Histopathology of a threatened surf clam, toheroa (Paphies ventricosa) from Aotearoa New Zealand

Testing a health baseline during a bivalve mollusc mortality event: An investigation into die-offs of pipi Paphies australis from Aotearoa New Zealand

Disease is a major threat to the economic, ecological and cultural services provided by wild bivalve populations. Over the past decade anecdotal reports on declining health of native bivalve populations around Aotearoa New Zealand have been supported by increasing observations of mass die-offs. Causes of declining health and mass die-offs of wild bivalves are not clear and could be due to a number of interactive and cumulative factors, including declining water quality, climate change, or disease. Pipi/kōkota (Paphies australis) within the Whangārei area (northern New Zealand) have suffered repeated die-offs and declining health since at least 2009. Baseline health data for wild native bivalve populations are scarce making it difficult to identify changes in pathogen infection prevalence and intensity and infer their importance to host health. This research aimed to examine and document the health of pipi in Whangārei with the objective of identifying factors that may contribute to their ill health and lack of population recovery. We sampled pipi from four sites within Whangārei, eight times across two years (total n = 640) to establish a health baseline using histopathology, general bacteriology, and qPCR for the intracellular bacteria Endozoicomonas spp. Three pipi mass die-offs occurred during the sampling window that were opportunistically sampled to compare against the health baseline established using healthy pipi. An increase in bacterial growth and a decrease in the abundance of Endozoicomonas spp. in mortality pipi was observed compared with the health baseline. Establishing a health baseline for pipi from Whangārei provided a benchmark to assess changes in a pipi population experiencing high mortality. Such data can help identify factors contributing to die-offs and to help inform what mitigation, if any, is possible in wild shellfish populations.

Bacteriology & bivalves: Assessing diagnostic tools for geographically remote bivalve populations

Two sampling approaches for the growth of common or dominant bacteria from bivalve haemolymph were compared: (1) samples processed in the field immediately after collection (field samples), and (2) samples processed in the laboratory at least 24 h after collection (laboratory samples). The sampling approaches were compared on 210 marine bivalve molluscs Paphies subtriangulata and P. australis from two shallow intertidal sites in North Island New Zealand. The approaches were evaluated for the amount of bacterial growth, type of growth, and diversity of growth. Differences in amount and type of growth between the two sampling approaches were observed. Samples processed in the field from P. subtriangulata had significantly more bacterial growth, and a higher diversity of bacteria, including more common or dominant bacterial species. Laboratory samples had a higher proportion of samples with no growth, however common or dominant bacteria were still isolated from these samples. For P. australis, field samples more often had no bacterial growth and laboratory samples had a significantly higher number of common or dominant growth present. Field samples did however contain a higher diversity of bacteria. By conducting bacteriology on bivalves in either the field or the laboratory only, there may be limitations to determining the significance of a bacterial agent isolated. Sampling of both field and laboratory samples should be carried out where possible to optimise detection of important bacteria.