Photographer: Paul Osmond
(see this users gallery)
Commonly found on Fire Urchins (Asthenosoma varium),this a parasitic Eulimid Shell (Luetzenia asthenosomae). This is not a symbiotic relationship, but a true parasite which can negatively effect the urchin over time especially when the number of shells on an individual urchin increases.
Image taken at night on the muck shore dive at Basura in Anilao, Mibini in the Batangas region of The Philippines.