Look at me? Still looking at me? Look again...

If a conch was staring at you, how would you feel?

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The conch is watching secretly. Source: gizmodo

Do you feel like the snail in SpongeBob SquarePants is looking at you with his big eyes?

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Many people may not know that conchs have eyes, but in fact, conchs not only have eyes, but they are also very large as shown in the picture. The eyes of related shellfish are much smaller than those of conchs.

Don’t think that its eyes are just decorations. In fact, its eyes are similar in shape to those of cephalopods such as octopuses. Because the structure of the eyeball is similar to a camera, it is also called a camera type eye.

The picture below is the eyeball of the eagle-wing conch (Strombus raninus), which is approximately 1.9 mm × 1.6 mm.

Source document 1

This eye is like a small snail, with a long, retractable eyestalk connected to the eyeball, which is composed of the cornea, lens, vitreous body, pigment cells and sensory rod bundles.

The locations of the various organs of the conch, with the location of the eyes marked.

But the eyes of snails are still different from those of humans and other vertebrates. It can even be said that the eyes of these lower creatures are more "advanced" in structure.

The eye of a conch

As shown in the figure below, in the eyes of vertebrates (left), the nerve fibers (2) are in front of the retina (1), which means that light must pass through the nerve fibers to reach the retina, blocking some of the light and causing an area on the retina to be unable to sense light, thus forming a blind spot (4). In the eyes of snails and cephalopods, the nerve fibers are behind the retina, so they do not block light or damage the retina.

In other words, human eyes are not as well designed as conch eyes, after all, their eyes have no blind spots.

Human eye structure source: thoughtco

The lens of a conch eye is closer to a sphere, while the lens of a human eye is closer to an ellipse and is flatter than that of a conch.

This difference occurs because snails live in water, while humans live on land. When humans see objects, they see through air, while snails see through water. The different refractive indices of the media have led to the evolution of spherical lenses with higher refractive indices in aquatic snails.

Scientists have also discovered that the refractive index of the conch's spherical lens is not uniform, but rather decreases from the center outward to form a gradient.

So the question is, what is the use of eyes for such a slow-moving creature?

First of all, we need to know what it can see? And what it can see depends largely on the sensitivity of the eyeball to light.

Scientist Kirschfeld deduced a formula to calculate the absolute sensitivity of the eye to light sources using the pupil diameter, eye focal length, rod bundle length, rod bundle diameter, and photoreceptor absorption coefficient.

A is the pupil diameter, f is the focal length of the eyeball, d is the diameter of the sensory rod bundle, x is the length of the sensory rod bundle, and k is the photoreceptor absorption coefficient. Source: Reference 1

Then scientists calculated that the average absolute sensitivity of the eagle-winged conch to light is 9.9μm2, while the human eye is only 0.023μm2 (the unit means the number of photons that can be received per square meter). The absolute sensitivity of the eagle-winged conch to light sources is about 430 times that of humans. In other words, the conch is much more sensitive to light than humans.

High sensitivity to light means that this creature can see more clearly in low light. Snails like to move around at dusk and evening, so their eyes must be more sensitive to light. But if the light is a little stronger, the snail's eyes may not be able to bear it.

Comparison of eyeball values ​​between different organisms. The resolution column is the retinal resolution. The human eye resolution is about 60 times that of the snail. Source: Reference 1

But in terms of retinal resolution, the snail cannot compare to humans (as shown in the table above). So, in simple terms, the snail has a pair of eyes that are sensitive to light but have extremely low resolution, and can't see anything clearly. Rather than calling this visual organ "eyes", it is more appropriate to call it "photoreceptors". If a fish swims in front of it, its eyes will probably only see a black shadow passing by.

But this vision is enough for it. As long as it senses a change in the light and shadow outside, it will immediately retract its head into the shell that provides protection. It doesn't need to run away at all, it just needs to hide when it senses danger.

The eyes of scallops and conchs have similar functions. However, although the eyes of scallops are smaller, they are much more numerous. A scallop has about 200 eyes, so its light-sensing range is wider than that of conchs. Moreover, when it senses danger, it will immediately swim away like clapping. In terms of escape ability, scallops are slightly better.

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In addition, the eyes of the snail have a special function that you might not expect: they can regenerate. In 2018, a scientist conducted an experiment on a snail and cut off its two eyes. But eight weeks later, it grew a pair of new eyes on its eye stalks.

The regeneration process of the conch eye Source: Reference 2

From this it can be seen that in terms of regeneration, human eyes will never be able to compare with the eyes of snails.

References:

Seyer J O. Structure and optics of the eye of the hawk‐wing conch, Strombus raninus (L.)J. Journal of Experimental Zoology, 1994, 268(3): 200‐207.

Clark J M. Restoration of visual performance and opsin expression within the retina during eye regeneration in the Florida fighting conch (Strombus alatus)J. 2018.

Source: This article is produced by Science Popularization China and supervised by China Science Popularization Expo

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