Wikipediaposting 2: animal eyes
While longer blog post ideas I have are still slowly finding their shape, I can't resist some sillyposting about fun trivia on animals, after a first post on this format almost a year ago.
(I also need the silly distraction to survive the increasingly authoritarian and severe repression against the nation-wide protests currently going on in France, so please indulge me.)
So here's some more surprising stuff I found throughout my wikidives! There's even a common theme this times: eyes.
Content warning for somewhat disturbing facts about the number of eyes of a variety of animals, including spiders (no picture of spider is shown).
To improve this post's accessibility, the more unsettling pictures are under clickable spoilers.
Three eyes
A wide variety of animals possess a photoreceptor at the top of their head, called the parietal eye, which is able to perceive light changes to some degree. It is more or less visible depending on the species1.
Click to see a picture of the regal horned lizard, where the phenomenon is particularly striking.
This "third eye" is not present in birds or in mammals, but it can be found in many other vertebrates (which surprisingly does not include snakes, despite including adjacent species).
Funnily enough, among this list, the lamprey is a bit of a special case: it has two parietal eyes in succession, effectively having four eyes. While that evolutionary trait has been lost on humans, I think it's a fascinating organ that more people should know about.
I mean, I wish I had been told sooner that sharks or lizards had opened their third eye.
Four eyes
We already mentioned the lamprey, which would fit the bill for this section, but... I couldn't believe it when I first found the page for this fish. The Anableps. The four-eyed fish2.
It's not about parietal eyes this time: it's because this fish literally has two pupils in each eye, one to see above the water surface and one below, as the eye is divided into two retinas to process the two parts of the information at once.
The following excerpt is long but deserves the read, I think. It starts with the amazing sentence "four-eyed fish have only two eyes, but" followed by some of the wildest stuff about eyes I've ever read.
Anyway, this is such a specific adaptation it's fascinating. I still don't know if I'm more unsettled or more amazed about it.
As some extra trivia about them, these fish also have some peculiar nonsymmetric mating habits that they share with their cousin species Jenynsia - whose Wikipedia page feels the need to precise, funnily but quite normatively, "Unlike their cousin Anableps, their eyes are normal."
Zero eye
Having no eye - or undevelopped eyes - is a common thing for a lot of animals living mostly in the dark, like many abyss-dwelling or cave-dwelling creatures (like some cave spiders or deep-sea-vent-based shrimps), or some much more expected species like moles.
Still, in some animals there's something a bit eerie to it: see for instance the delightfully weird eyeless axolotl-looking creature called the olm3.
Click here if you want to see what the olm looks like.
Whatever happens with most insects
Insects have two big compounds eyes, like how you'd picture a fly's eyes4... but apparently a lot of them often also have a lot of "simpler" eyes called ocelli, located somewhere between those bigger eyes. It's a bit disturbing when you think about this for too long, so let's not.
Whatever happens with some sea animals
Some of the sea animals least thought of as animals, like corals or anemone, don't have eyes. But it gets very interesting with a lot of others. I honestly didn't expect any of those while I was researching stuff for this post.
Starfish, for instance, have around a hundred ocelli at the end of each arm.
Several species of jellyfish also seem to have ocelli - but the box jellyfish in particular apparently has eyes more or less in the same sense as mammals?!
- According to some article, sea urchins seem to use their whole body as a compound eye. I'm not sure how that works or what it implies biologically, but that's quite the statement.
Despite what it looks like though, the urchin below that looks 100% like a videogame boss doesn't have a big eye; apparently that's just their butt.
- Finally, a few bivalves seem to have ocelli on the rim of their shell, and the scallop's is particularly striking, with around 200 such eyes. This is honestly equally impressive and creepy, and I'll never see scallops the same.
Click here if you want to see a striking cluster of iridiscent blue scallop eyes from up close.
One eye
Look, I'm absolutely out of my depth at this point of the post but I found that apparently some very odd tube creatures called tunicates may have exactly one eye? Also the strange not-fish below, called the lancelet? The latter's page even has a dedicated subsection called "Front eye"!
These two groups are the weird branches of the Chordata phylum... and honestly I'm unable to tell you more but I sure have some great Wikipedia pages to read for later now. These creatures are phylogenetically closer to humans than, say, the octopus or any insect!
Six eyes
You thought spiders always had eight eyes? Well... there's a whole Wikipedia page here to contradict this idea.
As a consequence, some spider groups from this list are described in a somewhat chaotic way - here a paragraph from the Haplogynae:
You'll notice in this paragraph a mention of the Tetrablemmidae (armored spiders), a group of spiders who have only four eyes, which is also a fun thing I didn't know about before stumbling onto this.
But wait... there's even better.
BE NOT AFRAID
Meet the Oonopidae, the goblin spider family, who can have 6, or 4, or 2, or 0 eye(s) depending on the species.
But the big winner in the "biblically accurate spider" category is the Caponiidae family. First, because they have an even wider range of available eye options, including odd numbers of 3 or 5 eyes5 - at least visually because some of their eyes almost meet in the middle.
... But even more importantly, because some Caponiidae have this:
I was absolutely floored by this. There are spiders whose number of eyes increases throughout their lives?!
I feel a whole spectrum of weird feelings knowing this, and now you may too! Yay!!
(note: After further investigation, I haven't found any actual source for this fact in the Wikipedia article for Caponiidae or any of its species'. Similarly, all the pages for the species with an apparent odd number of eyes didn't provide any more details about when this visual phenomenon happened. I suppose more research is needed, and [source?] brackets are probably deserved in the meantime. But I sure hope this is correct!)
You survived this post so here's a little guy
... No relationship with the eye stuff, I just thought you could use a little guy. They're the dumbo octopus, often conflated with the extensively mediatized flapjack octopus because the two were somehow mixed up in media articles and even sea floor livestreams for a while.
I love them.
Anyway! Eyes!!
... You know, I suppose this makes for a decent spooky season post6.
Don't mind me, I was just looking at the page for the Regal horned lizard out of curiosity, because of their prominent parietal eye; and I stumble upon the sentence "their main defense when coming in contact with predators is the ability to squirt blood from their eyes"?! aaaahgfjhgjdhgfj. Technically, it seems to come from the lower eyelid pores of their regular eyes, but this sure is a sentence to read...↩
all other animals called "four-eyed" merely have spots on their bodies that make them look like they have extra eyes... but this can get a bit unsettling too, see for instance in the case of the four-eyed turtle. Still, the Anableps is clearly something else.↩
also consider: the similar-looking Texas blind salamander, which belongs to the fascinating lungless salamanders, which breathe through their skin!↩
I remember extensively studying the human eye in biology class, but never insect eyes. Now I wonder how compound eyes work.↩
the French version of the same page doesn't mention the trivia of a potential odd number of eyes, but mentions at least one species with zero eye.↩
The less enthusiastic conclusion of this post is that we don't need dystopian sci-fi: sentient alien-looking lifeforms are actually here all over our planet and so fascinatingly different from us! and yet we kill them en masse, either through the exploitation of their biomes or straight-up for human consumption, because we consider their lives negligible. Maybe such a conclusion is too spooky for spooky season though.↩