Life on the surface of the water
Creating wobbly and shifting upside-down images of the real world, water surfaces can be beautiful in their own right, especially during a moonlit night. Some of my most beautiful photographs were of reflections from water surfaces. But they seem so bare of life (disregarding aquatic plants like duckweed, pond and water lilies and their inhabitants for the moment). In reality, however, there are actually numerous organisms that live, hunt and feed on this two-dimensional horizon dividing the wet and dry worlds.
In Finland, my second home so-to-speak (or is it my third, fourth or even fifth?), some of the first insect species to be seen after the long and severe winters are the water-striders, long-legged bugs (yes, true bugs belonging to the order Hemiptera) that glide on top of the elastic surface film of the meltwater puddles. Even on the surfaces of the open oceans a number of related species make their living. Water striders (or pond skaters as they are also called) feed on insects and compete with hunting spiders, e.g., the impressive Dolomedes fimbriatus, also light enough to run across the water surface without falling through it. These two long-legged arthropods may be joined by the whirligig beetles, streamlined, shiny black insects that are so wonderfully well adapted to their existence at the interface between air and water that their eyes are even horizontally divided with one half peering at the underwater world while the other sees what’s going on above.
Animals calling the surface of the water their home respond to water surface waves in various ways. Very strong waves (during a storm perhaps) cause them to seek shelter along the water’s edge until the water has calmed down again. With their mechanoreceptors on their legs or antennae they distinguish abiotic waves caused, for instance, by a falling leaf and permanent wind-generated “background noise” from biotic stimuli caused by con-specifics or small insects and other terrestrial arthropods that have fallen into the water as well as bigger animals like frogs, fish, water fowl and others. Waves generated by small, struggling insect prey, due to the filter properties of the water surface, lose their highest frequency components during propagation. A predatory surface animal, like the pond skater or water spider, therefore waits for ripples of the critical higher frequencies (up to 140 Hz) indicative of struggling insects and then determines from the curvature of the ripples and the height of the waves location, distance and possible size of the prey. Their mechanoreceptive sensors detect incredibly minute disturbances and their tiny brains can make sense of the signals and initiate appropriate action: quite an amazing feat.
Sending out a bow wave during swimming and registering its echo also helps the surface dwelling animals to avoid obstacles, for example when they move along the water surface at top speed, something that is fascinating to watch when dozens of whirligig beetles mill around on the water surface at tremendous speed without ever crashing into each other. Surface dwellers even transmit messages to other individuals, communicating territorial claims and amorous intentions to one another simply by “stirring up the water”, creating the right vibrations and -as the Beach Boys sang- for recipients to pick up “Good Vibrations”. (Listen to the song sometime, if you haven’t heard it for a while).
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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