Myzostomids are a group of animals whose phylogenetic relationships are still contentious, though an annelid affinity is increasingly favored (Rouse and Pleijel 2007). All are ecto- or endosymbionts (either commensals or parasites), and the majority of the species live in association with echinoderms, mainly crinoids (Lanterbecq et al. 2006).

An endosymbiosis with a Caribbean black coral (Hexacorallia, Anthozoa, Antipatharia) was previously reported (Goenaga 1977).

Morphological and histological analysis of the polyps of the Indonesian black coral Cirrhipathes cf. rumphii (Fig. 1a) revealed the occurrence of a myzostomid in approximately 30 % of the coral colonies that were sampled. The specimens (usually one worm per polyp when present) were found only in distal, large zooids (2–3 mm in diameter; Fig. 1b, c). The worms had rounded, flattened bodies (1.0–1.8 mm in diameter; Fig. 1d) with a cylindrical extensible pharynx, similar to ectosymbiotic Myzostoma (Lanterbecq et al. 2006). Typical of myzostomids, there were five pairs of parapodia with protruding chaetae (Fig. 1e) and a densely ciliated ventral surface. Histology showed that the larger myzostomids were sexually mature females. The myzostomids occupied the entire basal portion of the gastric cavity (Fig. 1c), and no traces of digestion by the host were seen, indicating a true endobiotic lifestyle. Myzostomids were found throughout the year in female and male Cirrhipathes cf. rumphii. The analyzed parasitized polyps were never fertile, even when the neighboring zooids contained mature gametes, suggesting parasitic castration.

Fig. 1
figure 1

a The whip black coral Cirrhipathes cf. rumphii from Bunaken, Sulawesi (Indonesia, Celebes Sea); b close-up of the large polyps arranged all around the stem; c longitudinal section of a polyp showing a myzostomid (white arrow) completely occupying the gastric cavity (m polyp mouth, t polyp tentacle); d an isolated specimen showing the small parapodia and the dorsal sense organs; e SEM image of a parapodium showing the acicular chaetae extruding from the distal extremity. Scale bars a 50 cm, b 5 mm, c, d 0.5 mm, e 25 μm

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Histological observations on an additional 15 Indonesian black coral species suggest that this myzostomid lives exclusively in Cirrhipathes cf. rumphii. However, the occurrence of a myzostomid in the Caribbean Stichopathes sp. suggests that these endosymbioses may be more widespread, especially in species with large-sized polyps, like those of unbranched genera such as Cirrhipathes, Stichopathes, and Pseudocirrhipathes.