Longitudinal study of winter mortality disease in Sydney rock oysters Saccostrea glomerata

Abstract

Winter mortality (WM) is a poorly studied disease affecting Sydney rock oysters Saccostrea glomerata in estuaries in New South Wales, Australia, where it can cause significant losses. WM is more severe in oysters cultured deeper in the water column and appears linked to higher salinities. Current dogma is that WM is caused by the microcell parasite Bonamia roughleyi, but evidence linking clinical signs and histopathology to molecular data identifying bonamiasis is lacking. We conducted a longitudinal study between February and November 2010 in 2 estuaries where WM has occurred (Georges and Shoalhaven Rivers). Results from molecular testing of experimental oysters for Bonamia spp. were compared to clinical disease signs and histopathology. Available environmental data from the study sites were also collated and compared. Oyster condition declined over the study period, coinciding with decreasing water temperatures, and was inversely correlated with the presence of histological lesions. While mortalities occurred in both estuaries, only oysters from the Georges River study site showed gross clinical signs and histological changes characteristic of WM (lesions were prevalent and intralesional microcell-like structures were sometimes noted). PCR testing for Bonamia spp. revealed the presence of an organism belonging to the B. exitiosa−B. roughleyi clade in some samples; however, the very low prevalence of this organism relative to histological changes and the lack of reactivity of affected oysters in subsequent in situ hybridisation experiments led us to conclude that this Bonamia sp. is not responsible for WM. Another aetiological agent and a confluence of environmental factors are a more likely explanation for the disease.

Bonamia parasites: a rapidly changing perspective on a genus of important mollusc pathogens

“…The parasite observed in this oyster by Farley et al (1988) does not seem to have been a Bonamia sp. , Spiers et al 2014, and while a Bonamia sp. was reported to have been observed using TEM by Cochennec-Laureau et al (2003b), it is unclear whether these authors actually visualized a Bonamia sp.…”

“…While some of these parasites were explicitly identified as B. exitiosa upon their discovery, these identifications were inconclusive given the presence of B. roughleyi in the same clade (Hill et al 2010b). Recent work with Australian samples, however, has indicated that the parasite visually observed as Mikrocytos roughleyi (Farley et al 1988) is not a Bonamia or Mikrocytos species at all , Spiers et al 2014, and that the parasite genetically identified in S. glomerata from Australia (Cochennec-Laureau et al 2003b) could be a mis-identified B. exitiosa. et al 2014, this DAO Special, M. A. Kroeck et al unpubl.…”

History, status and future of Australia’s native Sydney rock oyster industry

“…2013). The exact cause for winter mortality remains unknown but environmental factors provide a partial explanation for the disease (Spiers et al 2014).…”

Microcell parasites of molluscs: introduction to DAO Special 7

“…Work in recent years, however, has produced important insight into Mikrocytos. First, novel members of the genus were observed in C. gigas in China (Wang et al 2010 , Spiers et al 2014.…”

“…The fourteenth, by Spiers et al (2014), points to continuing uncertainty concerning the identity of B. roughleyi and reminds us that 'microcells' may well include more taxa than Bonamia and Mikrocytos. Like the recent publications of Burki et al (2013) and Hartikainen et al (2014), the contributions to this DAO Special also highlight the dynamic nature of research into the microcell parasites and the emergence of new investigators in the field and suggest that the next decade of microcell research will be just as productive as the last.…”