**3. Results**

A total of 21 host-coral species—19 scleractinians (Anthozoa) and two milleporids (Hydrozoa)—were recorded, divided over 11 families and 14 genera (Table 1). A few worm snail specimens were found on dead unidentified coral. All records were from the southern Caribbean islands Bonaire and Curaçao, with none from St. Eustatius in the eastern Caribbean. The worm snails with a clearly visible operculum were identified as *Petaloconchus* sp. (Figures 2 and 3; Supplementary Materials Figures S1–S23). Its operculum has upward-folded margins, giving it a tapering appearance, and its diameter was smaller than that of the shells' aperture, which prevented a total shutting of the tube (Figure 3). A small, concave operculum is characteristic for the genus *Petaloconchus* H.C. Lea, 1843 [24].


**Table 1.** Records of stony corals as host species (by family) for the vermetid gastropod *Petaloconchus* sp. based on photographs made at Curaçao (a: 2021; b: 2017; c: 2015; d: 2014), Bonaire (e: 2019), and St. Eustatius (2015).

Owing to its symbiotic nature, the coral-dwelling worm snail of the present study cannot be confused with previously reported Caribbean species, such as the reef-building *Petaloconchus varians* (d'Orbigny, 1839) in Venezuela [5] and Brazil [25]. The invasive vermetid *Eualetes tulipa* (Chenu, 1843) has also been recorded in the West Atlantic and the Caribbean, but its operculum is much darker than that of our specimens and it has not been reported as a coral symbiont but as colonies on rock and artificial substrate [26,27]. *Dendropoma corrodens* (d'Orbigny, 1841) is a small worm snail species (ca. 1 cm long), known from the Caribbean and the mid-Atlantic, which forms aggregations on dead coral substrate [26].

Several snail tubes were covered by algae and surrounded by faecal pellets (Figure 3). Many were surrounded by dead coral tissue (Figure 3B,C,E) or attached to dead coral next to the host's margin (Figure 3A). A few snails showed remnants of mucus webs (Figures 2A and 3D,F). In some corals, the snail tube was killing the polyps underneath and did not become overgrown by coral tissue (Figure 2C,D; Supplementary Materials Figures S17 and S19). In other ones no damage was observed, such as in *Cladopsammia manuelensis*, *Eusmilia fastigiata*, *Madracis auretenra*, *M. decactis* and *Millepora alcicornis* (Figure 2A,B; Supplementary

Materials Figures S4 and S7–S9). *Cladopsammia manuelensis* has recently been discovered as a shallow-water coral in the Caribbean [28,29].

**Figure 2.** Worm snails (*Petaloconchus* sp.) hosted by the branching corals *Madracis auretenra* (**A**) and *Millepora alcicornis* (**B**), and the massive corals *Porites astreoides* (**C**) and *Siderastrea siderea* (**D**). The snail tube in *M. alcicornis* is entirely overgrown by the coral (**B**), whereas the snails in both massive corals have caused considerable damage to their hosts (**C**,**D**). Tube diameter: ca. 4 mm.
