Hematodinium sp. infection alters hemolymph bacteriome composition and function in shore crabs, Carcinus maenas.

Hematodinium sp. is a parasitic dinoflagellate that infects and causes disease in a large fraction of decapod crustaceans in the Northern Hemisphere. The shore crab Carcinus maenas is susceptible to this parasite in its native range of European coastlines and along its invasive fronts. The parasite enters the host from the surrounding water, and fueled by the hemolymph (blood) resources, proliferates until host tissues are overrun. To improve our knowledge of the shore crab-Hematodinium pathosystem, we profiled the hemolymph microbiome of control and Hematodinium-positive crabs with low (early clinical) and high (late clinical) parasite levels through bacterial 16S rRNA V3-V4 meta-barcoding (n = 64). Concurrently, we assessed hemolymph condition through quantification of circulating hemocytes (immune cells) and bacterial colony forming units. Progression of Hematodinium sp. infection coincided with gradual changes in bacterial community composition. These changes were driven by shifts in relative abundances of specific taxa, e.g. Corynebacterium, Flavobacterium. Furthermore, alterations in bacterial communities were accompanied by marked shifts in metabolic signatures from stress and virulence dominated (including biofilm formation) to catabolism, peptide transport, and tissue putrefaction at the terminal stage of parasitemia. We present the first account of microbiome dynamics in healthy shore crabs and those overcome by Hematodinium sp. Our experiments reveal a clear link between bacterial community structure-function and the advancing disease state.