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DISPATCH 7: - 9.19.2006 | Tammy Frank Many people think that the animals living in the deep-sea are blindly stumbling through the depths, hoping to bump into their prey, or perhaps sensing them by chemical 
signals or motion. While it is true that deep-sea animals use a variety of senses to detect prey, many of them
are by no means blind, and have wonderful adaptations that make their eyes considerably more
sensitive than ours. So, while we perhaps would be stumbling around down there in the dark,
unable to see anything, they can actually see things at great depths due to some of the visual
adaptation described below.
There are a group of remarkable crustaceans called hyperiid amphipods that have some of the most unusual eyes we've seen. Phronima, seen in side view on the left, and an end-on view on the right, has huge eyes covering virtually the whole head. Based on the shape of the eye and 
the enormous claws, many of us who have seen Phronima (about an inch long in real life) are
convinced that this animal was the model for Alien. Phronima actually has four eyes - the two
huge fused lobes that you can see in the picture, and two
smaller lobes, one of which is pointed out by the arrow. The huge lobes present a very large
area for collecting light, but this animals need to get the light down to the tiny retinas
(4 of them - the black dots where the red arrow is) without losing any out to the sides.
These retinas are so tiny because Phronima is virtually transparent (an excellent predator
defense, as it's hard to see transparent animals in the water column), and big black eyes
would make it much more visible to predators. The way it transmits light down to the retinas
was discovered in the 1960s, when a scientist studying the structure of the eye found that
biological light guides fill the eyes and funnel light down to the tiny retina. These work
identically to man-made light guide, which weren't developed until the 1950s - just think
how much further we'd be along in fiber optic technology if we'd studied this animals in
the 1930s!
Cystisoma a is hyperiid amphipod that's quite large - the largest one we've collected on this trip was 3 inches long. The eyes of Cystisoma also cover the entire head. However, rather than compressing visual pigment into tiny dense black retinas, its retinas are broad flat plate like structures, as seen by the dim orange plates in the picture on the right. Spreading out the visual pigment like this still leaves their eyes reasonably transparent, and the enormous size of the eyes provides a vast area for collecting light. Gigantocypris is the world's biggest ostracod. Ostracods are sometimes called clam shrimp, because they resembled a clam, and are generally the size of a tomato seed. Gigantocypris, found at > 800 m, can be the size of a large marble. Living at such deep depths, it must collect as much light as possible to be able to see anything at all. Therefore, its eyes are enormous parabolic reflectors, designed to funnel as much light as possible to the retina. With this design, these remarkable eyes have an F-number of about .25 (the lowest F-stop on a standard camera is about 2.8). 
Some animals have eyes that have different functions. For example, in a squid called Histioteuthis, the eyes are dramatically different sizes. When these squid originally came up in trawls, scientists thought that one eye was damaged. However, as trawl technology got better, and these animals started coming up alive and in excellent shape, they soon realized that this was an adaptation to the light field in the deep-sea environment. In this environment, downwelling light (coming from the sun) is the light you see when looking upwards in the water column, and is relatively bright compared to the upwelling light (coming from backscatter) that you see when looking below you. he big eye of Histioteuthis has a lens, which helps to focus light and increase resolution, but lenses absorb and therefore cut down on light hitting the retina. The little eye doesn't have a lens, and while the image it obtains won't be at clear, it should have considerably higher sensitivity than the bigger eye. Scientists have hypothesized that the squid swims with the big eye pointing upwards towards the brighter downwelling light field when hunting for prey, while the little eye is pointing downward towards the dimmer upwelling light field, serving basically as a shadow detector. 
Some of the deeper living euphausiid species (krill are euphausiids) have similar adaptations for viewing bright downwelling and dim upwelling light, but in the same eye, rather than having one eye for downwelling and one eye for upwelling light. They have what are called "bilobed" eyes, because each eye is divided into two distinct regions, with a clear separation between them. This picture is of a species with equal sized upper and lower lobes, but in some species the upper lobe is considerably bigger, while in other species, the lower lobe is considerably smaller. As in Histioteuthis, the upper lobe has finer focusing structures, so it's not as sensitive to light but has better resolution and is probably used for tracking prey. The lower lobe has much cruder focusing structure, so it's considerably more sensitive to light, but has lower resolution and is probably used mainly for detecting shadows against the background light. 
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