Fig. 4

Temperature effects on the ultrastructure and cell density of the bell tissue of Cassiopea andromeda medusae. A–D Light microscopy images and E, F, H SEM images of the medusae in control (A, C, E, i.e., left column) and heat stress conditions (B, D, F, H, i.e., right column). E and F are artificially colored for better visualization with oral epidermis and gastrodermis in beige, mesoglea in blue, amoebocyte(s) in yellow, amoebocyte nuclei in purple, and symbionts in green. G Impact of heat stress on the density of symbiont cells and host nuclei in the oral (delineated by green dotted line) and inner mesoglea (delineated by red dashed line) illustrated in C and D. Asterisks indicate significant differences between treatments (*p < 0.050, **p < 0.010, ***p < 0.001). H SEM image exhibiting the intracellular degradation (white arrowhead) and disorganized thylakoids (black arrowhead) in heat-stressed symbionts. SW: seawater, oral epi: oral epidermis, m: mesoglea, am: amoebocyte(s), n: amoebocytes nuclei, s: symbionts, gast: gastrodermis, gc: gastric cavity)