But how do animals know their “ups” and “downs”?
You are fortunate (and so am I of course) that we have no problem to detect what is up and what is down. We, and most other vertebrates, have gravity receptors, sense organs in the inner ear, which consist of a cushion of sensory hairs with small crystals of calcium carbonate and calcium phosphate as weights spread across them. Depending on whether vertical acceleration of the vertebrate animals is towards or away from Earth, this earstone material either de- or increases excitation of the sensory hairs it rests on; thus, creating the sensations of rising and falling and an awareness of being lifted or dropped.
However, the visual input is, of course, also important as natural light tends to come from above. It is really the information provided by the eyes as well as that of the gravity receptor together that the brain uses and to some extent this scenario holds true also for humans as tests with volunteers have demonstrated. Fish show this beautifully: swimming quite normally when the light illuminates them from above, they become lopsided as soon as the light hits the aquarium from the side. You can also observe this in fish swimming into caves or grottos, getting illuminated from the side. Light from below is ignored as long as the gravity receptors of the fish are intact, but when destroyed the fish will turn in a way that its dorsal side will face the direction of the light.
In the crayfish and many other crustaceans the gravity receptors alone, under all conditions of the direction of the impinging light, are responsible. A simple experiment, devised more than a hundred years ago by the Austrian Alois Kreidl proves that. What did Alois Kreidl do? The gravity particles in the statocyst (as the gravity sense organ of crustaceans and other invertebrates is often called) must be replaced after every moult. If a newly moulted crayfish is given not sand, but nickel or iron filings to place in its statocyst instead of the usual sand grains and a magnet is then held against the side of the aquarium, the crayfish will turn on its side. The tiny force of the iron- or nickel filings being attracted to the magnet (thereby deflecting the sensory hairs and creating a nerve-transmitted signal so beautifully examined by the South-African-born David C. Sandeman), fools the crayfish in believing that this force is gravity and the magnet is Earth’s centre.
Invertebrates, however, those that lack the statocyst’s gravity receptors, but possess eyes like waterfleas, for instance, exclusively use the direction of the light to determine where’s “up” and where’s “down”. If a light is allowed to shine into an aquarium from below, all waterfleas in the tank will change their swimming direction and turn upside down to approach the light. Obviously, this dorsal light reaction can be used to test the waterflea’s sensitivity to different light intensities, differently coloured lights, and different degrees of linearly polarized light. In Nature, of course, light used to be a very reliable indicator of the upward (= skyward) direction, but human inventiveness has muddled things up a bit and now not even the dorsal light reflex is any longer what it used to be. Artificial lights have created “photic pollution” that all sorts of animals are subjected and need to adapt to. And it has changed human behaviour as well, as I notice looking at my watch right now: it’s exactly 0.26 h: bedtime.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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