ABSTRACT
At all times and in all places, the Earth is awash in electromagnetic radiation, sound waves, vibrations, and chemical stimuli. No species can detect and respond to all that sensory information; each has carved out a small portion of the whole to form its own sensory niche. Technology has allowed us to describe these niches for other species even if we cannot truly appreciate or experience them. For example, try to envision being a mormyrid and singing to your mate with electricity or being any fish and touching an object without being in physical contact with it. This is the essence of
umwelt
, the term used by von Uexkull
to call our attention to these unique sensory worlds, many parts of which are beyond the perceptual capabilities of mere biologists. In the following pages I shall discuss the umwelt of the threespined stickleback, with particular reference to how that world interacts with and shapes intraspecific communication during the breeding season. When data are scarce for threespines, I shall fall back on information from other gasterosteids, under the assumption that the conservative nature of (some) evolutionary diversification will allow extrapolation from one close relative to the other. By the end of this chapter, I hope to have shown that sticklebacks are immersed in a unique world of images and scents, and possibly tastes and sounds. These different sensory modalities are employed to a greater or lesser extent throughout their lives, but it is during the breeding season that multimodal and multicomponent signals really shine.
Light transmission is more complicated in aquatic ecosystems than on land. Photons crossing the air-water interface will eventually be absorbed or scattered, either by organic particles suspended in the water or by the water molecules themselves. Absorption of light on its way from the sender to the receiver leads to image degradation because information is lost. Scattering of light leads to image degradation because some information is lost (from the object), and irrelevant information is added to the visual pathway (from the background). This extraneous information decreases contrast between the object and the background: distant objects appear faint and blurred, as if seen through fog (the veiling effect). The upshot of all this absorbing and scattering is that the composition of light in any given environment depends on the distance that light has travelled since entering the water and what is in the water. For example, chlorophylls shift the transmission maximum of light to wavelengths of around 500 to 600 nm (greenish yellow: majority of coastal waters, lowland ponds and rivers). Add tannins and lignins to the picture, and little light penetrates below 3 m, with the transmission maximum pushed to well over 600 nm (reddish brown: some swamps, marshes). Because of this differential transmission, the quality (intensity and wavelength composition) of light may vary dramatically along different lines of sight radiating from the same point.
