Light has trouble penetrating beyond 1km into an ocean, but animals deeper down still manage to see. How do they do it?
Our cruise involved a pretty mixed bunch, representing seven nationalities and 14 research institutions, with the single objective of exploring and interpreting the role of light in the lives of deep-sea animals.
The feeblest vestiges of daylight hardly reach 1km into the clearest oceans, which have an average depth of 3.8km. The result is that all the world's oceans are dark some of the time and most of this colossal living space is dark all of the time, lit only by flashes of bioluminescence.
Yet most deep-sea animals have eyes, whose often astonishing optical complexities have evolved to view this living light.
Interpreting the diversity and ecology of the deep sea requires knowledge of how the eyes and the light are used to find prey, to attract mates and to elude predators.
To get this knowledge we must go to the animals, in our case by working on the Natural Environment Research Council research vessel RRS Discovery. Our experience ranges from first-time students to veterans of almost 60 cruises and we trawl for our quarry in the depths of the eastern Atlantic, close to the Cape Verde Islands.
Animals from trawls are usually dead, but it is essential for our purposes that we collect live ones. Special buckets on our nets bring the animals up in the same cold and darkness that they normally experience. Their exquisitely sensitive eyes are easily damaged by our lights; they must remain in the dark if we are to carry out realistic studies of their capabilities. Working in the dark (with an infra-red viewer) in an enclosed space on a rolling ship has interesting side-effects - but is a necessary infliction.
We have two main objectives: the first is to establish what the different light-sensing structures in our fish, shrimp and squid can do and the second is to find out what bioluminescent signals the animals are producing. We are recording from the eyes of our deep-sea shrimp to find how they respond to defined light sources.
In our fishes, we are labelling and counting the nerve fibres that run from the eyes and other sensory systems to the brain, and preparing tissues from the pineal organs and the eyes for later study of their light-sensitive pigments. Molecular analysis of the genes that code for the opsin component of these pigments will be carried out ashore.
This will tell us how they are related and explain their diversity. Direct measurements on the filters and reflectors in the eyes of fish, squid and shrimp are showing us how the visual systems are tuned to wavelengths.
Light signals in the deep sea are recorded with an image-intensified video camera sent to the seafloor and later recalled to the surface for recovery of the data. The bioluminescent displays of live specimens are imaged on board. We are studying their light-emitting organs, their chemistry and the nature of the luminous bacteria used by female anglerfish to light their lures and attract their prey.
As our cruise continued, the signs of success became increasingly apparent: the ripe aroma from the trawl net on the afterdeck, the chronic lack of sleep from the round-the-clock fishing, the mounting samples in the deep-freezers.
Alongside the scientists, two previous colleagues were on board to film the deep-sea animals for the BBC Blue Planet series; on the morning we caught a large, live, female anglerfish with an attached parasitic male, it was as if we had won the deep-sea lottery.
Peter Herring, Southampton Oceanography Centre, RRS Discovery, recently in the eastern Atlantic.