Fishy secret of the deep

MOST people have never heard of the two most common fish.

Bristlemouths, of the Cyclothone family, are the most numerous vertebrate on Earth, with quadrillions of these inchlong fish swimming in the ocean’s twilight zone, 1,000ft down.

The 6 in lanternfish of the Myctophidae family make up more than half of deep sea creatures.

Bristlemouths have needle-like fangs and can open their mouths extraordinarily wide. They have rows of photophores, glandular organs that appear as luminous spots, along their underside. These provide countershading or camouflage from predators and prey. Their swim bladders are lipidfilled to maintain a stable position.

Lanternfish rise at night to 350 ft to feed and descend to 3,000 ft during the day to avoid predators. This daily migration can be tracked using sonar bouncing off their air-filled swim bladders.

Their bioluminescence comes from bacteria they culture and they can control how much light they emit, which is where they get their name.

Max Smith, St Andrews, Fife.

QUESTION How do active noise-cancelling headphones work?

WHEN wearing headphones in a noisy environment, the sound that passes to your ears consists of a combined waveform of what you want to listen to plus noise leaking in from outside.

noise-cancelling earphones have microphones that pick up only the outside noise. The electronics then inverts the phase, to make a mirror-image waveform, then adjusts its amplitude and mixes it with the audio stream of your device.

What reaches your ear is the music you want to hear with the noise signal cancelled out. The sophistication lies in how well the electronics carries this out. Better, faster and more intelligent algorithms work on the derived cancelling signal to adapt and anticipate the outside noise waveform.

At the simpler end, white noise — a steady hiss or drone, such as in an aircraft cabin — is dealt with well due to the fact it is regular and predictable. White noise generators are sometimes included in

modern office design where they can cancel random background noise. Counter-intuitively, adding noise makes a quieter environment.

Keith Matthews, Ferndown, Dorset.

QUESTION What has been the best reply to someone who pulled rank with: ‘Do you know who I am?’

FURTHER to the earlier answer, Ben Elton penned the best answer to this.

In his police station TV sitcom, The Thin Blue Line, a group of hooray henry types have been arrested for being drunk and disorderly. One turns to a WPC and drawls: ‘Do you know who my father is?’

unimpressed, she replies: ‘I’m sorry I can’t help you there, Sir. have you tried asking your mother?’

Phil Alexander, Farnborough, Hants.

IS THERE a question to which you want to know the answer? Or do you know the answer to a question here? Write to: Charles Legge, Answers To Correspondents, Daily Mail, 2 Derry Street, London W8 5TT; or email charles.legge@dailymail.co.uk. A selection is published, but we’re unable to enter into individual correspondence.