Future Predators Avoid Becoming Prey
Electroreception is the ability of certain marine vertebrates to detect minute electrical charges transmitted through the water. The shark electroreceptor system is composed of sensory organs called ampullae of Lorenzini, that lie within small pores across the head of the shark. All animals emit low level electrical fields generated by the activity of their cells, and this highly sophisticated system allows sharks to detect minute changes in the surrounding electrical field. These electrical signals help them localize and identify nearby prey, enhancing their visual hunting abilities.
A prey species finding itself in the vicinity of a hungry shark therefore seems doomed to become a meal. After all, he can’t alter his electrical field. Or can he? If that prey is a developing shark, with its own sensory system beginning to come online, it seems he can use these early electroreception abilities to avoid becoming a meal himself, at the teeth of another shark or other predator.
In work recently published, Kempster et al examined the electroreception abilities of embryos of the Brownbanded bamboo shark (Chiloscyllium punctatum). These small sharks are oviparous, completing their development within egg cases, the leathery structures that female sharks attach to sea grasses and other substrates. The egg-encased, tethered embryos have no ability to flee from approaching predators. As they near the end of their development, the shark egg cases open to the water, and both transmit and receive electrical signals.
When Kempster et al exposed embryonic bamboo sharks to small electrical fields that resembled those of other sharks or large fish, the embryos immediately became very still and stopped all gill movements – in effect they “held their breath”. These actions are likely to diminish both the electrical changes caused by the respiration of the embryos, as well as any physical movement of the water that might give away their presence. The response was strongest in the older embryos with more fully developed electroreception systems. The young sharks couldn’t be fooled again, however, when the same artificial stimulus was given half an hour later – perhaps realizing that no actual predator appeared the first time – they no longer responded.
Kempster et al suggest that a better understanding of electroreception might inform the development of more efficient shark repellant devices, but even on its own this is an key insight into the survival mechanisms of developing embryos.
The article is: Kempster RM, Hart NS, Collin SP (2013) Survival of the Stillest: Predator avoidance in shark embryos. PLoS One 8(1): e52551.
And can be found open access at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0052551
Photo credit: www.abc.au