Fig. 2
Electron microscopy (EM) micrograph of pelagic swimming larva of Thoosa mismalolli. (A) Scanning electron microscopy (SEM) of pelagic swimming larvae showing the single layer of imbricate monoaxonic flat disc (spicules; s) and their long radial protrusions (p) with a radial arrangement. (B) Transmission EM section of the body of pelagic swimming larva. Note the loss of regional differentiation, the low cell density in the larval body, and the remanent intracellular bacteria (ib) engulfed by gray cell (grc) with well-developed pseudopodia (white arrows). (C) Detail of gray cells (grc) of the peripherical layer in the MZ. Note the prominent anucleolate nucleus (n) filled with diffuse euchromatin while the cytoplasm contains numerous yolk granules (y) and lipid droplets (l). (D) Detail of a section in the MZ with low cellular density. Note the progressive degeneration of both the peripheral gray cell (grc) layer and the wide alveolar-like layer composed by spherulous cells. (E) A detailed view of the deeper region of the MZ reveals a population of overcrowded extracellular bacteria (eb) and archeocytes (arc) containing numerous yolk granules (y) and lipid droplets (l) as well as phagosomes with 1–2 bacteria (pg). (F) Detail of bacteriocytes (bac) with abundant phagosomes (pg) still containing recognizable intracellular bacteria (ib). (G). Detail of inner zone (IZ) densely populated with extracellular bacteria (eb). (H) Archaeocytes (arc) exhibit the reorganization of the cytoplasmic membrane and the formation of phagosomes (pg). Notably, these cells can efficiently ingest large quantities of extracellular bacteria (eb).
