Cruise diary

1 July 2009
Location: NE Atlantic

Nathalie writes (with a little help from Will and Veerle)….

Very late last night/very early in the morning ISIS was sent on another mission. This time, rather than investigating life on the seabed, she was making a map of the seafloor through a swath bathymetry survey. The multi-beam swath system works by sending out a fan of sound waves to the seafloor. We know how fast sound travels through water, so by recording how long it takes for the sound to be reflected back up, we can calculate how deep the seafloor is at that point.

The resulting data produces a map of the changes in terrain; including the shape of the canyons we’ve been investigating on this cruise. We already have a map of the area, produced by a system on board the ship, but ISIS can produce a better map, as she can be manoeuvred 40m above the seafloor. This is better because the sound waves have less distance to travel so there is less attenuation (loss of quality) and the fan of signals can be much smaller, which can provide more, smaller-scale detail.

As ISIS dived to the seafloor, Brigitte and Veerle spotted bioluminescence, and a school of bright orange scorpion fish, which kept ISIS company for quite a while!

What is bioluminescence?
In the deep sea, light is absent, so a lot of predators (like fish & squid) have big eyes to collect as much light as possible, so by producing short flashes of intense light smaller creatures like shrimp dazzle whatever may be trying to eat them. Once the predator recovers, it attacks the spot where the light appeared, by which time the shrimp has moved out of the way.

Other deep-sea creatures that use light:

Hatchet fish use light to match their surroundings so they can disappear – like camouflage.
Deep-sea angler fish have a light, which hangs in front of them, that attracts other critters, which they can then eat.
There’s another deep sea fish (called Malacosteus) that uses red light like a searchlight to hunt – lots of other deep sea creatures are coloured red (e.g. crabs, lobsters and anemones) as in the dark re d is a very hard colour to see, so with it’s red light this fish can spot more than most… yum!