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The photoreceptors mediating light sensitive behaviours in Lepidochitona cinerea

Brown, Chloe (2020) BION03 20192
Degree Projects in Biology
Abstract
Simple vision and the non-visual photoreceptors precursors have given rise to an evolutionary arms race in spatial vision. Chitons have non-visual photoreceptors, called aesthetes, but in some species aesthetes lack pigmentation therefore the function is uncertain, although prevailing theory is that they are photosensory. Here I provide support for this theory using the robust shadow response of Lepidochitona cinerea to determine the contrast thresholds. The targeted shading of either the whole chiton, aesthetes or girdle with stimuli differing in Weber contrast stimuli shows that aesthetes are involved in mediating the shadow response and are nearly as sensitive as the whole chiton. Surprisingly, the aesthetes are not the only... (More)
Simple vision and the non-visual photoreceptors precursors have given rise to an evolutionary arms race in spatial vision. Chitons have non-visual photoreceptors, called aesthetes, but in some species aesthetes lack pigmentation therefore the function is uncertain, although prevailing theory is that they are photosensory. Here I provide support for this theory using the robust shadow response of Lepidochitona cinerea to determine the contrast thresholds. The targeted shading of either the whole chiton, aesthetes or girdle with stimuli differing in Weber contrast stimuli shows that aesthetes are involved in mediating the shadow response and are nearly as sensitive as the whole chiton. Surprisingly, the aesthetes are not the only photoreceptors involved in this; there was a robust response, albeit a less sensitive threshold, from the girdle as well. I propose that these provide accumulative information, acting as a dispersed visual system. Additionally, I demonstrate for the first time that a chiton with simple aesthetes has a looming response. This response is robust in older individuals and may have developed because of differential predation pressure throughout ontogeny. Aesthetes and unidentified girdle photoreceptors mediate the shadow response and are likely involved in the looming response, although further research is necessary to determine this. (Less)
Popular Abstract
Chitons can see even without eyes

Looking under almost any rock on a rocky shore around the world you will find a chiton hiding from the sunlight. It is possible you have never even heard of a chiton and you would not be alone, even in scientific research they are really understudied.

This does not mean they are not fascinating little things though. Chitons move incredibly slowly and are really tough, making them the tiny tanks of the sea. They have a hard, segmented shell and around the bottom of this is the soft outer edge called the girdle. On the shell are hundreds of tiny channels, called aesthetes, that go through the shell and connect the simple nervous system with the outside world. Most species of chitons do not have eyes,... (More)
Chitons can see even without eyes

Looking under almost any rock on a rocky shore around the world you will find a chiton hiding from the sunlight. It is possible you have never even heard of a chiton and you would not be alone, even in scientific research they are really understudied.

This does not mean they are not fascinating little things though. Chitons move incredibly slowly and are really tough, making them the tiny tanks of the sea. They have a hard, segmented shell and around the bottom of this is the soft outer edge called the girdle. On the shell are hundreds of tiny channels, called aesthetes, that go through the shell and connect the simple nervous system with the outside world. Most species of chitons do not have eyes, but previous research shows the aesthetes might be used for checking for changes in light, caused by a predator.

Imagine you are this slow-moving snail-woodlouse hybrid, so slow that you cannot get away from anyone trying to eat you. But, you have this ninja fast reflex allowing you to clamp onto a rock so well that they will not be able to detach you. In order to do this, you are going to want to know if someone is coming at you. This would require being able to see, without eyes, what is coming before it is too late.
This research confirmed that the grey chiton can do this by using the aesthetes. Not only this but they can use a structure in the girdle to also sense changes in light that elicits the ninja fast clamping to the rock. To determine this, I used pictures of the chiton itself to create tailor made shading of either the aesthetes, the girdle, or the whole chiton. The shading on each part included different amounts of greyness and showed that the aesthetes are more sensitive than the girdle part to lighter shadows. However, neither of these are as sensitive as the whole chiton. Therefore, the aesthetes and girdle might work together to be as sensitive as the whole chiton, essentially making the grey chiton one giant eye. This would maximise the chance of spotting a predator before it is too late.

Without proper vision, the grey chiton can tell if a predator is approaching, although there is a lot of shadow forming, non-threating things in the sea. So, further research would be needed to understand if chitons can properly distinguish between a predator approaching or if they are scared of any moving shadow. Understanding vision in unusual creatures like chitons leads to a better understanding of the evolution of the eye.

Master’s Degree Project in Biology, 60 credits.
Department of Biology, Lund University

Advisor: Dan-E. Nilsson
Lund Vision Group. (Less)
Please use this url to cite or link to this publication:
author
Brown, Chloe
supervisor
organization
course
BION03 20192
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
9038383
date added to LUP
2021-01-27 14:52:25
date last changed
2021-01-27 14:52:25
@misc{9038383,
  abstract     = {Simple vision and the non-visual photoreceptors precursors have given rise to an evolutionary arms race in spatial vision. Chitons have non-visual photoreceptors, called aesthetes, but in some species aesthetes lack pigmentation therefore the function is uncertain, although prevailing theory is that they are photosensory. Here I provide support for this theory using the robust shadow response of Lepidochitona cinerea to determine the contrast thresholds. The targeted shading of either the whole chiton, aesthetes or girdle with stimuli differing in Weber contrast stimuli shows that aesthetes are involved in mediating the shadow response and are nearly as sensitive as the whole chiton. Surprisingly, the aesthetes are not the only photoreceptors involved in this; there was a robust response, albeit a less sensitive threshold, from the girdle as well. I propose that these provide accumulative information, acting as a dispersed visual system. Additionally, I demonstrate for the first time that a chiton with simple aesthetes has a looming response. This response is robust in older individuals and may have developed because of differential predation pressure throughout ontogeny. Aesthetes and unidentified girdle photoreceptors mediate the shadow response and are likely involved in the looming response, although further research is necessary to determine this.},
  author       = {Brown, Chloe},
  language     = {eng},
  note         = {Student Paper},
  title        = {The photoreceptors mediating light sensitive behaviours in Lepidochitona cinerea},
  year         = {2020},
}