Adult individuals of the pearlfish Carapus mourlani (Petit, 1934) occur commonly in the mantle cavity of the opisthobranch mollusc Dolabella auricularia (Lightfoot, 1786) in shallow marine waters of the Gulf of Chiriquí, Pacific Panamá. Nearly 30% of the molluscan hosts collected during the day on a coral reef contained one or two fish. Feeding observations of a captive fish as well as the intact condition of the host's ctenidium and other internal organs suggest that C. mourlani is an inquiline commensal and not parasitic. Fish curl around the ctenidium during the day and capture microcrustaceans when the fish emerge from their host at night to feed. From low-light infrared video recordings, Carapus was observed to accurately grasp rapidly swimming amphipods in nearly total darkness and ingest them. This symbiotic relationship appears to benefit Carapus by allowing the fish to avoid predators during the day and to forage at night.