Bioluminescence of the marine worm Chaetopterus variopedatus was first investigated several decades ago mainly using tissue extract. Light production of the worm, however, originates from a secreted mucus only. Here, we report the optical and physicochemical properties of the luminous mucus. We show that the produced light occurs as a long glow in the blue range (455 nm), which is an unusual color for a shallow benthic invertebrate. We also show that the light originates from a photoprotein whose light production is independent of molecular oxygen yet somewhat related to the physicochemical (rheological) characteristics of the mucus itself. Indeed, the mucus seems to polymerize and become more viscous on exposure to H2O2, which in turn seems to inhibit the light production. Ferrous iron was not associated with any strong stimulatory effect. This is in contrast to past studies on worm tissues showing that the light production is strongly stimulated by H2O2 and ferrous iron. Overall, our results highlight the fact that working on the luminous mucus only (vs. worm tissues) provides the ability to study its chemical properties possibly involved in the fine control of light production-as well as its rheological properties-and identify the possible interactions between these two properties.