This episode was brought to you by Curiosity Stream (+ Nebula!)
Hey everyone, it’s Inés! And look! I found a snail! Do meet Siput!
I happened upon them as I was crossing a road in the rain, and my foot very narrowly missed
their big shell. So concerned, I decided to pick it up to save it from the perils of crawling
by a rainy main road, and I was also curious to see how its size would compare to the Giant
African Land Snail.
...it’s Achatina fulica.
Yes, that’s the Giant African Land Snail… and yes…
I know they’re invasive. But they have grown on me, literally and figuratively. How could
you resist the cute eyes of this cabbage-chomping machine?! And speaking of their cute eyes,
truth be told I have often wondered how well Siput actually sees what the world is like
for a tiny, or in this case, rather large snail.
[Draw Curiosity intro music]
Truth be told, mollusks are some of the most interesting animals out there as far as eye
evolution is concerned. Although the human eye can seem mindbogglingly complex on the
surface, with the retina, the cornea, the lens... how such an organ evolved from scratch
is actually pretty decently understood and it has happened convergently on multiple occasions
- and many different mollusks actually represent quite a few of those intermediate steps.
Firstly, a small layer of pigmented cells connected to nerve fibres appear. These are
photoreceptors, as they are sensitive to light and allow the organism to perceive light and
darkness. With time, this layer of photoreceptors becomes larger, and the surface begins to
invaginate, allowing the pigment cup eye to discern directionality from the light. With
time, the invagination deepens, forming an optic cup or pinhole lens. The small opening
allows light to be focused well, allowing for basic shapes to be discerned, but with
the caveat that it is prone to infection given that the interior of the optic cup is in contact
with the water from the outside, and also that it doesn't really perform well in low-light
environments, because the pinhole doesn't really let through that much light. However,
such a small opening does mean that the growth of a transparent epithelial layer over it
is a very easy adaptation to evolve, and this allows the separation of the interior of the
eye from the outside. From here, it's a lot easier for the eye to develop its own more
fine-tuned adaptations, such as vitreous humour that refracts the light more optimally than
water, a contractible pupil to control the amount of light that enters, a lens to focus
the light, specializations on the retina and so on.
And, as said, across the mollusks, and to a lesser degree within the gastropods, different
species represent different steps across this spectrum, and in this case, Siput actually
places around here as snails do have a closed eyecup with a cornea and a very basic lens.
However, despite their large eye stalks and careful eyeballing of the world around them,
land snails actually don’t see that well, and they don’t rely on their eyesight as
their primary sense. If you’ve ever picked up a snail, you’ve probably noticed that
unless they’ve slowly and carefully placed their eye in front of an object and then avoided
it, they more often than not bump their eyestalks into objects around them. Their retinas are
quite shallow compared to species of marine snails, which are known to see substantially
better, suggesting that terrestrial snails probably aren’t using their eyes to recognise
objects, because they can't focus them as well, but instead they might be measuring
light intensity at large, to get a sense of their surroundings. Nonetheless, despite their
vision seeming quite unfocused, their eyes do in fact have some very interesting features
that you may not have known about! Firstly, each tentacle-bearing eyecapsule
doesn’t contain one, but two eyes! the main eye and the accessory eye. They both share
the same cornea and optic nerve, but they have separate lenses and separate ultrastructures.
That being said, you probably wouldn’t be able to see the two eyes easily with a naked
eye given that the main eye is only about an ⅛ of the size of a pinhead, and the accessory
eye is a fifth to a third of the size of the main eye. Initially, the function of the accessory
eye was not quite clear. It was hypothesised to be sensitive to infrared light, even though
snails, in fact, do not respond to stimuli with a wavelength longer than 750nm. And interestingly,
although the main and accessory eye both have two types of photoreceptors in similar concentrations,
the accessory eye lacks what is known as a “screening pigment”, a layer of pigment
that is behind the retina which absorbs light and allows for image formation, meaning it
would be unlikely enhancing an image in any way given that the lack of screening pigment
means the accessory eye will detect light from any direction. However, when the eye
is retracted within the tentacle, the accessory eye may be the perfect way to detect the changes
in light intensity levels at large, and alert the snail to find out what is going on. Although
not all terrestrial snails have accessory eyes, some species of slugs and annelids have
convergently evolved similar structures.
Secondly, snails are achromatic, which means they are not able to perceive colour. Now,
you may have just overheard me saying that snails have 2 photoreceptor types and might
have thought that this means they could perceive colour. After all, we have three cone types,
and each one has a different peak spectral sensitivity that peaks at red, green and blue.
This means that the same wavelength of light stimulates each cone type differently, and
our brain does the wonderful job of integrating those different signals into what we perceive
as colour. But, despite snails having two types of photoreceptor, the spectral sensitivity
is the same in each one, meaning that they won’t be useful in detecting colour. They
do, however, react differently to light. One type of photoreceptor increases its firing
rate with more light intensity, and they decrease it when it is reduced, so in essence, they
depoalrise in response to light, whereas the other type of photoreceptor does the opposite,
it hyperpolarises in response to light, and depolarises in darkness. So the former could
be called “lightness” neurons, and the latter “darkness neurons”. Now, different
photoreceptor types that depolarise and hyperpolarise in response to light and dark are not unique
to snails, but in the absence of photoreceptors that allow colour detection, this system allows
them both to be able to distinguish different light intensities quite precisely, and to
be capable of light detection in both light and dark environments, as the darkness cells
will be able to dark-adapt and allow the snail to see in a low light environment. Although
snails aren’t thought to rely on eyesight for finding food, they are capable of learning
and recognising different patterns. They also use light to orient themselves, and they are
known to seek out the darkest locations in their environment, presumably to avoid being
eaten by predators and not drying out. However, seeking out dark environments doesn’t mean
they are merely avoiding light, but they're actually using their enhanced ability to detect
relative light intensities to scout out and move towards that dark spot, even if it actually
means that they have to cross through a brighter section temporarily.
And finally, snails have the most extravagant blink of the animal kingdom, capable of pulling
back their eyecapsule into their retractable tentacles to protect them from harm. That
being said, in the unfortunate event that harm should come to their eyes, their final
superpower is their ability to regenerate them. Should a Giant African Land Snail lose
its eye, it will be able to grow back the tentacle and its eye over the course of a
few weeks or months. Young snails are able to repair damage faster, whereas older snails
that have ceased growing generally take a lot longer to restore their eye and their
tentacle, and they actually don’t always recover their vision entirely. Some of their
behaviour that is triggered by vision, such as reacting to changes in light and scoping
out those darker nooks in their environment, is occasionally compromised in older individuals,
suggesting that even if the eye structures regenerate, the optic nerve connecting them
to the cerebral ganglion may not retain the same regenerative capacity and the point at
which this switch happens in snails is certainly very interesting from the perspective of regeneration
and ageing.
So overall, snails do a pretty good job with what they have even if they can’t see very
well. Whilst their eyesight might make it harder for them to enjoy today’s sponsor
- your eyesight certainly won’t! Curiosity Stream is the first streaming platform devoted
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If this video has piqued your interest in animal vision and you want to… *see* more,
I can recommend the title “What Animals See” to learn more about precisely that
from the mouth of Professor Thomas Cronin, who incidentally, is easily one of the leading
experts in mantis shrimp vision. A curiosity stream subscription also includes a free Nebula
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As ever, thank you to my wonderful patreons on patron for supporting Draw Curiosity
These are my lovely patrons
and as always, thank you so much for watching me and I’ll see you
in the next one! Bye!
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