*3.2. Morphological Analyses*

Histological sections could only be produced for the *S. maldivensis* samples. Symbiodiniaceae-like cells could be observed in both the polyps and the gastrovascular gastrodermis of the sections examined (Figure 4). From most paraffin blocks containing samples of *C. abies*, sections of the tentacles and oral cone were not obtained because there is only a thin layer of tissue surrounding the skeleton, but cells likely to be Symbiodiniaceae could not be detected in the few successful sections that were made. Ultrastructural

(TEM) observations were possible for all 18 samples examined from the two species, but no Symbiodiniaceae-like cells could be identified in any of them. Mucous cells, zymogen granules, and a large number of cnidocytes, including spirocysts and b-mastigophores were evident (Figure 5). On SEM examination, round cells of different sizes (3–10 μm) were observed, among which the larger round cells inside the gastrodermis of the polyp gastrovascular cavity (~8 μm; Figure 6a) could be dinoflagellates cells. More abundant were smaller round cells (3–4 μm), which are likely to be the vesicular mucous cells also observed on the ultrastructural analysis (Figure 6b).

**Figure 4.** Histological cross-sections of a *Stichopathes maldivensis* polyp. Symbiodiniaceae-like cells (arrows) are observed in the gastrodermis of the gastrovascular cavity (**a**) and in the gastrodermis of a lateral tentacle (**b**). (**c**) Inset for comparison is an image from Wagner et al. ([34], Figure 2c) in which Symbiodiniaceae cells were identified through histological examination and molecular analysis. Scale bars = 20 μm.

**Figure 5.** Transmission electron microscope (TEM) images of a cross-section of a *Cupressopathes abies* polyp tentacle. (**a**) Ultrastructural view of numerous cnidocytes, including: spirocysts (sp) and b-mastigophores (b-m), as well as spumous mucous cells (sp.m). (**b**) Longitudinal section of mature spirocysts (sp). (**c**) Close view of zymogen granules (zym) and spumous mucous cells (sp.m) inside the gastrodermis. (**d**) Cross-section of mature b-mastigophores. Scale bars = 2 μm.

**Figure 6.** Scanning electron microscope (SEM) images of a *Stichopathes maldivensis polyp.* (**a**) A round potentially dinoflagellate-cell of about 8 μm inside the gastrodermis of the polyp gastrovascular cavity (white arrow). Numerous b-mastigophores can be observed in the ectoderm (blue arrows). (**b**) Smaller round cells (3–4 μm) that are likely to be mucous cells (white arrow) in the polyp ectoderm. The image also shows several b-mastigophores aligned among which one has a broken capsule which exposes the cnidocyte tubule (black arrow). (**c**) Closer view of spirocysts in a polyp tentacle ectoderm. Scale bars: a = 10 μm, b = 2 μm, c =1 μm.
