Once considered a hazard to mariners, the mysterious and reclusive giant Pacific octopus (Enteroctopus dofleini) is a remarkable predator even if it doesn’t attack boats. As one of the largest of the extant family of Octopodidae, this beautiful creature is an efficient and mostly nocturnal hunter that spends the day concealed in rocky caves in the northern Pacific. One specimen was weighed at 272 kg with a tip to tip spread of its arms of 9.6 metres, but most are in the 15 kg range with a much smaller arm spread. It is a primarily visual predator, relying upon sharp visual acuity honed with remarkable intelligence to subdue well fortified prey, and to protect itself against its own predators.

The octopus eye (an invertebrate eye) resembles a fish eye (a vertebrate eye) and is a good illustration of convergent evolution. On closer examination, however, there are substantial and critical differences illustrating that the eyes of octopuses and of fish do not have a common camera-style eye ancestor, but rather each evolved its individual eye separately.

Both the octopus and the fish eye have a camera-style eye with an iris, nearly circular lens, vitreous cavity, and photoreceptor cells lining the interior of the cavity. There …