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Mass mortalities of juvenile Pacific oysters (Crassostrea gigas) in aquaculture operations have been linked to the ostreid herpes virus 1 (OsHV-1) or its variants. This oyster has heritable resilience to the virus which can provide a basis for selective breeding for higher OsHV-1 resistance, however, controlled challenge methods for improving selection response towards increased OsHV-1 resistance are required. The objective of this study was to compare the survival of sibling juvenile oysters exposed to either a laboratory virus challenge or a heat shock challenge (as proxy for the virus challenge) and the survival of the same families on two oyster farms where they were exposed to natural field conditions including the presence of OsHV-1. A strong correlation between the family rankings was observed for the two farm sites. Estimated narrow sense heritabilities (h2) on the underlying liability scale were 0.38 (on-farm survival), 0.45 (laboratory virus challenge survival) and 0.15 (heat shock challenge survival). Only on-farm survival and laboratory virus challenge had a high genetic correlation. The method for measuring heat shock resistance in juvenile oysters produced high variability in survival among families but low estimates of heritability. Overall, the results of this study indicate that the laboratory virus challenge is a simple and relatively effective tool for selective breeding specifically towards OsHV-1 resistance or as part of a controlled multi-trait program. Statement of relevance The OsHV-1 virus is a serious problem for commercial aquaculture of Pacific oysters in numerous countries. This paper tests two ways of evaluating oysters for resistance to OsHV-1 that can be incorporated into selective breeding programs.

A lack of understanding regarding the feeding biology of post-settlement mussels has considerably constrained the hatchery culture of many commercially important mussel species, including the Greenshell™ mussel, Perna canaliculus . Therefore, we evaluated how particles of different size (MySpat artificial diet full particle size range 2–60 μm, sieved MySpat diet with a size range of 2–15 μm; polystyrene beads, 3–15 μm, and microalgae, 4–6.5 μm) affected the retention and capture efficiency in post-settlement juveniles (PSJ). The response of PSJ of various sizes (PSJ-I: 0.3 mm shell length, PSJ-II: 1.0 mm, and PSJ-III: 7.0 mm) was different. Overall, smaller mussels (PSJ-I and PSJ-II) were unable to selectively capture particles by their size on all diets, conversely larger juveniles actively captured smaller particles between 2 and 8 μm. Independently of the PSJ size, the full particle size range MySpat artificial diet had a negative effect on the feeding activity of the mussels, with particles blocking the pallial organs in the non-selective PSJ-I and PSJ-II, and completely inhibiting the feeding activity in the more selective PSJ-III. These results provide useful information to understand the feeding biology and ecology of PSJ of this species that can be used for the improvement of hatchery feeding methods. Artificial and live food particle size can be chosen in order to avoid larger harmful particles (> 15 μm), and the exclusive use of smaller food particles should be considered for PSJ Statement of relevance This article provides information to better understand particle capture process in post-settlement juvenile mussel in order to improve hatchery/nursery feeding practices. The results of this research will allow making informed decisions regarding selection of food particle size in relation to the size of the juvenile mussels and their ability to deal with particles of different sizes. Furthermore will also help to decide the adequate size of transfer for grow-out as well as being useful for the development of suitable artificial diets.