Nassarius snails are common prosobranch gastropods and are eurythermal and euryhaline species. They typically inhabit sandy or muddy substrates in intertidal zones and shallow coastal waters, as well as tide pools and rocky areas within coral reef environments; a few species live in deeper seabeds. Usually buried beneath the sand, they extend a siphon above the surface to draw in fresh seawater while using highly sensitive chemoreceptors to detect environmental changes, including temperature, water quality, conspecifics, potential mates, and predators. Most importantly, they are able to detect the direction of food sources. Nassarius snails primarily feed on fish or other animal carcasses; they may also prey on polychaete worms, sipunculans, and in some species, even the eggs of opisthobranchs.
Within the sandy bottoms of the inner lagoon reefs of Dongsha, there lives a group of swift-moving “sand ninjas” — Nassarius snails, with the black-topped Nassarius being the most representative species. Hidden beneath the sand, black-topped Nassarius snails detect the scent of nearby food in the water and quickly emerge from the substrate, rapidly approaching their target. They then extend their elongated proboscis and use the radula at its tip to scrape off food. In a complete and healthy ecosystem, Nassarius populations do not allow animal carcasses to remain on the seabed for long. Within a short time, they mobilize in large numbers to remove these “delicacies,” playing the role of scavengers and maintaining nutrient cycling within the ecosystem.
An interesting phenomenon observed in Dongsha’s inner lagoon is that some black-topped Nassarius individuals have their shells entirely covered by colonies of hydroids, giving them the appearance of wearing a feathery cloak. These hydroids belong to the class Hydrozoa, family Cytaeididae, genus Cytaeis. Although commonly referred to as jellyfish, they are actually sessile hydrozoans. Research on this symbiotic relationship between Cytaeis hydroids and snails is still limited, with records reported only from Kagoshima, Japan (Cytaeis sp.) and northern Sulawesi, Indonesia (Cytaeis capitata)… (to be continued).
(Text and photos provided by the Department of Biomedical Science and Environmental Biology, Kaohsiung Medical University: Chiu Yu-wen, Huang Yen-ming, Su Chun-yu, and Tsai Cheng-ta.)