The world’s biggest squid species have developed over time huge eyes to give them advanced warning of approaching sperm whales. Colossal (15m) and giant squid (13m) have eyes that measure 27cm (11″) across, which is much bigger than any fish on record.
The only benefit of such huge eyes in the murky depths of the oceans is to spot enormous shapes, such as the ones of sperm whales. The findings were published in the journal Current Biology and they could explain the equally large eyes of the ichthyosaurs, which could have helped them evade the even larger pliosaurs.
Lead researcher Dan Nilsson from Lund University in Sweden was himself present at the necroscopy of a colossal squid, four years ago in New Zealand. He was able to examine the eyes and the hard parts of the lens. Predation, mostly by sperm whales and their forebears, has driven the evolution in the eyes of these squids.
While whales can spot squid using sonar, squids can only rely on vision, which could suggest a powerful evolutionary pressure towards developing better eyesight.
Various species of Architeuthis and the much bigger colossal squid (Mesonychoteuthis hamiltoni), can grow to more than 10m in length, when measured from the tip of their body to the end of their tentacles.
M. hamiltoni has barbed, swiveling hooks, which have been documented on the bodies of sperm whales. There are quite a number of colossal squid beaks in the stomachs of sperm whales, indicating that sperm whales often triumph in the underwater battle for survival.
Nilsson showed that there is no advantage in developing eyes larger than the ones belonging to swordfish, about the size of oranges, except when used to detect large, moving masses underwater.
Reference: “A Unique Advantage for Giant Eyes in Giant Squid” by Dan-Eric Nilsson, Eric J. Warrant, Sönke Johnsen, Roger Hanlon and Nadav Shashar, 15 March 2012, Current Biology.