PHYSIOLOGICAL-MECHANISMS IN THE CONTROL OF BIOLUMINESCENT COUNTERSHADING IN A MIDWATER SHRIMP

In the oceanic midwater environment, most animals have evolved an extr aordinary anti-predation behavior using bioluminescent countershading (counterillumination) to help them remain cryptic to visual predators. For the midwater penaeid shrimp, Sergestes similis, the interaction o f both both hormonal and neural systems may be involved in the control of counterillumination. S. similis responds to downward-directed illu mination, 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 bioluminescence may involve a hormonal pathway mediat ed by the light-adapting retinal distal pigment dispersing hormone. On ce induced, the rapid control of counterillumination may involve a neu ral pathway. Because counterilluminating animals directly respond to t heir optical environment, an understanding of the control of biolumine scence provides an insight into the poorly understood visual processin g capabilities of deep-sea animals.