Figure 4.

The benefit of huge eyes in giant and colossal squid. An illustration of eye size (a) shows the continuous range of eye size in fish, cephalopods and whales, up to that of swordfish (3 cm pupil diameter [28]), and the leap up to 9 cm pupil diameter in giant and colossal squid [10]. Visual volume as a function of pupil diameter (b), calculated for stimulated bioluminescence below 800 m depth where there is no influence of daylight and performance is independent of viewing direction (from [11]). The ability to detect very large luminous objects (2 m wide) is superior and grows steadily for eye sized up to those of adult giant and colossal squid. The shaded areas indicate the range of eye sizes found in animals except giant and colossal squid (grey), and the range unique to these large cephalopods (light blue). Modelling values (from [11]): clear oceanic water, 10¹⁰ quanta s⁻¹ point source intensity, 0.55 m nearest-neighbour distance between point sources, approaching target triggering a water volume calculated as a cylinder of target width (0.1 m, 0.5 m, 2 m) and a length of 2.5 target widths, receptor diameter 0.4 µm.