<p>Mass mortality events (MMEs) occur when a disproportionate part of a population dies in a single event. The frequency of MMEs is increasing globally. In the past, MMEs have been linked to starvation, changes in environmental conditions and disease outbreaks. However, it is often unclear what the underlying cause of these events are. In New Zealand several MMEs have occurred in the bivalve species Austrovenus stutchburyi (Wood 1828) and Paphies subtriangulata (Wood 1828) with little known about the cause. Both of these species are recreationally harvested for consumption in New Zealand and have cultural significance.  In order to better understand MMEs in these species we must first gain a better understanding of stress expression. Bivalves have few observable features and it is difficult to classify them as healthy or stressed without investigating immune change which can be quite costly. Some research has looked into how different cell types change in response to pollutants but few studies have researched how cell types change in response to environmental conditions. The aim of this research was to find novel ways of assessing if shellfish were healthy or stressed. Little is known about how shellfish respond to environmental stressors and this is the first study to look at several novel stress expressions simultaneously, in New Zealand shellfish.  Histological, morphological and behavioural responses were measured in both A. stutchburyi and P. subtriangulata after treatment with increased temperature, lowered salinity and increased fine sediment input for up to 5 weeks. Temperature stress was the main stressor for P. subtriangulata (85% of overall mortality occurred in the heat treatment), salinity was the main stressor for A. stutchburyi (46% of overall mortality occurred in the salinity treatment), and fine sediment stress did not seem to have an effect on either species in this study. Overall, A. stutchburyi were more robust to the treatments, but low mortality occurred in both species (≤8%). Mortality correlated with time of year and was believed to be related to spawning in P. subtriangulata (48% of overall mortality occurred from October-November). Both species had a single histological marker, in A. stutchburyi this was change in gill morphology, and in P. subtriangulata this was change in digestive gland morphology. Several individual morphological features were identified as potential stress markers in A. stutchburyi and P. subtriangulata. Additionally, when removed from aquaria P. subtriangulata had impeded foot retraction time in the salinity treatment. The differences in stress markers shows the diversity of reactions to stressors even within New Zealand bivalves. This study provides a useful baseline in investigating how P. subtriangulata and A. stutchburyi respond to environmental stress. The histological slides produced during this investigation are an invaluable resource that can be used in future studies and in comparisons with archived specimens from known MMEs. Knowing how to detect signs of stress in these bivalves will help to predict MMEs in the future and aid in implementing processes to combat these events.</p>