The Re-Acquisition of Shrimp Black Gill Infections by Penaeid Shrimp; Oceanic or Estuarine Sources?

Penaeid shrimp support one of the most valuable commercial fisheries in the southeastern U.S. In Georgia and South Carolina, however, the fishery has experienced a significant decline within the last two decades. This decline is coincident with the emergence of an undescribed species of apostome ciliate infection causing gill tissue melanization (Black Gill). This ciliate is currently referred to as the shrimp Black Gill (sBG) ciliate. The sBG ciliate is present in shrimp populations annually from May through January with peak prevalence of visibly melanized gills occurring in late summer and fall. Molecular and histological studies indicate that the sBG ciliate is absent from shrimp populations during the winter and spring. Where the ciliate goes and how the shrimp is re-infected each year are questions that remain unanswered. In this study we examined post-larval shrimp during ingress to the estuary from the ocean, to determine whether ciliates are acquired at sea or within the estuaries. Post-larval penaeid shrimp were collected in June and July 2018 during evening floodtides near the mouth of Charleston Harbor, SC. Other crustaceans were captured during floodtides at the mouth of Wassaw Sound, GA in June 2019. Penaeid post-larvae and other small crustaceans were screened for the presence of the sBG ciliate using a diagnostic PCR-based assay targeting apostome ciliates. The identity of the sBG ciliate was confirmed in positive samples by sequencing a hypervariable region of the 18S rRNA gene. The sBG ciliate was not detected in ingressing penaeid shrimp post-larvae. The non-pathogenic apostome ciliate H. chattoni and another undescribed apostome ciliate, however, were detected. This new ciliate was also detected in mysid shrimp that often co-occur with ingressing penaeid post-larvae. The absence of sBG ciliate infection in the penaeid post-larvae supports the hypothesis that sBG ciliate infections are acquired once recruited into the estuary.