In the Dongsha lagoon, colonies of hydroids attached to the shell surface grow along the reticulated ridges of the Nassarius shell. The hydrorhiza adheres and spreads across the shell surface, covered by a thin epidermal layer. Each cylindrical hydranth measures approximately 1 mm in length and about 0.2 mm in width, with ten tentacles at the tip. At the base of the hydranth, pear-shaped reproductive structures (gonophores) develop. These gonophores mature into free-swimming medusae; in some cases, immature medusae can be observed within the gonophores. Each hydranth bears ten tentacles, and a pear-shaped medusa structure is present at its base.
Studies of related hydrozoan species indicate that the symbiosis between Cytaeis hydroids and Nassarius snails is complex. The life cycle of these hydrozoans includes a sessile polyp stage and a planktonic medusa stage. The sessile stage attached to the snail shell reproduces asexually through budding, proliferating as the shell grows and producing additional individuals to form expanding colonies. During sexual reproduction, pear-shaped gonophores develop beside the hydranths and mature into sexual medusae. At this stage, the life cycle enters the sexual phase, releasing free-swimming medusae. Individual medusae produce sperm and eggs; fertilized eggs develop into planula larvae. When the planula larvae complete their planktonic phase, they attach to the shell of a Nassarius snail. Once successfully settled, they grow into small hydranths and, following the growth of the snail shell, develop into new colonies.
Each hydranth of the hydrozoan extends its small tentacles as far as possible in hopes of capturing plankton drifting past. When the Nassarius snail buries itself in the sand, the hydroids elongate their bodies so that their tentacles protrude above the substrate, or they feed by picking up organic particles between sand grains. Some individuals even emit a green fluorescence; whether this green glow is due to symbiotic algae remains unknown (End).
(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.)