Function and evolution of superficial neuromasts in an Antarctic notothenioid fish.

Extracellular recording techniques were used to measure frequency response functions of anterior and posterior lateral line nerve fibers innervating superficial neuromasts at five different locations on the head and trunk of an antarctic notothenioid fish, Trematomus bernacchii. Scanning electron microscopy was used to measure neuromast size according to location. Fibers innervating neuromasts from all locations were similar in showing equal responsiveness in the 10-30 Hz range to equal pk-pk velocity levels of a sinusoidally vibrating sphere. The mean cut-off frequency (CF) at which responsiveness declined to 50% of maximum was 46 Hz for all fibers combined. Superficial neuromasts located on the ventral trunk line were three to six times larger in surface area than most other neuromasts. The mean CF for fibers innervating these large neuromasts was 7-18 Hz lower than mean CF's corresponding to other superficial neuromast locations, but small differences in mean CF's were not consistently related to neuromast size. It is argued that fiber responses from different superficial neuromasts are more similar than dissimilar and that the evolution of large superficial neuromasts on the ventral trunk line is linked to a general paedomorphic trend among notothenioid fishes that may be essentially non-adaptive for the lateral line.