Published in Science on Friday 5th December, a new study has observed interesting behaviours in electric eel predation.
‘Electric eels’ (Electrophorous electricus – some awesome alliteration there) though not actually technically eels, are one of a few groups of marine organisms which harness electric discharges for prey capture and predator deterrence, though many lineages are able to sense these fields. Elasmobranchs of all genera detect electric fields using ampullae of Lorenzini, which are distributed in pores laterally along the fish’s body and predominantly around the head; the movement of prey item’s muscle in the water column generates electric fields which are detected to a very small threshold of sensitivity. Further, a number of Batoid elasmobranchs – the rays – have adapted to produce electricity proportional to their size, up to around 200volts. While the Pacific electric ray uses electricity to stun prey, the less electric ray uses theirs exclusively in predator defence. Elephantnose fish are a group of Osteoglossiformes, the group which contain giant arapaima, and are classified as producing either ‘waves’ and ‘pulses’ to sense their environment.
And finally, one of my favourite animals; the duck-billed platypus (Ornithorhynchus anatinus), uses electroreceptors embedded in the rostrocaudal rows of skin of the bill to determine the size and direction of prey in the water, in conjunction with touch and pressure sensitive mechanoreceptors. These are homologous to the electrosensors in the bills of the related terrestrial Echidna, who have significantly less (≤2000 vs 40,000). Both of these species are Monotremes, and in addition to being the only egg laying mammals, are the only mammals with these electrosensory capabilities.
Electric eels use electrically to ‘illuminate’ their environment, sending out low-voltage pulses and sensing the feedback, much like echolocating bats. However, they are also renowned for their ability to generate extremely high voltage shocks; shocks of up to 600volts, over double that of a UK mains plug.
This has consistently been interpreted as an adaptation to stun and immobilise prey. This makes them easier to eat, and stops otherwise trashing prey from damaging delicate respiratory surfaces around the gill area.
However, this new research by Catania suggests that the discharges eels can generate are specialised. The high frequency, high voltage shocks mentioned above allow immobilisation capture of free swimming prey, or defence against predators. Also known, however, are repeated pairs and triplets of high voltage discharges, which are used when hunting in complex environments.
Catania measured the responses to eels’ electric discharge in the bodies of fish, and found that motor neurone activation is required to produce muscle contraction of the prey fish.
This means that electric eels’ discharges act specifically to activate prey motor neurones, and hence muscle, and that this allows the eels to ‘remotely control’ their desired prey item. They not only immobilise prey, but by repeatedly sending out periodic waves or pulses of electric discharges, that the eel can cause full body involuntary twitching, causing the prey to move and reveal its location – the eel then strikes 10-15 seconds later. These volleys of impluses are advantagous, as an initial twitch and the resulting electrical feedback will inform the eel if the prey item is alive, and worth the energy investment of pursuing it. This also acts to explain why this behaviour is used particularly in cryptic environments, where prey needs to be ‘flushed out’.
With this insight, there is further evidence to support suggestion that organisms that are highly sensitive to and which produce electric fields may use them for additional reasons, including those not exclusively relating to predator-prey interactions, such as sociosexual selection and communicating with conspecifics.