Fig. 3

Distribution of microbial communities over the gill lamellae of fiddler crabs. a–c Scanning electron microscope (SEM) imaging of a representative gill of the fiddler crabs Tubuca urvillei. Representative SEM images of the other crab species gills are provided in Supplementary Figure S5. a Magnification of the gill lamellae. b Detail of the bacterial layer covering the lamellae of the gill of each species investigated. Asterisks indicate the gill lamellae. c Rod-like bacteria covering the entire lamellae of the gills. d–f Transmission electron microscope imaging of fiddler crab gill (indicated by a “g” letter) of Austruca albimana: white asterisks show the gill cuticles, while orange arrows the bacterial layer. d Section of the gill that shows both side of the gill lamellae covered by bacteria. e Magnification of the bacterial layer. f Gill lamellae surface (indicated by a g letter); arrowheads: electro-dense area where bacteria are attached to the gill; black arrows: bacterial pili. g–p Localisation of bacteria in crab gill by confocal laser scanning microscopy (CLSM) of fluorescence in situ hybridisation (FISH)-stained gill lamellae of T. urvillei (g–j) and A. albimana (k–n) with the different probes (see supplementary figure S5 for the bright field and FISH negative control). o, p IMARIS 3D-structure reconstruction of the gill lamellae and the bacterial layer (frontal and lateral view) of T. urvillei. Note the absence of signal inside the gill lamellae that support the evidence that bacteria live on the surface of the lamellae without entering them. Red arrow indicates the heterogeneous inner morphology of the gill lamellae. “High CG bacteria” indicates cells with high guanine-cytosine content typical of Actinobacteria. The images are meant to be typical of the range of observations made