Physiological mechanisms in the control of bioluminescent countershading in a midwater shrimp

In the oceanic midwater environment, most animals have evolved an extraordinary anti‐predation behavior using bioluminescent countershading (counterillumination) to help them remain cryptic to visual predators. For the midwater penaeid shrimp, Sergestes similis, the interaction of both hormonal and neural systems may be involved in the control of counterillumination. S. similis responds to downward‐directed illumination, detected by the eyes, with light emission from five hepatic light organs. Dark‐adapted specimens undergo a slow induction process prior to production of the conventional counterillumination response. The induction of bio‐luminescence may involve a hormonal pathway mediated by the light‐adapting retinal distal pigment dispersing hormone. Once induced, the rapid control of counterillumination may involve a neural pathway. Because counterilluminating animals directly respond to their optical environment, an understanding of the control of bioluminescence provides an insight into the poorly understood visual processing capabilities of deep‐sea animals.