Sponges are known to host a variety of invertebrates, including polychaete worms (Koukouras et al. 1996). Host sponges offer space and can serve as a refuge for such animals if they excrete chemical compounds, which may serve as defense against predators (Waddell and Pawlik 2000). Some encrusting sponges on coral reefs use toxins as an aid to overgrow and kill live corals to create suitable substrate space, but they are not necessarily lethal to the Christmas tree worms that live inside the corals (Hoeksema et al. 2016). Christmas tree worms are polychaetes of the genus Spirobranchus(Serpulidae), which produce calcareous tubes as dwellings that are attached to hard substrate or embedded inside the coral skeletons.

During a survey on sponge–coral interactions at Curaçao (southern Caribbean) in June 2017, the serpulid Spirobranchus giganteus(Pallas, 1766) was observed as associate of at least 10 sponge species with various growth forms (Fig. 1; Electronic supplementary material Figs. S1–15). A worm tube attached to a vase sponge (Fig. 1a) was covered by an orange-veined encrusting sponge (Fig. 1b, c). Dissection of the sponges revealed that worm tubes could be tracked down to the corals that acted as original substrates for the worms. Apparently, the sponges had overgrown the corals that contained the worms. As substitute hosts, they allowed the worms to continue their growth. Spirobranchus worms have also been reported from various Mediterranean sponges, but the nature and mechanism of their association was not explained (Koukouras et al. 1996).

Abstract

Snails of the genus Coralliophila (Muricidae: Coralliophilinae) are common corallivores in the Caribbean, feeding on a wide range of host species. In the present study, the morphological and genetic variation in C. galea and C. caribaea were studied in relation to their association with host coral species at Curaçao. Differences in shell shape among snails living on different hosts were quantified using geometric morphometric and phylogenet-ic relationships were studied using two mitochondrial markers (12S and COI). Based on these analyses, a new species, C. cura-caoensis sp. nov., was found in association with the scleractinian coral Madracis auretenra. Both C. galea and C. caribaea showed host-specific differences in shell shape, size, and shell allometry (i.e. changes in morphological development during growth). Shell spire variability contributed foremost to the overall variation in shell shape. In C. caribaea minor genetic differences existed between snails associated with scleractinian and alcyonacean cor-als, whereas in C. galea such intraspecific variation was not found. These results shed more light on morphological and genetic differences among coral-associated fauna living on different host species.