Surface wave detection by animals is the process by which animals, such as surface-feeding fish are able to sense and localize prey and other objects on the surface of a body of water by analyzing features of the ripples generated by objects' movement at the surface. Features analyzed include waveform properties such as frequency, change in frequency, and amplitude, and the curvature of the wavefront. A number of different species are proficient in surface wave detection, including some aquatic insects and toads, though most research is done on the topminnow/surface killifish Aplocheilus lineatus. The fish and other animals with this ability spend large amounts of time near the water surface, some just to feed and others their entire lives.
Description of behavior Certain species of fish spend a substantial portion of their lives near the surface of the water in order to feed, usually on insects that are struggling at the surface. Species that detect surface waves typically use them to localize such prey. When the hunting posture is assumed (which may be neutral posture) as specific mechanosensitive organ is held in contact with the surface of the water in order that mechanoreceptors can receive surface waves. The animal will wait a small amount of time (typically <1s) before initiating a response towards the prey, should the surface waves perceived fall within the preferred stimulus range. Response towards prey typically follows the pattern orientation towards prey, swimming towards prey, and then prey capture. This ability is sometimes referred to as a sense of "distant touch."Schwarz, J.S., Reichenbach, T., and Hudspeth, A. J. (2011) "A hydrodynamic sensory antenna used by killifish for nocturnal hunting". The Journal of Experimental Biology 214:1857-1866
Several species have been shown to use surface wave detection for prey capture. Among these are many species of freshwater fish, notably the groups hatchetfish (Gasteropelecidae), freshwater butterflyfish (Pantodontidae), halfbeaks (Hemiramphidae) and killifish (Aplocheilidae)(list from ). For its consistently stellar performance at the task, the topminnow/killfish (both terms are used in the literature) is one of the primary models for investigation. These species tend to live in small bodies of freshwater, as well as creeks and swamps.
Surface waves
The ripples which surface-feeding fish detect are known more technically as capillary waves. Capillary waves are generated by movement of an object at the surface of the water or from the brief contact of an object with the surface from either medium (air or water). Waves radiate outward in concentric circles from the source, and the waveform of each train of waves changes in very specific and predictable waves, as dictated by surface tension and gravity.Bleckmann H, and Kase R. H. (1987) "Prey localization by surface wave ray-tracing: fish track bugs like oceanographers track storms". Experientia 43: 290-293 The water surface has a dampening effect which causes an abnormal dispersion pattern in which waves decrease in amplitude, speed and frequency with distance from the source.Müller U., and Schwartz E. "Influence of single neuromasts on prey localizing behavior of surface-feeding fish, Aplocheilus lineatus". Journal of Comparative Physiology A 149 (1982): 399-408 Short-wavelength (higher frequency) waves disperse faster than longer wavelength waves, resulting in higher frequencies at the front of the wave-train and lower frequencies at the tail; a fish detects this as a downward-sweeping frequency modulation.
Surface wave detection in Aplocheilus lineatus A vast amount of the research on surface wave detection has been done in the surface-feeding topminnow/killfish Aplocheilus lineatus. Schwartz (1965) demonstrated that this species has exceptionally well-developed surface wave detection ability, and it is easily housed and trained in laboratories.Schwartz, E. (1965) "Bau und Funktion der seitenlinie des streifenhechtlings Aplocheilus lineatus". "Z Vergl Physiology A 50": 55-87, cited and translated in Coombs S., Görner P., and Münz H. eds. The Mechanosensory Lateral Line: Neurobiology and Evolution (New York: Springer-Verlag New York Inc., 1989), pp.501-524 Pantadon bucholzi (a surface dwelling butterfly fish) is used less often though is very similar in its anatomy and behavior. The rest of this article will focus on research done with A lineatus.
Experimental methodology (See the work of Bleckmann, Schwartz, Müller, etc., 1965-present).
The experimental setup for testing A lineatus abilities is very standardized. Subject fish are often blinded so that visual cues cannot be used. Stimuli are delivered to the surface of the water in a test tank via one of two methods (experimenters often use both): for the first, a loudspeaker is set up facing the water surface, while a plastic disc covers the front of the speaker cone with a small hole in the center, allowing air to be pushed through the hole to stimulate the water surface in a pattern controlled by a square-wave generator. Alternatively, a small rod a few millimeters in diameter is dipped briefly a few centimeters into the water. Either setup can be moved around the tank to deliver stimuli at various locations. The loudspeaker setup offers the advantage that stimuli can be precisely controlled to mimic natural stimuli or to test certain wave properties.
