The eye is one of the most marvellously complex results of evolution, and our eyes are more complicated than most. Innumerable organisms use eyes to differentiate the world around them, and, as we mentioned before, lots of animals see in different colours (https://www.seriousreaders.com/blog/mantis-shrimp-proud-owners-of-the-strangest-eyes-in-the-animal-kingdom). But, as any fiduciary financial consultant will tell you, “More is always better.” So, we ask the most important question, which animal has the most eyes?

Hang on a minute - what is an eye?

Okay, okay - let’s start by defining what an eye is, because animals use these organs in very different ways. For the purpose of this discussion today, we will be referring to eyes as individual ‘organs of vision’.

Why is this definition important? Well, there are some interesting critters like the dragonfly, which have very large, segmented eyes. If you were to count all of these segments, then you could say that dragonflies have a whopping 30,000 eyes. However, these segments are connected to the nervous system via only two points, and so these 30,000 segments are more correctly defined as a large array of lenses, rather than individual eyes.

Excellent - we now know what an eye is.

But who really is packing it in the eye department?

Lampreys

Let’s start off with one creature you wouldn’t want to meet in a dark rockpool - the lamprey. According to Wikipedia (and more importantly, several scientists), the lamprey is the only still-existing vertebrate with four eyes. Why is this? Well, it is generally regarded that vertebrate organisms with greater than two eyes, actually possess eyes which are more primitive than their binocular (two-eyed) counterparts.

Judging by the vastly complex eyes of mammals, it is better to have just two well-developed eyes that detect more light and colours, than four eyes with individual optic nerves, which serve to only detect one or two wavelengths. The lamprey is a part of the oldest living group of vertebrates, called cyclostomes, an evolutionary line which never even developed a jaw. They have remained relatively unchanged for hundreds of millions of years, and their visual system reflects this. Not many of their relatives survive today, and perhaps those eyes are part of the reason why.

How many eyes? - four

Spider

Now, when you ask most people about the animal with the most eyes, they’ll often tell you it’s the spider, with eight eyes. Ironically, spiders are much more reliant on their sense of touch, than their sense of sight. Most spider species can’t see in a great amount of detail, getting a rather fuzzy image. However, due to their lack of balance and acceleration sensors, they actually can’t tell which way up they are without their eyes - a trait shared with many arthropods.

So, if a spider is blind, it literally can’t tell which way is up.

How many eyes? eight

Box jellyfish

Box jellyfish are some of the most dangerous animals around, with poisonous, 15ft stinging tentacles which can cause a cardiac arrest in minutes. If that wasn’t disconcerting enough, they’ll also watch you with a whopping 24 eyes as you try to swim past them. Box jellyfish technically aren’t “true jellyfish”, as they are capable of slowly trudging towards their prey (at about 4mph), which may be what their eyes are primarily used for.

All twenty-four eyes of the box jellyfish are considered “true eyes”, each possessing a retina, cornea and lens. Its visual system can be split into two unique sections. The first section can be referred to as the “rhopalial ocelli”, forming a cluster of two pairs of eyes. These are able to discern directional light, with one pair pointing upward, and the other downward. The second section, using twenty eyes, exists purely to delineate sensations of light and dark and these are much simpler organs.

The eyes of these terribly toxic, tentacled terrors are much more advanced than those of other jubilant jellyfish - which is just another reason to steer well clear of them.

How many eyes? Twenty-four

Scallop

Yes - I know what you’re thinking. Scallops really are delightful, pan seared with a bit of garlic butter, but they also have the most eyes of any organism, with an almost unbelievable 200 eyes.

Scallops have tiny, blue iridescent eyes all the way around the edge of their mantle, lining up against its shell. The truly fascinating part of their eyes are the guanine crystalline structures inside the eye, which act as mirrors, reflecting light into two retinas on each eye. This is the same way that light is reflected in a telescope.

All of that remarkable evolutionary biology, stored inside a little, ocean-bound mollusc without a brain.

How many eyes? Over two hundred……

What an eye-ronic turn of events….

We hope you’ve learned something about the eyes of animals today, and that you aren’t too disappointed that a brainless mollusc has the most eyes in the animal kingdom. While humans typically only have two eyes, they are quite adequate for what we require, whether that is admiring others, glaring angrily at social delinquents, or reading very interesting articles on the internet.

Sources:

Suzuki DG, Grillner S. The stepwise development of the lamprey visual system and its evolutionary implications. Biol Rev Camb Philos Soc. 2018 Aug;93(3):1461-1477. doi: 10.1111/brv.12403. Epub 2018 Feb 28. PMID: 29488315..

1. Funtrivia.
2. Wikipedia.
3. Smithsonian Magazine.