FUNCTION AND EVOLUTION OF SUPERFICIAL NEUROMASTS IN AN ANTARCTIC NOTOTHENIOID FISH

Extracellular recording techniques were used to measure frequency resp onse functions of anterior and posterior lateral line nerve fibers inn ervating superficial neuromasts at five different locations on the hea d and trunk of an antarctic notothenioid fish, Trematomus bernacchii. Scanning electron microscopy was used to measure neuromast size accord ing to location. Fibers innervating neuromasts from all locations were similar in showing equal responsiveness in the 10-30 Hz range to equa l pk-pk velocity levels of a sinusoidally vibrating sphere. The mean c ut-off frequency (CF) at which responsiveness declined to 50% of maxim um was 46 Hz for all fibers combined. Superficial neuromasts located o n the ventral trunk line were three to six times larger in surface are a than most other neuromasts. The mean CF for fibers innervating these large neuromasts was 7-18 Hz lower than mean CF's corresponding to ot her superficial neuromast locations, but small differences in mean CF' s were not consistently related to neuromast size. It is argued that f iber 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 a mong notothenioid fishes that may be essentially non-adaptive for the lateral line.