Water-Insoluble Black Pigment Released from the Octocoral Sinularia flexibilis

Coral reefs are the most diverse and productive marine ecosystems on earth. The National Museum of Marine Biology and Aquarium has cultured more than 35 species of corals for research. When we conducted the asexual propagation of corals, the octocoral Sinularia flexibilis released a black pigment that stained the operator’s hands black. This is the first reported case of the skin being dyed black while propagating corals. We quantified the blackness of the stain by using the RGB value of the color. The longer the coral contacted the skin, the darker the skin became. Incubating the tentacles of S. flexibilis in high-salinity filtered seawater increased the amount of the black pigment released. However, collecting 100% of the black pigment was exceedingly challenging because it was very sticky and was constantly entangled with Symbiodiniaceae. Furthermore, we were unable to identify any solvents that could dissolve the pigment. The structure and function of the black pigment merit further study as it has the potential to become a new black dye for human industries.