Fig. 3

Transmission electron micrographs documenting the variety of microbes in sponges and the main mechanisms of transference. A–C Diversity of microbes in an undescribed Thymosia sp. (an HMA demosponge), where bacteria+archaea (A) as well as and cyanobacteria (B) freely co-exist in the mesohyl and with microbial population in bacteryocites (C) [81]. D, E Mesohyl of HMA demosponges Petrosia ficiformis (D) and Aplysina cavernicola (E). The former shows a pocket bacteriocyte that has a mix of microbes while the latter is a regular bacteriocyte with a single strain of a Spirochaete-like bacteria [73, 81]. F Cytoplasm of a Chondrilla caribbea (HMA demosponge) oocyte filled with bacteria+archaea, cyanobacteria, yeasts, and lipid bodies [82]. G Young oocyte of Corticium candelabrum (HMA homosclerophorid) phagocytosing a bacterium from the mesohyl [74]. H Oocyte of Aplysina aerophoba (HMA demosponge) phagocytosing a nurse cell. I,J Nurse cell of C. caribbea charged with bacteria+archaea, yeast, and yolk precursors. Note that the nurse cell is also in the process of phagocytosing two bacteria (J). K, L Morula of A. aerophoba that has incorporated bacteria+archaea into the blastomeres (K) and with nurse cells being incorporated by immigration through the cleavage furrows (M). Morula of C. candelabrum in which both free-living microbes and nurse cells are migrating through the cleavage furrows between the blastomeres. Note that blastomeres have also phagocytosed groups of bacteria to make them intracellular. N–P Swimming larva of A. erophoba that has accumulated bacteria+archaea below the larval epithelium (N) and making of a diverse microbial array (O), which is also phagocytosed and used as a food source by amoeboid cells in the larval interior (Q). Swimming larva of C. candelabrum, showing a dense a diverse assemblage of bacteria+archaea within its internal, central cavity. R Coccoid bacterium in the mesohyl of Thymosia sp. nov being infected by a bacteriophage virus. Labels: “b”= bacteria+archaea, “c”= cyanobacterium, “y”= yeast, “v” virus, “im”= intracellular microbes, “em”= extracellular microbes, “m”= mesohyl, “ns”= nurse cell, “o”= oocyte, “n”= cell nucleus, “l”= lipid body, “k”= yolk bodies, “bl”= blastomere, “le”= larval epithelium. All samples studied were collected and pictured by MM