Hello to you lovely people ! This blogpost is about a one-week experiment our team conducted during the Biosensors course, which consists of a wonderful month giving birth to a myriad of very diverse scientific group-projects about biosensors. It was organised by our interdisciplinary bachelor program “Frontiers of Life Sciences” hosted by the CRI!!!
Where the light shines, the shrimp goes!
(credits to https://youtu.be/XmajoRnBW30)
Changes in light intensity are known to have a strong effect on Daphnia's behavior.
Like many species in the marine environment, Daphnia - a small planktonic crustacean - migrates to upper luminous water layers during the night and return to deeper layers during the day, before sunrise.
This behavior is due to their transparency: even though transparency protects Daphnia from being seen by their predators, it enhance the exposition of their molecular components (proteins, DNA) to solar irradiation. As a result, they react to UV light with negative phototaxis : they move away from the sunlight.
But Daphnia also perform positive phototaxis, which means they move towards sources of light. During the night, when the aggressive sunrays can't harm them, Daphnia return to surface seeking for their main source of food : photosynthetic phytoplankton. Phytoplankton floats near the surface for they need sunlight to accomplish photosynthesis. Daphnia use light from stars and moon as a trusted guide towards their food. All they need to do is swim upwards where the beaming, harmless moonlight spreads all over the ocean.
Fig 1 - Daphnia’s compound eye: It is made of thousands of photoreceptor units, and enable them to detect different light intensities. (credits to http://arthropoda.southernfriedscience.com/wp-content/uploads/2011/02/daphniaeye.jpg)
We were wondering how sharpened these receptors are, and therefore how precisely Daphnia can trend towards a located source of visible light. We decided to answer that question, or at least to get more insights about it, by comparing Daphnia and an electronic light sensor (LDR standing for Light Dependent Resistor). We focused on angular precision,
LDR is a light-controlled variable resistor, which means that its resistance varies with light intensity. Indeed, its resistance decreases when the light intensity increases. This features enables the device to detect light and measure the luminosity
So far, we supposed that the precision of both Daphnia and LDR will increase when the intensity rises. We assumed that the angular precision of the electronic sensor will overtake the precision of the Daphnia.
To investigate this, we ran two different experiments, a biological and an electronic one. We had six light intensity tested for both (including no light and maximal light we could produce with a white LED).
For the biological part, we first measured the average position of ten daphnia (disposed in a thin layer of water) at each of the six different values of intensity we exposed them to. Then we compared the resultant angular position with the value of the angle of the light source.
In the meantime, we tested the precision of the electronic sensor we’ve engineered in such a way that it takes measurements every 1° in 180° and gives us the angle of the highest perceived value of intensity.
Fig 2 - Our experimental setup: We exposed both daphnia and a LDR (disposed on a movable arm) to six different light intensities. The light was produced by a LED that was disposed randomly but at a known angle (135°). Then we measured the angle between the source of light and the position of both Daphnia (their average position) and the LDR.
We expected the Daphnia to move towards the light, but they actually moved away from it. Anyway, we still could exploit our results : indeed, to avoid the light Daphnia have to go as far as possible from it. Their ability to do so still depends on the precision of their light-sensitive organs. Instead of moving towards the maximum of intensity they would move towards the minimum of intensity perceived. And where would the minimum of intensity be ? Well it would be at the farthest point of the position of the light source, which means at the exact opposite point !
Fig 3 - Data analysis for daphnia: The average position of daphnia was calculated (cross) using ImageJ. The measured angle is between the light ray and the ray passing through the averaged position. It is the same wether the daphnia go to the light or if they escape from it.
Many studies have been conducted regarding the behavior of Daphnia when exposed to UV light, but little is known about the effect of visible light on marine zooplankton. Our results show that for Daphnia, an increase in light intensity goes along with a decrease in the measured angle. It means that the accuracy of daphnia photoreceptors increases. Our graph shows that a limit of accuracy of 10° is reached from 500 a.u.
Regarding the the electronic device, it seems that accuracy doesn’t depends on light intensity, but more measurements need to be done. Plus, the precision of the sensor depends mostly on the precision of the mobile arm.
Fig 4 - Our results: Angle in function of intensity for both tested devices.
Comparing the response of two different sensors to different light intensities appeared as the primordial aspect of our study. From our results, it appears that the LDR device is precise but not accurate, and that it’s accuracy does not depends on light intensity. As for daphnia’s accuracy, it depends on light intensity, and their photoreceptors are not super precise.
References:
The positively phototactic reaction of Daphnia magna Straus: a contribution to the understanding of diurnal vertical migration J Ringelberg - Netherlands Journal of Sea Research, 1964 - Elsevier
Interaction of polarized light and turbidity in the orientation of Daphnia and Mysidium TH Waterman - Zeitschrift für vergleichende Physiologie, 1960 - Springer
Individual swimming behavior of Daphnia: effects of food, light and container size in four clones Stanley I. Dodson, Shanna Ryan,Ralph Tollrian and Winfried Lampert
Phototaxis in water fleas (Daphnia magna) is differently influenced by visible and UV light
U. C. Storz, R. J. Paul
Some physical factors influencing the feeding behavior of daphnia magna straus J. W. McMahon
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