Counterillumination reduces bites by Great White sharks.

Abstract

In the open ocean, achieving camouflage is complicated by the fact that the downwelling light is generally much brighter than the upwelling light, which means that any object, even if its ventral surface is white due to countershading, will appear as a dark silhouette when viewed from below.^1,2,3 To overcome this, many marine species employ counterillumination, whereby light is emitted from photophores on their ventral surface to replace the downwelling light blocked by their body.^4,5,6 However, only a single behavioral study has tested the efficacy of counterillumination as an anti-predation strategy.⁷ Counterillumination is predicted to be particularly useful against predators that have poor visual acuity and lack color vision,⁸ like the Great White shark (Carcharodon carcharias), the species responsible for most human shark-bite fatalities globally.⁹ Here, we take inspiration from nature to show that counterillumination can prevent Great White sharks from attacking artificial seal decoys. Using seal decoys fitted with LED lights and towed behind a boat, we explored the efficiency of different light configurations on the deterrence effect, showing that visual shape and motion cues are critical for prey recognition by Great White sharks. Counterillumination that is brighter than the background is most effective in deterring sharks, implying that, in this context, counterillumination works through disruptive camouflage rather than background matching. Our results reveal the importance of a dark silhouette against a lighter background in predatory behavior in Great White sharks and that altering the silhouette may form the basis of new non-invasive shark deterrent technology to protect human life. VIDEO ABSTRACT.