*3.2. Specimen Morphological and Molecular Analyses*

Morphological analyses and comparisons (see Table 1) showed specimens of *Palythoa caribaeorum* colonies (Figure 1A) with various levels of coenenchyme thickness (see description in [4,21]). *P. caribaeorum* oral disks and tentacle colors varied between brown and green, in comparison to that observed for *Zoanthus* spp. at all sites, where colors ranged from bright green, orange, grey, blue, and dark green. Maximum mean oral disk diameter for *P. caribaeorum* was 12.8 mm ± 0.96 (Mean ± SD, *n* = 15 polyps; 3 polyps per colony), which was larger than for *Zoanthus* spp. 10.2 mm ± 0.10, *n* = 15 polyps (Table 1). Although *P. caribaeorum* and *P. grandiflora* (Figure 2B) could easily be identified through morphological analyses using maximum oral disk size, tentacle color, and tentacle numbers, analyses of *Zoanthus* spp. (Figure 2C–F) were challenging with the aforementioned characteristics. *Zoanthus* spp. colonies showed variation in coenenchyma thickness.

DNA was successfully amplified for 21 COI amplicons, which were approximately 780 bp in length. Phylogenetic analysis of the mitochondrial cytochrome oxidase subunit I (COI) gene identified zoantharians as *Zoanthus pulchellus*, and *Zoanthus sociatus* (see Figure 3) and harboring *Symbiodinium* sp. (formerly Symbiodinium clade A), whereas *Palythoa caribaeorum* and *Palythoa grandiflora* (Verrill, 1900) (see Figure 3) harbored *Cladocopium* sp. (formerly Symbiodinium clade C) in the symbiont family Symbiodiniaceae. The COI tree (Figure 3) distinguished *Palythoa* and *Zoanthus* spp., which belong to families Sphenopidae and Zoanthidae, respectively. Blasted specimens aligned with sequences from Florida (GenBank accession numbers JX119156, JX119154, JX119165) and Brazil (Gen-Bank accession number KT454365). Other Atlantic Ocean sequences (GenBank accession numbers JX119160, JX119164, JX119167, JX119157, JX119159, JX119168 AB214177 KF499705, KF499712) aligned with closely related zoantharian species, such as *P. caribaeorum* and *P. grandiflora*; *Z. pulchellus* and *Z. vietnamensis* (Pax and Müller, 1957); *Z. sociatus* and *Z. sansibaricus* (Carlgren, 1900). For instance, *Palythoa* sp. (MZ150797-801, MZ147090-91) aligned with *Palythoa caribaeorum* (KT454365) and *Palythoa grandiflora* (JX119165). Additionally, *Zoanthus pulchellus* (JX119156) aligned with MZ150802-805, MZ147092-094, MZ156026, and was distinguished from *Zoanthus sociatus* (JX119154) aligned with MZ147095-097 and MZ150806-807, and were well supported by phylogenetic analyses (ML bootstrap% > 60%) (Figure 3).

Results for specimens in family Sphenopidae revealed moderate support for subclade *P. caribaeorum* (KT454365) and *P. grandiflora* (JX119165) (63%). Results for specimens in family Zoanthidae revealed moderate support for *Z. pulchellus* (JX119156) (69%), and high support for *Z. sociatus* (JX119154) (94%) (Figure 3). *Zoanthus pulchellus* specimens (MZ147092-094, MZ150802-805, MZ156026) also matched sequences from *Z. vietnamensis*, which is widely distributed across Indo-Pacific coral reefs, and *Zoanthus sociatus* specimens (MZ147095-097, MZ150806-807) sequences matched *Z. sansibaricus*, which is also widely distributed across Indo-Pacific coral reefs.

**Figure 2.** Zoantharian colonies along the northeastern coast of Toco, Trinidad (**A**) brown color morphotype of *Palythoa caribaeorum* specimen P3-br-SB, (**B**) green color morphotype of *Palythoa grandiflora* specimen P4-gr-GA, (**C**–**E**) color morphotype variation in *Zoanthus pulchellus* specimen Z9-org-GA, Z5-gr-SB, Z6-gr-SB, respectively (**F**) color morphotype in *Zoanthus sociatus* specimen Z15-blu-GA.

**Figure 3.** Phylogenetic maximum likelihood tree generated from the mitochondrial COI sequence alignment. GenBank accession numbers from other studies of species shown in parentheses. Numbers above branches represent maximum likelihood probabilities.
