Pacific oysters are sessile bivalve benthic filter feeders therefore are constantly exposed to challenges and pathogens in the surrounding environment (Ben-Horin et al., 2015; Burge et al., 2016; Fisher et al., 2003; Harvell et al., 2004; Labreuche et al., 2006; Lelong et al., 2007). The bivalve immune system is complex and is comprised of both physical and biological immune components (Allam and Raftos, 2015). The first line of defense is the physical components of the oyster; this includes the shell, mucosal layer and skin (Allam and Raftos, 2015). The shell is critical as it provides a physical barrier to physical, biological and chemical interactions that pose a threat to the oyster (Allam and Raftos, 2015). The mucosal layer and the skin provide a second layer of protection aiding in the entrapment of microbes and other pathogens and the assistance of their removal (Allam and Raftos, 2015). There is increasing evidence of the immune factors often associated and incorporated in these structures, for example haemocytes and the phagocytic activity in mucosal secretions (Allam and Raftos, 2015; Allam et al., 2001; Lau et al., 2017).
