Unveiling the predator–prey interaction between a newly discovered nudibranch and sponge with molecular evidence

Benthic invertebrates often use secondary metabolites as chemical defenses, which are key to understanding marine ecosystem dynamics. Nudibranchs accumulate biologically active compounds from sessile prey such as sponges and bryozoans. While many sponge-feeding nudibranchs show specific dietary preferences, the molecular details of their predator–prey relationships remain largely unknown. In this study, a new nudibranch species, Rostanga confitura sp. nov., was discovered consistently attached to the encrusting sponge Phorbas sugashimaensis sp. nov. The nudibranch is identified as a new species of the genus Rostanga on the basis of morphology and phylogenetic analyses. Similarly, the sponge was identified as a new species, Phorbas sugashimaensis sp. nov., characterized by its spicule characters. Furthermore, 11 shared compounds between the nudibranch and sponge, along with the detection of sponge DNA in the nudibranch’s feces, indicate a predator–prey relationship. This study sheds light on the chemical ecology and feeding behavior of nudibranchs, emphasizing their intricate interactions with marine sponges.