The three Caribbean Coralliophila species clustered into three well-supported, separate clades on the phylogenetic tree (posterior probability (PP) = 0.997-1.000; Fig. 15). Phylogenetic trees constructed for both markers separately showed the same pattern (Online Supplementary Material 5). No monophyletic Caribbean cluster was found: Coralliophila curacaoensis sp. nov. is the sister species of a group consisting of both C. fontanangioyae Smriglio and Mariottini, 2000 and C. meyendorffii (Calcara, 1845) from the Eastern Atlantic and the Mediterranean; C. galea is sister of C. mira (Cotton and Godfrey, 1932) from the Indo-Pacific, and C. caribaea is sister of the Indo-Pacific Leptoconchus sp. Hence, the genus Coralliophila is also not monophyletic.
No host-associated genetic divergence was found within Coralliophila galea (p = 0.458 and p = 0.342 for 12S and COI, respectively). In C. caribaea on the other hand, a small divergence (mean uncorrected distance of 0.6% for 12S and 2.9% for COI) was found between snails associated with Alcyonacea and those associated with Scleractinia (p < 0.0001 for both 12S and COI). This genetic structuring within C. caribaea was also visible on the phylogenetic tree, as C. caribaea collected from Scleractinia cluster all on a single branch (with one exception), although the support value for this branch was low (PP = 0.803). There was no host-associated divergence within C. caribaea from scleractinian corals (p = 0.213 and p = 0.971 for 12S and COI, respectively) or alcyonacean hosts (p = 0.941 and p = 0.945 for 12S and COI, respectively).
The small divergence among C. caribaea individuals associated with either scleractinians or alcyonaceans was also observed in the haplotype networks constructed for both markers (Fig. 16). With one exception for COI, haplotypes were unique to snails associated with hosts from either Scleractinia or Alcyonacea. In addition, haplotypes of snails associated with alcyonaceans mostly (with one exception for both markers) clustered together on a single branch in the haplotype network. Snails associated with scleractinians formed (again, with one exception for both markers) the other branches in the networks. The two clusters were separated by two mutations for 12S and eight mutations for COI. No clear correlation between host species and haplotype was present within C. galea.
The ABGD analysis based on COI, using a more extensive dataset of coralliophiline snails, proposed nine different groupings of specimens into MOTUs, depending on the a priori boundary between intra- and interspecific divergence. A histogram of the frequencies of pairwise Kimura two-parameter (K2P) distances revealed a multimodal distribution, with the lowest minimum frequency around a K2P distance of 0.05 (Fig. 17). Using this value as a boundary between intra- and interspecific divergence, the coralliophiline dataset can be subdivided into 16 MOTUs. At this value, the specimens of C. galea, C. caribaea and C. curacaoensis sp. nov. are clustered into three different MOTUs, in agreement with the phylogenetic tree. The divergence within C. caribaea was confirmed to be intraspecific, not revealing any cryptic species, using this boundary value of 0.05.