**4. Discussion**

Since Caribbean coral-dwelling vermetids previously were not recognized in the scientific literature, they may have become introduced recently or they may have been overlooked. The tubes of the snails can be confused with those of polychaete worms of the serpulid genus *Spirobranchus* Blainville, 1818, which are common in the Caribbean, where they have a wide host range [30]. Both groups, coral-dwelling worm snails and serpulid worms, have their tubes partially embedded in the coral skeleton and both possess an operculum that is used to close the tube for the protection of soft bodyparts [2,31]. *Spirobranchus* worms are eye-catching because of their high densities and colorful, twin-conispiral branchiae [31,32]. Vermetid snails, on the other hand, use transparent mucus nets to catch food (Figure 3D,F) [33–35], which makes them less remarkable (Supplementary Materials Figure S19).

**Figure 3.** Close-up images of worm snails (*Petaloconchus* sp.) and their hosts: *Siderastrea siderea* (**A**), *Porites astreoides* (**B**,**D**–**F**), and *Madracis auretenra* (**C**). The tubes are partly or entirely overgrown or surrounded by green and red algae (a). The operculum (o) appears to be tapering towards one side because of the margins being turned upwards next to the tentacles. Some worms show remnants of mucus nets (m). Faecal pellets are common around the tubes (fp). Tube diameter: ca. 4 mm.

The habitat of *Petaloconchus* sp. resembles that of *P. keenae*, which has been reported as an associate of the Indo-Pacific coral genera *Porites*, *Montipora*, and *Pavona* at the Hawaiian islands [21], and possibly as *Petaloconchus* cf. *keenae* living in corals at Kwajalein Atoll in the Marshall Islands [36]. It is therefore reasonable to speculate that the coral-associated *Petaloconchus* sp. in the southern Caribbean is the same species and that it has been introduced from the tropical Indo-Pacific. As long as its identity cannot be confirmed by molecular analyses and morphological studies of the radula, protoconch, egg capsules, and coloration of shell and body [21,22], we consider the present species to be cryptogenic in the southern Caribbean. An earlier presence of such Caribbean worm snails could be verified with the help of coral collections in natural history museums [37]. Most museum collections of stony corals consist of dry specimens, but remnants of vermetid shells may still be present and recognizable.

Considering the poor knowledge of *Petaloconchus* sp. in the Caribbean, it is relevant to know of possible natural enemies that may be able to remove evidence of coral-dwelling vermetids. The carpiliid crab *Carpilius convexus* (Forskål, 1775) has been reported to prey on the vermetid *Ceraesignum maximum* in the Red Sea by breaking its shell and the coral in which it lives [38]. A possible predator of *Petaloconchus* sp. would therefore be *Carpilius corallinus* Herbst, 1783, which has been observed to crush large tubes of serpulid worms and also parts of the host coral [39].

The host-coral range in the present study (21 species) is more extensive than recorded for any other coral-associated vermetid. The host ranges of the Indo-Pacific vermetids *C.*

*maximum* and *P. keenae* (mentioned above) are based on miscellaneous records and might be much larger in reality. Previous field surveys specifically targeting host-coral ranges (and prey preferences) of gastropod families and genera also yielded various additional host records and showed that some species are very host-specific, e.g., *Leptoconchus* [40] and Epitoniidae [41], and that others are generalists, such as some species of *Coralliophila* [42] and *Drupella* [43,44]. Research on coral-dwelling nudibranchs, such as the well-camouflaged *Phestilla* spp., also demonstrates that an ongoing search for possible hosts results in new association records and species discoveries [45–47].

The occurrence of coral injuries (dead coral surface and shells overgrowing coral polyps) suggests that the snails are harmful to their hosts, which is relevant for coral reef conservation [48,49]. It is possible that coral polyps are killed by the snail's mucus webs (Figures 2A and 3D,F), which may smother and even poison them if the mucus is toxic, as reported for *Ceraesignum maximum* [50]. The occurrence of turf algae on the shells is expected to increase damage to the hosts, as observed in *Spirobranchus* tubes overgrowing coral polyps [31,32]. Future field research with a focus on reef-dwelling vermetids will likely result in more information on their densities and additional host records. Molecular analyses may reveal the actual identity of the present species and its native range, and also whether cryptic speciation has taken place across the various host corals.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/d14030196/s1, Supplementary S1: photographic host records of *Petaloconchus* sp.: Figure S1: *Agaricia agaricites* at Bonaire (2019); Figure S2: *Agaricia humilis* at Curaçao (2021); Figure S3: *Agaricia lamarcki* at Curaçao (2021); Figure S4: *Cladopsammia manuelensis* at Curaçao (2017); Figure S5: *Colpophyllia natans* at Curaçao (2021); Figure S6: *Diploria labyrinthiformis* at Curaçao (2021); Figure S7: *Eusmilia fastigiata* at Curaçao (2014); Figure S8: *Madracis auretenra* at Curaçao (2021); Figure S9: *Madracis decactis* at Bonaire (2019); Figure S10: *Madracis senaria* at Curaçao (2021); Figure S11: *Madracis senaria* at Bonaire (2019); Figure S12: *Meandrina meandrites* at Curaçao (2021); Figure S13: *Millepora alcicornis* at Curaçao (2021); Figure S14: *Millepora complanata* at Curaçao (2015); Figure S15: *Montastraea cavernosa* at Bonaire (2019); Figure S16: *Orbicella annularis* at Bonaire (2019); Figure S17: *Orbicella faveolata* at Curaçao (2021); Figure S18: *Orbicella franksi* at Bonaire (2019); Figure S19: *Porites astreoides* at Curaçao (2021); Figure S20: *Pseudodiploria strigosa* at Curaçao (2021); Figure S21: *Siderastrea siderea* at Curaçao (2021); Figure S22: *Stephanocoenia intersepta* at Curaçao (2021); Figure S23: Unidentified dead coral at Curaçao (2021).

**Author Contributions:** Conceptualization, B.W.H. and R.J.v.d.S.; methodology, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; validation, B.W.H.; formal analysis, B.W.H.; investigation, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; resources, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; data curation, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; writing— original draft preparation, B.W.H.; writing—review and editing, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; visualization, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T.; supervision, B.W.H.; project administration, B.W.H.; funding acquisition, B.W.H., C.E.H., S.J.L.-D., R.J.v.d.S., A.S.-M., R.S., R.F.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The field research at Curaçao was funded by the Alida M. Buitendijk Fund, the Jan-Joost ter Pelkwijk Fund, the Holthuis Fund, and the Dutch Research Council (NWO) Doctoral Grant for Teachers Programme (nr. 023.015.036). Fieldwork at Bonaire was supported by the World Wildlife Fund (WWF) Netherlands. The Treub Maatschappij (Society for the Advancement of Research in the Tropics) funded research at both Bonaire and Curaçao.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Acknowledgments:** We are grateful to the funding agencies mentioned above. We thank Rudiger Bieler (Field Museum of Natural History, Chicago) for the identification of the worm snail. We thank the staff of CARMABI (Curaçao) and the Dive Shop for their hospitality and assistance during the fieldwork. BWH is also grateful to Stichting Nationale Parken Bonaire (STINAPA), Dutch Caribbean Nature Alliance (DCNA) and Dive Friends (Bonaire) for logistical support at Bonaire, and to the Caribbean Netherlands Science Institute (CNSI), St. Eustatius National Parks Foundation (STENAPA), and Scubaqua Dive Center, for facilitating research in St. Eustatius. We also want to thank two anonymous reviewers for their constructive comments, which helped us to improve the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
