Synchronous and asynchronous counterillumination by three types of photophores in the firefly squid, <i>Watasenia scintillans</i>

Countershading—animal body coloration that is darker on the dorsum than the ventrum— is a form of crypsis achieved by a gradient in skin pigmentation. Counterillumination is somewhat similar, achieving crypsis in midwater marine organisms by the emission of bioluminescence directed downwards. Although mesopelagic squids are assumed to use counterillumination, high-resolution spatiotemporal changes in illumination, particularly for each type of photophore, have not been elucidated. Watasenia scintillans has three types of photophore on its ventral surfaces: blue-emitting and green-emitting abdominal photophores, and ocular photophores. It is demonstrated here that all three types of photophore are involved in counterillumination synchronously, reversibly, and repeatedly in response to changes in dim overhead lighting. The green abdominal photophores exhibited a gradual decay and maintained their glow in the dark. Conversely, when animals were exposed to air, green luminescence alone was quickly extinguished. There were no observed color shifts (such as from green to blue) in any of the photophores. The ocular photophores emit blue light irregularly, partly because they are concealed by the epidermal chromatophores. While synchronous illumination does suggest the crypsis hypothesis, the ability to independently control each type of photophore could also support a previous hypothesis of conspecific signaling. Findings Firefly squid counterillumination was recorded on video. Synchronous illumination of all the ventral photophores in response to dim, overhead light supports the crypsis function hypothesis. Asynchronous illumination of three types of photophores in the dark suggests an additional function besides a crypsis effect. No color shift was observed in any photophores.