Several hundred species of fish have evolved the ability to generate electric fields, which they use to navigate, communicate and home in on prey. But this ability comes at a cost – the electric field is generated continuously throughout life, so consumes a great deal of energy, and it can also attract predators which are sensitive to it. Electrogenic fish species therefore utilize various strategies to save energy and to minimize the likelihood of being detected. Some generate irregular pulses of electrical discharges whose rate can be modulated; others can also modulate the strength of the electric field.
The cellular and molecular mechanisms underlying one of these behavioural adaptations are now revealed in a beautiful study published in the open access journal PLoS Biology. It shows that in one species of electric fish, circadian cues and social encounters regulate the movements of proteins called voltage-gated sodium channels – which are crucial for generating the electric field – in cells of the electric organ. At night, low light levels and social interactions drive the insertion of sodium channels into the cell membranes, leading to a dramatic increase in the strength of the electric field.
THIS weird and wonderful creature is the star-nosed mole (Condylura cristata), a small, semi-aquatic mammal which inhabits the low wetlands of eastern North America. Like other moles, it ekes out an existence in a network of narrow underground tunnels, and digs shallow surface tunnels where it forages for insects, worms and molluscs. Living as it does in almost complete darkness, the star-nosed mole has poorly developed eyes, and is virtually blind. Instead, it relies heavily on its remarkable star-shaped nose. This organ enables the star-nosed mole to decide whether something is edible with astonishing speed – in fact, it recently entered the Guinness Book of Records as the world’s fastest forager – and also to sniff out food underwater.
The star-shaped nose is a highly specialized sensory-motor organ, which consists of 11 pairs of fleshy finger-like appendages, or ‘tendrils’. The star, which is less than half an inch in diameter, is divided into a high resolution central fovea region and less sensitive peripheral areas. It is much larger than the nose of other mole species, covering 0.92 cm2 per touch, compared to 0.11 cm2 covered by the noses of other mole species. The star also contains a far higher density of receptors than the noses of other mole species; its surface is covered with 25,000 mechanoreceptors called Eimer’s organs. (That makes it about 6 times more sensitive than the human hand, which contains about 17,000 receptors.) This makes the star ultrasensitive – it is, in fact, the most sensitive organ in the entire animal kingdom.