Seeing through the pond snail’s eye
In this time and age of global information flow, internet and machine translations, it is unthinkable that a discovery no matter where and in which language published, even if it were Basque, Haussa, or Aymara, would remain unnoticed as had happened in the 19th century with Mendel’s epoch-making laws of inheritance.
However, that interesting observations will sometimes not receive the attention they perhaps deserve is a fact and has to do with the enormous amount of new information that modern scientists have to cope with. I’m afraid one of our discoveries on the photoreceptors of the giant pond snail Lymnaea stagnalis falls into that category, even though the work was reported in an excellent and international journal. What did we discover? Many people will have seen the spiral shells of the freshwater ram’s horn snail Planorbaris sp. and the pointed, trumpet-like shells of the giant pond snail Lymnaea stagnalis should also be familiar to many.
Both of these species need to come to the surface of the water from time to time to breathe and to fill their lungs with air. Occasionally they may even venture a little above the water line. However, as all of us know when we use our eyes under water, the underworld we see is blurred. Water has a higher refractive index (1.3) than air (1.0) and our eyes are designed to see in air.
The fovea, the area of our sharpest vision, lies in a small depression of the retina behind the lens, where colour-sensitive cones are plentiful and rods are absent, blood capilleries don’t interfere, and an almost 1:1:1 ratio of receptor to bipolar to ganglion cells guarantees the highest possible resolving power. In the above mentioned aquatic snails, as Dr Marina Zieger found, the situation is reversed: to see in air their retina possesses a tiny dome-like swelling, which is closer to the snail eye’s lens than the rest of the retina, which is devoted to seeing under water and has its own separate resolving power centre.
Having a place of the retina specifically for vision in air, small excursions above the water line, therefore, need not result in poorer vision quality, and yet what would such slow and seemingly primitive creatures want to see at all, given that their eyes are superior to ours in terms of adaptability? What about resolving power, for that of land snails with 25° or so is known to be extremely poor. Measurements by Dr Gal in my lab at the time show that the eyes of the giant pond snail have a resolving power of 2°, which is not bad at all and allows this snail to visually detect thin vertical stems that it can ascend to reach the water surface to breathe. These snails are, however, not the only aquatic snails with reasonably good vision: the prize for the best eyes amongst them may go to stalked eyes of the fish-eating and highly poisonous marine cone shell snails.
Anybody willing to work on them? We are quite proud of our results coming from research on the eyes of a harmless species; results which are the fruits of several years of dedicated work. But will they receive some recognition? To see that with our own eyes would certainly please us.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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