Physiological and microbiological determinants of the subtropical non-indigenous Rangia cuneata health and condition in the cold coastal waters of the Baltic sea: the Vistula lagoon case study.
The clam Rangia cuneata originating from the Gulf of Mexico has recently been introduced in the Baltic Sea waters. In the Polish coastal waters R. cuneata colonized riverine (the Vistula Delta) and the lagoon (the Vistula Lagoon) areas. In the Vistula Lagoon population strong fluctuations in clam biomass and density was observed being a consequence of reoccurring yearly spring mortality. In the present work we examined the condition and general health of R. cuneata from the Vistula Lagoon and the potential role of microorganisms as causative factors of mortality events. Overall, 102 adult clams were collected in March 2019. Out of those, 50 clams were used for microbiological and histological analyses while 52 for biochemical examination. Histological examination revealed that 47 clams were characterized by infiltration of hemocytes in various internal organs with mantle and gills most commonly affected. Observed inflammatory response was morphologically classified as mild to severe infiltrative hemocytosis as seen in severe systemic infections (septicemia) with the latter occurring in 40% of clams. Bacteria isolated from the clam tissues, characterized by a massive growth, included those from the genusAeromonas (A. sobria, A. caviae, A. hydrophila), all classified as potential pathogens. Additionally, the presence of Kocuria spp. (G+) and pathogenic Shewanella putrefaciens (G-) was documented in one individual. Bacteria identified in water samples were almost identical to those from the tissue and included all the threeAeromonas species, S. putrefaciens and Staphylococcus pseudintermedius (G+). Gram staining revealed the presence of G(+) bodies in hepatopancreas, while G(-) bodies were often observed in the epithelial tissue of gills and mantle. Biochemical examination revealed the lack of glycogen, low carbohydrate and lipid contents indicating scarce levels of stored energetic materials. Hence, predominant bacteria isolated from R. cuneata together with low energetic reserves may greatly explain poor condition of these clams, and periodical mortality outbreaks occurring in the studied area. We thus hypothesize that the disease outbreak potentially elevating species mortality and affecting its reproduction may limit rangias' success in creating a stable population in some areas with microorganisms forming mechanism participating in ecosystem resistance against introduced species during early colonization of the new environments.