Host Species Diversity,seasonal Occurrence,and Tissue Distribution Of Perkinsus Spp. In The Shellfish Along The Coastal Area Of Southern China

Perkinsozoa,a group of parasitic protozoa,is known to infect marine mollusks,particularly bivalves,worldwide.Perkinsus spp.infections may cause a decrease in gametogenesis and egg production and have a negative impact on host growth.High Perkinsus spp.infection intensities can result in mass mortalities,thus causing direct economic and ecological consequences.As a dangerous parasitic pathogen,Perkinsus spp.was described as internationally reportable or notifiable pathogens for mollusks on the list of the World Organization for Animal Health(OIE,2017).The research on Perkinsus spp.started relatively late in China compared to the other foreign countries.However,information about the occurrence of Perkinsus spp.along the coast of China,where the diversity and production of shellfish are very high,is not yet available.In the present study,investigations on species diversity and host diversity of Perkinsus spp.were performed in different bivalves along the coast of southern China.Seasonal occurrence of Perkinsus spp.was also performed to monitor the variation of Perkinsus infection in the clam Soletellina acuta(Mollusca: Rsammobiidae)along the coast of Wuchuan County,southern China over one year.Tissue distribution of P.olseni was confirmed using the real-time polymerase chain reaction(qPCR)to evaluate the tissue tropism of Perkinsus.The results provided useful date for quarantine work and contributed to the scientific prevention and control of Perkinsiosis.The main results are summarized as follows.1.The traditional RFTM method and PCR assay were carried out to investigate the species diversity and host diversity of Perkinsus spp.in different bivalves along the coast of southern China.A total of 208 different bivalves samples were collected from 6 sites along the coast of southern China.Our results showed that Perkinsus spp.infections occurred commonly in these samples,and the total prevalence was 47.6%.The infected samples included Crassostrea hongkongensis,Meretrix meretrix,Chlamys nobilis,S.acuta,Pinctada martensii and Cyclina sinensis.The sequencing results indicated that P.olseni and P.beihaiensis coexisted along the coast of southern China.The highest prevalence of Perkinsus spp.(66.9%)occurred in C.hongkongensis,which were Perkinsus-infected among different sites.2.To the best of our knowledge,we identified Perkinsus infection in the clam S.acuta for the first time.A total of 180 wild clams(S.acuta)at a time were collected in September 2016(fall period),December 2016(winter period),March 2017(spring period),and June 2017(summer period),respectively.Sequencing results indicated that P.olseni and P.beihaiensis coexisted in the same S.acuta population,but not in the same individual clam among different sampling months.Between these two species,S.acuta is more prone to be infected by P.olseni than by P.beihaiensis.The prevalence of Perkinsus spp.in the clam was 21.9%,which was relatively lower among cultured bivalves in the same water,as the clam S.acuta is not farmed currently.The prevalence of Perkinsus spp.in S.acuta differs significantly among seasons as found in the present study.The prevalence of Perkinsus spp.was higher in September(fall)and March(spring),while it was the lowest in June(summer).Interestingly,reverse seasonal patterns of infection rate occur between P.olseni and P.beihaiensis infection in the clam S.acuta.Especially,in the March sampling period,only P.olseni was present,while P.beihaiensis was absent in the clams.3.It’s the first time to evaluate the tissue distribution of Perkinsus spp.using qPCR assay.The results of qPCR assays demonstrated that P.olseni could infect all types of tissues,including gill,mantle,digestive gland,and siphon.Concentration of P.olseni was not significantly different among four tissues.Concentration of P.olseni in mantle was almost equal to that in gill,while higher than that in siphon.The positive rate of P.olseni in the gill and mantle tissues reached 100%,and was not significantly different between them,while it was significantly higher than that in the digestive gland and siphon tissues.In summary,this study reports the presence of Perkinsus in a new host,the clam(S.acuta),for the first time.The results of the seasonal periodicity of Perkinsus infection,the tissue distribution of Perkinsus and so on,were useful for quarantine work and contributed to the scientific prevention and control of Perkinsiosis,which provided reliable dates and laid the foundation for Perkinsus detection in southern China.