*4.3. Dinoflagellate Density Difference between Depths*

Contrary to expectation, no statistically significant difference in dinoflagellate density was found between shallow and mesophotic depths, although the mean dinoflagellates density at 20 m depth was greater than at 40 m depth in both species (the mean differences between depths being 0.200 and 0.016 cells mm−<sup>3</sup> for *S. maldivensis* and *C. abies*, respectively). Differences in microalgae density can be inferred from previous studies. For instance, among the 14 samples from different colonies of *A. griggi* from Hawaii for which the cell density was reported, the highest densities were from the two colonies sampled on shallow reefs [34]. The samples of *Cirrhipathes* collected in Bunaken, Indonesia, were from two different depths (15 and 38 m) [37]. While it was not reported whether there were any differences in Symbiodiniaceae density related to depth, the Kd 490 (proxy of turbidity) values at 15 m and 38 m depth in Bunaken have been estimated to be very similar [50]. In Toliara, Madagascar, where the Kd 490 values indicate high turbidity (Figure 1b) due to sedimentation derived from river runoff, light penetration at a 40 m depth is likely to be considerably reduced (as was noticeable during field work). However, it is not inevitable that higher densities of Symbiodiniaceae should be found in association with antipatharians in shallower water since it is believed that the uptake of dinoflagellates in the coral–algae symbiotic association can be controlled by the coral host [25,31]. More studies on antipatharian colonies exposed to higher radiations are therefore necessary to assess the significance of high densities of dinoflagellates in antipatharians (assuming they occur), and to understand the mechanism behind the association.
