In the open sea, there is nowhere to hide. Predators scanning the depths can spot potential victims by how they scatter the light shining through the water. However the different shapes, sizes and transparencies of the creatures swimming around in the ocean affects how easy they are to see. So Yakir Gagnon, Nadav Shashar and their colleagues measured how light-scattering by zooplankton, a common prey, affects their visibility in the Gulf of Aqaba().
The team collected a variety of shrimp-like crustaceans, marine worms and acorn worm larvae, mostly 1–3 mm long, using light traps or very fine mesh nets. Gagnon delicately glued individual zooplankton with a tiny amount of superglue to a very fine glass needle before re-submerging them in seawater. To measure scattered light, the team shone white light directly downwards onto the back of each animal. A semi-circle of fibre optic cables placed at different angles relative to the light source and linked up to spectrophotometers measured the spectra of the light scattered off the animals at each of the different angles. They could also calculate how much light passed through the creatures, and therefore how transparent they were, finding that they ranged in transparencies from 1.5% to 75%.
As light travels through the water column, it is scattered or absorbed: red wavelengths are filtered out first, and blue light travels the furthest. Light intensity decreases logarithmically with depth, such that differences in light intensity with depth are much greater at shallower depths than at deeper ones. So the depth of the water influences which wavelengths of light are present,and therefore which wavelengths are scattered.
With this in mind, the team wanted to know how visible each of the species were at different depths and viewing angles. To do this they combined the information they had collected about the unique light scattering patterns of the different zooplankton species with previously developed mathematical models that calculated how light intensity changes in the sea according to the viewing angle of an observer, the angle of the light travelling through the water, and also the depth. Using these models, the team calculated how depth affected the sighting distance, which is the distance between an observer and the zooplankton.
At depths of up to 40 m, most of the animals were brighter than the background, and were easiest to see when observed horizontally or from diagonally above and below. This means that at less than 40 m depth, it is much harder so see the animals by looking directly down onto them, or directly up. In general, the less transparent animals could be seen from further away.
At greater depths, however, there is a cross-over such that the background is brighter than the animals, but the point at which this happens varies depending on the viewing angles and the depth. Theoretically, at this point the animals should be very difficult to see as they have a similar brightness to the background. A little deeper, though, and they become less bright relative to the surrounding water. The best viewing angle also changed at depths below 40 m, to looking directly upwards. These results suggest that the predators that feed on the zooplankton will have to pick their viewing position carefully if they want to spot their next meal.
