Electrical food-sensing fish!

A new study in PLoS Biology describes a fish which uses an electric field to sense its prey. While these sorts of prey-sensing strategies have been known for quite some time, this black ghost knife fish from the Amazon is unusual because its sensory range is omnidirectional, meaning that it can sense prey in a cylinder-shaped volume around its body, compared to other animals like bats and dolphins which generally only sense what is in front of them. The authors introduce two concepts: Sensory Volume (SV) - the volume of space in which the animal can detect objects and prey, and the Motor Volume (MV) - the volume within reach of the animal in a certain length of time by activating its musculoskeletal system. The SV of a lion, for example, is long and extends far beyond its MV (the space it can reach by moving and running a certain degree) because it can see and hear for long distances. In contrast, this fish has a small and omnidirectional SV and MV, meaning that it is sensitive to a small volume immediately surrounding its own body, but can react quickly to anything that comes within this space too. Therefore, the SV to MV ratio can be a taken as a measure of the strategy an animal takes to react to prey and other stimuli - close and immediate vs. far and long-term.

Another evolutionary 'theme' relevant here is the idea of cephalization - having a head. The usual story goes that sessile animals (fixed to a substrate, like corals and sea anemones) are usually radially symmetrical so that they can respond equally to stimuli (prey, danger, water currents) which might conceivably come from any direction. On the other hand, mobile animals tend to be bilateral and cephalized, the head being where the sensory, feeding, and gas-exchange organs are concentrated, because it is most advantageous to have awareness and the capacity to react to external stimuli in the front of the body, facing the direction of movement. Therefore, an omnidirectional sensory field like this one described here shows that the above explanation might be oversimplifying matters, and that animals are possibly more aware than we give them credit for.