Turning loss into opportunity: the key deletion of an escape circuit in decapod crustaceans.

Decapod crustacean escape responses are adaptive behaviors whose neural bases are well understood. The escape circuit is composed of giant neurons. Lateral giant interneurons (LGs) respond to posterior stimuli by generating a somersaulting tailflip; medial giant interneurons (MGs) respond to anterior stimuli with a backwards tailflip. Both sets of interneurons connect to giant fast flexor motor neurons (MoGs). Most features of the escape circuit are thought to result from strong selective pressure to respond to stimuli in the shortest possible time. Despite the apparent advantages of the escape circuit, it has been lost in multiple taxa independently. Some losses of the escape circuit may be rare cases of disaptation, where organisms are less well adapted than related species (i.e., those with the escape circuit). The losses of the escape circuit might be key deletions that promoted the radiation of decapod crustaceans by increasing selection pressure for species to evolve new anti-predator strategies and removing constraints against change.