Luminescent defensive behaviors of polynoid polychaete worms to natural predators

The intertidal polynoid scale worm Harmothoe imbricata emits a bright green luminescence from its elytra when provoked. These worms are hypothesized to use both luminescence and autotomization (voluntary loss of body parts) as specialized defenses against predators. Previous studies had focused on the mechanisms of light production or used human manipulation to provoke luminescent displays, rather than using actual predators. Worms and local crustacean predators collected in Cumberland County, Maine at three intertidal locations between June 2011 and March 2013 were placed in a small chamber with a removable center divider and their interaction was recorded with infrared cameras, a night vision device with an infrared barrier filter, and photomultipliers. Three different luminescent responses were observed: attached scale flashing, autotomized glowing scales, and autotomized flashing/glowing posterior segments. In some cases, autotomized scales/segments acted as successful decoys, with the anterior portion of the worm crawling away while the attacker focused on the posterior glowing portion. Worms appeared to use different combinations of defensive behaviors in different scenarios, such as when attacked by different predators. Luminescent defensive displays were also more complex than predicted and suggest that the predator’s actions during and following an attack may inadvertently aid in the scale worm’s escape. The use of live predators was pivotal to describing the worm’s defense mechanism and providing insight into how luminescence may be used in nature.