The Spiny Squat Lobster (Galathea strigosa) is one of the more striking denizens of the depths in UK waters. This specimen I photographed at the Farnes was not to keen to pose, but I was able to capture some of the brilliant blue patterning that characterizes this species.
Its interesting to speculate what the purpose is of the bright blue lines, considering that without the benefit of a camera flash, the murk and darkness soaks up much of the color and that these crustaceans are most active at night. The key, I suspect, is that the color blue is one of the last colors to fade out as you go deeper, and would be one of the few still visible at depth (the orange, in contrast, would be indistinguishable from other nearby colors). Some crabs have been shown to make use of coloration patterns to distinguish conspecifics and to evaluate potential rivals, while the lines (appearing as dark streaks on a light background) could help confuse predators by breaking up the body pattern. Other than some speculative observations on the related Galathea intermedia I haven't come across any studies that try to test this, however.