Snails associated with the coral-killing sponge Terpios hoshinota in Okinawa Island, Japan.

Terpios hoshinota is a thin encrusting sponge that overgrows live scleractinian corals and it is linked to coral loss in many reefs. However, our knowledge of the species associated with this sponge species is poor. During a periodical survey of T. hoshinota in 2020, we found tiny snails crawling on the sponge in the subtropical waters around Okinawa Island, Japan. We observed egg capsules inside the sponge tissue and veliger larvae released from the egg capsules. Molecular analyses of both the snails and veliger larvae (cytochrome oxidase I, COI) showed that they were identical and belonged to Joculator sp. (family Cerithiopsidae). There was no direct observation of predation on the sponge by this snail; however, to the best of our knowledge, this is the first report on a close association between a snail and the sponge T. hoshinota.

Contact Reactions between Individuals of the Coral-killing sponge, Terpios hoshinota.

Outbreaks of the coral-killing sponge Terpios hoshinota Rützler and Muzik, 1993 have become a threat to corals and result in coral reef deterioration. This species has an increasing distribution in the Indo-Pacific Ocean and thrives in patches on some reefs in Okinawa, Japan. However, the dispersal process and mechanisms involved remain unknown. We observed the self and non-self recognition capabilities of T. hoshinota by performing contact assays in aquarium and in the field. In the contact assays (indirect and direct contact), allogeneic sets did not fuse and showed a rejection reaction as they formed boundaries (approx. 0.2 mm width) between their tissues. Although the initial reaction between individuals involved adhesion in allogeneic sets, the two individuals remained distant from each other. Histological observations showed that soft tissues (such as collagen) were not present in the boundary zones. These boundaries were maintained for more than 2 weeks. Boundary formations were also confirmed at three field sites in Okinawa, Japan. Our results suggest that T. hoshinota can distinguish self and non-self individuals. Contact assays are a useful method for evaluating the spatial distribution and local population structures of T. hoshinota in coral reefs.