Insights into a putative polychaete-gastropod symbiosis from a newly identified annelid worm that predates upon Conus ermineus eggs

Cone snails are a diverse group of venomous marine gastropods that have dioecious reproduction and internal fertilization resulting in egg deposition inside capsules. However, the observational studies conducted on their spawning behaviour and egg masses have left many open questions. Here, we analyse egg masses from a specimen of Conus ermineus kept in captivity for over 17 years. We present the first detailed description of the morphological features of the egg capsules and eggs (e.g., egg shape, size, and colour). The analysis of these capsules led us to the identification of a dinophilid worm (Polychaeta: Dinophilidae) living inside the snail egg capsules and likely feeding upon the snail eggs. This is the first report of such behaviours among dinophilids. Our analysis suggests that these worms belong to a new species, here described as Dimorphilus oophagus sp. nov., supported by both molecular and morphological data. Finally, we discuss the possibility of a putative symbiotic relationship between the worm and the snail.