Bedore, C. N., Kajiura, S. M., & Johnsen, S. (2015, December). Freezing behaviour facilitates bioelectric crypsis in cuttlefish faced with predation risk. In Proc. R. Soc. B (Vol. 282, No. 1820, p. 20151886). The Royal Society. DOI: 10.1098/rspb.2015.1886
Regardless of size or shape, all organisms in the ocean face the same three basic challenges: find a mate, find food and avoid being eaten in the process. Avoiding predation can be tricky because the ocean is full of many large predators and suitable hiding places are limited. However, over time organisms have evolved different ways to avoid predation. In the case of cephalopods, such as squid, cuttlefish and octopus, this evolution has resulted in some extraordinary camouflage techniques. Cephalopods are able to match the color and texture of their background to render themselves nearly invisible to potential predators (for more on camouflage, check out this post!).
Though effective, cephalopod camouflage does not offer a 100% success rate. For instance, sharks are very successful in hunting cephalopods and use passive electrolocation to find visually hidden prey. Sharks have sensors that can pick up electric signals, which are produced by every animal in the ocean. For fishes, these signals are created at the gills as a result of ion exchange with the seawater. More ventilation at the gills means more electrical signals are produced. This begs the question: how do cephalopods have a chance at survival when even basic ventilation works against them? Well, the evolutionary “cat-and-mouse” game between predator and prey has become even more interesting. It appears one species of cuttlefish has evolved to reduce its electrical output via a behavioral freeze response. A group of researchers set out to test the effectiveness of this freeze technique in cuttlefish and how it may lower predation from sharks.
Cuttlefish (Sepia officinalis) and two types of sharks (bonnethead and blacktip) were used in the experiments. The first objective was to record baseline electrical voltage and frequency measurements from cuttlefish ventilating by themselves in the tank. They placed electrodes in the tank and measured electrical output associated with each body cavity opening (siphon, funnel and mantle cavity) of the cuttlefish (Fig. 1). They also measured voltage and frequency when the openings were blocked by the arms, which helped determine the insulating effects of the skin.
Using these electrodes, the scientists tested the electrical response to a looming predator. No cuttlefish were harmed in this study – instead cuttlefish were exposed to videos of sharks on a nearby iPad (Fig. 1). The response of cuttlefish to this virtual predator was scored as no response, freeze, or jet (escape) depending on the electrical output. Live sharks were exposed to either a freeze, resting or jet-simulating current typically produced by cuttlefish. The sharks’ behavior towards the different electrical currents was observed.
When cuttlefish were by themselves in the tank they produced electrical voltages ranging from 10-30 µV at the siphon, funnel and mantle openings. These voltages were lowered when openings were closed and much higher when cuttlefish jetted away (Fig. 2). Cuttlefish drastically altered their behavior in the presence of virtual shark predators. Instead of trying to flee from the predator, cuttlefish froze until the stimulus was taken away. This freeze response significantly lowered both electrical voltage and frequency. In addition, “frozen” cuttlefish decreased their mantle height and slowed body movements during ventilation. So, when threatened by shark predators (albeit virtual), cuttlefish chose to freeze rather than escape.
What happens when we look at the freeze response from the viewpoint of the sharks? Both shark species bit at freeze-simulating electrodes only half as often as they bit rest-simulating electrodes. When sharks were exposed to jet-simulating electrodes, the bite response was even more pronounced (Fig. 3). Sharks responded more aggressively and attacked electrodes more often when they were emitting a resting or jetting-simulating current. It seems a freeze response offers cuttlefish the most refuge from shark predators.
Conclusion and Significance:
In the face of a looming shark predator, cuttlefish chose to freeze instead of flee. This freeze response lowers the electrical field of the cuttlefish, limiting the shark’s ability to use its sensory detection. In the ocean, fleeing away from a nearby shark may draw unwanted attention to the cuttlefish. Though cephalopods often perform inking when they dart away, many shark species are chemically attracted to ink products.
Elasmobranchs, such as sharks, place substantial predation pressure on cephalopods, thus electrical freezing may provide vital protection from nearby predators. This case is just one example of an evolutionary “arms race” between predator (shark) and prey (cuttlefish) in the ocean. Avoidance techniques such as the freeze response in cuttlefish help provide structure and stability to marine food webs. Further research on these predator-prey interactions is necessary to understand life in the ocean.