*3.1. Dinoflagellate Cells Count*

The isolation of Symbiodiniaceae-like cells with sodium hydroxide should have enabled efficient isolation of the microalgal cells if present within the coral tissues and also isolated cnidocytes (Figure 3). For both antipatharian species and both depths (all 66 samples), dinoflagellate cell density ranged between 0–4 cells mm−3. Microalgae density was different between the two species (*t* = 2.36, *p* = 0.029; Table 1), with a mean density of 0.037 ± 0.013 cells mm−<sup>3</sup> (mean ± SE) for *C. abies* and 0.121 ± 0.034 cells mm−<sup>3</sup> for *S. maldivensis*. The mean density difference between the two species was 0.084 cells mm−<sup>3</sup> regardless of the depth. No significant difference was detected based on the region of the colony using the quasi-Poisson GLM; therefore, the factor 'region' was removed from the model (the average dinoflagellate cell density from the three regions of each colony was calculated to obtain a mean cell density per colony for further analyses). No significant difference was detected in Symbiodiniaceae-like density between depths (*t* = −1.577, *p* = 0.131; Table 1). However, the statistical power to detect a significant difference in cell density between depths for each species was limited. The results of a power analysis showed a low power of 0.26 (Type II error rate 74%) for *S. maldivensis* and 0.06 (Type II error rate 94%) for *C. abies* to detect differences between depths.

**Table 1.** Results of the quasi-Poisson GLM test for differences between species and depths. The intercept represents *C. abies* dinoflagellate cell density. Significant *p*-value (*p* < 0.05) are shown in bold.

