(b) Transmission electron microscope (TEM)

For each species, sub-sample sections were made from the samples of the top, middle and bottom part of one colony from a 20 m depth and one colony from a 40 m depth. For *S. maldivensis*, due to the large size of the polyps (~6 mm, Figure 2b), sections of the tentacles and of the oral cone were analysed separately, making a total of 18 sub-samples in all. These sections, each about 1 cm long, were post-fixed for 1 h at room temperature with 1% osmium tetroxide in a 0.1 M sodium cacodylate and 2.3% sodium chloride buffer. They were rinsed several times in the same buffer and dehydrated in an increasing concentration series of ethanol. Samples were then placed in Spurr resin overnight before polymerisation at 70 ◦C for 24 h. Then ultrathin sections of 50–70 nm were cut on a Leica Ultracut (UCT) microtome equipped with a diamond knife and collected on formvar-coated copper grids. These were stained with uranyl acetate and lead citrate and observed with an LEO 906E (Zeiss) transmission electron microscope.

(c) Scanning electron microscope (SEM)

For each species, the samples corresponding to the middle sections of one colony from a 20 m depth and one from a 40 m depth (each about 1.5 cm long) were studied under a scanning electron microscope (SEM). The four samples stored in 70% ethanol were then dehydrated for TEM analysis. Sub-samples were also made in order to be frozen with liquid nitrogen before being cut randomly. The eight sub-samples were dried in a critical-point dryer using CO2 as the transition fluid (Agar Scientific Ltd.) before being mounted on aluminium stubs and coated with gold in a JFC-1100E (JEOL) sputter coater. These samples were observed and photographed with a JSM-7200F (JEOL) scanning electron microscope.
