frequency and intensity. The temperature elevations are also associated with increased frequency of extreme events such as storms and floods, which can have consequences on the local marine ecosystems. The Green-lipped mussel (GLM) (GreenshellTM mussel), Perna canaliculus (Gmelin 1791), and the Blackfoot pāua Haliotis iris (Gmelin 1791) are endemic marine molluscs in New Zealand. Both species play a key role in the marine environment, hold cultural value and support fisheries and aquaculture industries. The changing climate is impacting both species in terms of growth, reproduction, pathogen dynamics and survival and this trend is likely to worsen. This thesis aims to explore the environment/host/pathogen interactions in relation to changing climate in the selected molluscan study species, and their shared parasite Perkinsus olseni. Investigations aimed to identify pathologies and pathogens associated with host, phenology, vulnerability, and disease susceptibility. More broadly, to provide insights into deleterious processes and their detection methods that are applicable to the species studied and to molluscs in general. A forensic histopathology approach with supporting techniques was used to identify the microscopic changes at the tissue level. Field monitoring and laboratory scenarios were conducted to explore findings and attempt to extrapolate to real-world scenarios. Positive temperature anomalies of up to 3°C and summer temperatures above 22°C were regularly detected in one of the field survey sites (Chapter 2), with the others regularly above 16°C. Tissue conditions of H. iris and green-lipped mussels indicated physiological and reproductive stress associated to temperature, as well as several conditions correlating to pathogen presence. The gametogenesis cycle appeared to be prolonged for the green-lipped mussel (Chapter 2) and H. iris (Chapter 4), with both potentially using oocyte atresia (nutrient resorption from developing eggs) as a resource for maintaining condition. Ceroid was elevated in the slower-growing population of H. iris and may allude to advanced physiological age and increased vulnerability under a changing climate (Chapter 3 and 4). In the lab scenarios elevated temperature and prolonged exposure time resulted in an increase in the number of focal ceroid aggregations, haemocytosis, a decrease in energy reserve cells (glycogen) and a decline in reproductive condition (Chapter 5). Flood events, in addition to impacting our low-lying research facility, were also observed to impact reproductive condition by causing the mussels in the system to spawn (Chapter 6). Finally, under controlled challenge conditions and in the absence of thermal stress, the parasite Perkinsus olseni was still able to outcompete the host (Green-lipped mussel) immune response and continue to develop within the tissues (Chapter 7). The findings demonstrate that fine-scale tissue-level research can provide useful information on many levels, can be applied to other molluscan species, and highlight the importance of monitoring marine health. Finally, continued integration of field and laboratory research is critical in elucidating the effects of external and internal stressors on molluscs to provide early detection of non-infectious and infectious diseases.