They may not be apparent to the inexperienced observer, but snails do have eyes. They are not blind but are thought to be capable of only a very low level of vision.
For decades scientists have worked hard to unlock the mysteries of sight in snails.
Much of the challenge of establishing if snails can see comes from the difficulty in designing suitable experiments that can conclusively and repeatedly demonstrate sight in snails.
As gastropods that are wholly adapted to terrestrial life, snail eyes have to do a lot of work!
The remarkable scientific findings shared below demonstrate just how unique the structure and function of the eye of Helix species is.
In this article, let’s take a closer look at the eye of a snail!
Do Snails Have Eyes?
Snails have a head that does possess eyes!
The eyes of a snail sit on paired stalk-like tentacles with olfactory (smell) tentacles beneath.
The stalked eye of a snail is completely different to the human eye in structure.
Though there is nothing conventional about them they do have functional elements that are similar to those in the human eye and are capable of perceiving light and movement.
Investigating the Eyes of Snails
Study of the eye of a snail interests biologists because they, along with slugs, are the only members of the mollusc family that are adapted to terrestrial life both in their morphology and physiology.
Their distinct organs of vision and sight capabilities set them apart from their water-dwelling relatives.
Richard M Eakin was an American zoologist who became one of the world’s experts on the eyes of snails.
He and his team of students and researchers undertook extensive studies on the eyes of slugs and snails, accruing large numbers for investigation by snipping off the tip of the ocular tentacles which are completely soft.
For the Research: Because snail eyes are extremely small, he and his team stockpiled several thousand eyes, to provide enough ocular material for extensive biochemical studies.
The Structure of a Snails Eye
Through microscopy, scientists have been able to understand the anatomy and structures of a snails eye.
Components of a pulmonate (snail or slug) eye include:
- A transparent cornea
- The eye capsule is contiguous with the snail’s cornea.
- A secreted lens which a large composite lens of 150xm diameter.
- The retina is made up of four layers: villous, pigmented, somatic and neural layers.
In the retina of a garden snail, there are four distinct cell types.
Each eye has over 2 thousand photosensory cells which respond to light and send nervous signal messaging via their axons.
Light-sensitive Structures In a Snails Eye
Eakin and his team identified two key structures in the photosensory (light-sensing) cells of a common garden snail (Helix aspersa).
- Microvilli: these are light-sensitive organelles within the snail’s eye cells
- Microvesicles: these are bubble-like structures that transport the photopigments and minerals needed by the microvilli to chemically respond to the presence of light.
The presence and function of these ocular cellular structures in the snail have been demonstrated with a variety of experiments, including the uptake of radioactive retinol by both villi and vesicles, staining studies and comparison between light-tolerant and nocturnal snail and slug species.
Perception of Light in Snails
If you have ever observed garden snails, you will be well aware that they are efficient at getting away from open sunlight and making a swift retreat to darker locations with vegetation.
It is almost certain that such snails rely on light sensitivity to navigate to a safer environment, though scientists have also proposed that sails use chemoreception to know where to go.
So, Do Snails See? Do Snails Have Vision?
The sight of a snail is nowhere near comparable to the sophistication of human sight.
At best, they see very poorly, and have been demonstrated to detect objects at extremely close range.
Behavioural responses to visual stimuli in snails are not always consistent with a moving snail more likely to bump its optic tentacle against an obstacle before manoeuvring around it.
Here are some experiments that have shown that snails can see:
- As far back as 1982, it was discovered that snails could perceive small objects placed a few millimetres away from them and larger objects up to a centimetre away.
- Eakin and his team noted that garden snails were able to repeat avoid metal objects held close to one or other optic tentacles.
- Snails were shown to be able to recognise dark marks against a plain background.
Other Responses of Snails to Light
An interesting phenomenon observed in snails is the rising-up reaction.
This is where a snail responds to a sudden stark change in lighting by rising up with its body rising up from the ground at a 90-degree angle and then waving from side to side.
This happens within a few seconds of the lighting change and is prolonged with the optic tentacles fully extended and weaving too.
This reaction is repeated several times before the snail becomes acclimatised and can even take place on verticle surfaces.
Snails Rely on Smell and Touch Just as Much as Their Eyesight
The lower olfactory tentacles on either side of a snails mouth provide a snail with a keen sense of smell, which a snail relies on as it moves.
The smell is likely to be the best-developed sense of a snail. As the snail moves the lower tentacles behave like whiskers and are stretched out with maximum exposure to the air as it searches for food.
Less-known Fact: Alongside smell is the use of chemotaxis, as the snail picks up the presence of nearby snails from substances left in slime trails.
It is amazing that snails, being arthropods and without a central nervous system are capable of possessing and using eyes.
However, as their eyesight is not so great, they have their sense of smell to help them navigate their environment effectively.
As scientists learn more and more about how snails see, we’re sure that greater insights into the secrets of snail sight will be achieved.
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