Why does the “cockeye” squid have a teeny tiny eye, and a giant green eye?

molluscfacts:

I’m so glad someone asked this – the cockeyed squid is one of my favourite molluscs for this exact reason. Here’s what they look like, for those who don’t know:

Cockeyed squid is actually a genus (Histioteuthis) that comprises quite a few species, including the one pictured above which is known by the adorable name, strawberry squid (Histioteuthis heteropsis), but they all have the famous odd-sized eyes – something that’s not seen in any other bilaterally symmetrical animal. 

The reason for this is each of their eyes is adapted for a different type of vision. Cockeyed squid live in the mesoplegaic (or twilight) zone where sunlight is almost non-existent, and they generally float like in the picture above, with their large left eye facing slightly upwards, and their smaller right eye facing downwards. 

The role of the fairly normal-looking right eye is to pick up up the bright bioluminescence of creatures in the dark below them. Their left eye, however, is specially adapted to break the counter-illumination of creatures swimming above them. Counter-illumination is a strategy adopted by many creatures in the deep sea, where they illuminate the underside of their bodies just enough to “counter” their silhouette made by the small amount sunlight coming from above. The squid’s large eye sees past this as it is very sensitive to light, and the greenish-yellow lens helps to filter out the sunlight, making the creatures stand out.

This article from MBARI goes into a bit more depth

Also, here’s a good video showing how they swim: