Surface wave detection by animals
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.
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.
[2] 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.
[2] 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.
[4] Pantadon bucholzi (a surface dwelling butterfly fish) is used less often though is very similar in its anatomy and behavior.
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.
[7] The wave-detection system of A lineatus and other surface feeding fish is tuned to match the waves that signal prey in their environment.
[6] 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.
A common suggestion is that an animal performs a ray tracing calculation, similar to what human oceanographers use to locate ocean storms without a satellite.
[2] However, there is no evidence of a neural circuit that performs this calculation, and other researchers suggest that the distance between any pair of surface wave neuromasts is too small (A lineatus's head is only 1 cm wide) for the calculation to be made accurately, and then at distances from the stimulus less than 7 cm.
[11] A lineatus' possible use of the wavefront's curvature can be excluded since fish are still able to judge the distance of a wave's source with only a single functioning neuromast when all others have been ablated.
Each bundle of hair cells is covered in a gelatinous capulla which the capillary waves actually make contact with and in doing so cause afferent neurons to fire.
[7] Recent work has also revealed small ridges of tissue around each neuromast which direct water flow around it.
In this regard what research has been done shows considerable variation between species; in A lineatus, these neuromasts are considered the supraorbital line, and are innervated by ramus opthalmicus superficialis.
[17] In one of the few electrophysiology studies performed on A lineatus, it was found that of all the primary afferents from neuromasts that they recorded, half phase-locked to wave stimuli.
Since this was the extent of the feature representation found, it was concluded that further analysis of waves must happen at higher levels in the nervous system.
[19] Xenopus has approximately 200 lateral-line organs located along the sides of its body, and also around its eyes, head and neck.
It is claimed these are used to navigate and detect prey either striking the water or struggling[20] but the role of these organs in surface wave detection is not entirely clear; Xenopus with all of their lateral line organs destroyed are still able to respond to surface waves in an oriented fashion.
[21] It has been speculated that both Xenopus and the European grassfrog use their seismic sensory capabilities to locate conspecifics in breeding ponds.
[23][24] Females of the water spider Argyroneta aquatica build underwater "diving bell" webs which they fill with air and use for digesting prey, molting, mating and raising offspring.
