Christmas tree worms are serpulids of the genus Spirobranchus (Polychaeta: Serpulidae). These sedentary animals are common on shallow coral reefs, where they usually live in close association with scleractinian and milleporid corals (Dai and Yang 1995). In most studies concerning the taxonomy and phylogeny of Spirobranchus worms, little or no attention is given to their host’s identity (e.g., Kupriyanova et al. 2015; Willette et al. 2015). Therefore, it is not unexpected that new host species can still be discovered (Hoeksema and Ten Hove 2014).

During a survey of the distribution and host specificity of Spirobranchus species around Koh Tao, Gulf of Thailand (January 2015), two small individuals of Spirobranchus corniculatus (Grube, 1862) were found as epibionts on the shells of two fluted giant clams, Tridacna squamosa Lamarck, 1819 (both of them ~35 cm long). No epibiotic corals were present on the shells. Identification of the worms was based on recently updated information on S. corniculatus (Kupriyanova et al. 2015; Willette et al. 2015).

The clams and their worms were observed during two dives at 5 m depth off the southern coastline of Koh Tao: Hin Ngam (10°04′02.1″N, 99°50′24.9″E) and Chalok Bay (10°03′41.6″N, 99°49′37.0″E). Both tube worms had settled on the ventral side of their host’s shell, close to the mantle edge. One of the worms was hidden straight underneath the clam’s mantle edge, which was extended outward over the shell’s upper valve margin (Fig. 1a). When the mantle retracted, the worm became exposed (Figs. 1b and c). The worm on the second host was ca. 3 cm away from the mantle edge. These observations are noteworthy because previously giant clam shells were reported to host tubeworms but not specifically Spirobranchus species (Vicentuan-Cabaitan et al. 2014; Neo et al. 2015).

Fig. 1
figure 1

Spirobranchus corniculatus as an associate of Tridacna squamosa at Chalok Bay (scale bars: 1 cm). a Worm underneath the clam’s extended mantle edge (arrow). b Mantle slightly retracted, exposing the worm. c Mantle sufficiently retracted to reveal the worm’s operculum (arrow)