Wave characteristics are measured optically. This is done by shining a laser (often helium-neon) at the water surface. Reflections and distortions of the laser's beam are picked up by a photodiode.
Fishes' movements are videotaped with a high-speed camera from above the testing tank so that the precise timing and nature of their responses' to stimuli can be reviewed.
Behavior of A lineatus Aplocheilus lineatus assumes a hunting posture at the water surface in which it holds the top of its head within 200 μm of the water surface. On perception of appropriate wave stimuli, fish first turn towards the stimulus. This initial turn is made within 150ms of the wavetrain's arrival to the fish. Fish then swim to the wave's source. There are specific parameters for wave stimuli to be perceived which reflect the type of waves produced by prey. Fish are least sensitive to low frequencies, with a minimum threshold of 10 Hz and have peak sensitivity at 75–150 Hz; the highest frequency they can detect is 250 Hz.Müller, U. (1984) "Die morphologie und physiologie Anpassung des Seitenlinien-systems von Pantodon bucholzo an den Lebensraum Wasseroberflache". PhD Thesis, University of Giessen, West Germany, cited in Coombs S., Görner P., and Münz H. eds. The Mechanosensory Lateral Line: Neurobiology and Evolution (New York: Springer-Verlag New York Inc., 1989), pp.501-524 These waves have small amplitudes (on a micrometer-scale), and fish can detect waves with minimum peak-to-peak displacement of 1 μm at 10 Hz and 0.0007μm in their most sensitive range of 75–100 Hz. Fish are also sensitive to waves generated within 7–30 cm of the fish's location, though they can occur at any point within that radius. Because the water surface acts as a low pass filter, this radius is different for different wave frequencies: 70 Hz waves with 100μm peak-to-peak distance are subthreshold at 37 cm, and 140 Hz waves are subthreshold at 19 cm.Bleckmann, H. (1980) "Reaction time and stimulus frequency in prey localization in the surface-feeding fish Aplocheilus lineatus". Journal of Comparative Physiology 140:163-172, cited in Hoin-Radkovsky, Bleckmann, Schwartz 1984) Fish are able to tell apart concurrent waves of different frequencies when the frequencies are different by at least 15%, though at certain frequencies this difference can be as low as 8%.Hoin-Radkovsky, I., Bleckmann, H., and Schwartz, E. (1984) "Determination of the source distance in the surface-feeding fish Pantodon bucholzi pantodontidae". Animal Behavior 32:840-851
The wave-detection system of A lineatus and other surface feeding fish is tuned to match the waves that signal prey in their environment. A lineatus feeds on aquatic, semi-aquatic and terrestrial insects, often insects that have just touched the surface of the water or have fallen in and are struggling at the surface. Abiotic sources give off capillary waves of frequency 8–14 Hz, while biotic sources give off much higher frequencies, anywhere from 12–45 Hz and above.Bleckmann, H., Waldner, I., and Schwartz, E. (1981) "Frequency Discrimination of the Surface-Feeding Fish Aplocheilus lineatus - A Prerequisite for Prey Localization?" Journal of Comparative Physiology 143:485-490 This correlates well with A lineatus''' peak sensitivity at higher frequencies.
Wave stimuli are classified into two types: the first is click stimuli (short bursts of amplitude less than 100μm which contain many frequencies between 5 Hz and 190 Hz and are of short duration, as when an insect just touches the water rather than moves continuously at the surface). The other is continuous wave stimuli, which will contain many frequencies and in the wild are generated by fallen prey struggling at the water surface.
It has been pointed out that A lineatus and other surface feeding fish are able to locate prey even if the waveform is only in click form (just touching the surface once, as when surfacing for air or when a mosquito larva is hatching), which takes less time to complete than the fish will take to swim over.Lang, H.H. (1980a) Surface wave discrimination between prey and nonprey by the back swimmer Notonecta glauca L. (Hemiptera, Heteroptera). Behavioral Ecology and Sociobiology 6:233-246, cited in Bleckmann, H, and Schwartz, E. (1982) "The functional significance of frequency modulation within a wave train for prey localization in the surface-feeding fish Aplocheilus lineatus". Journal of Comparative Physiology 145: 331-339 This indicates that the fish are simply performing a form of sensory taxis, that they are getting all the information they need to locate the prey in the initial stimulus reception and are retaining the location while orienting and swimming. Fish also can begin orienting and swimming after receiving only the first few waves in a wavetrain or click stimulus.
Directional orientation When stimuli are presented within a detectable range, orientations toward the stimulus by A lineatus are extremely precise: the response is almost perfect with angles up to 150° in either direction from forwards, and precision falls off but not much when waves come from the rear of the fish.