The function of pectoral filaments (which are separate rays of the lower part of pectoral fin, and of which 3 pairs are longer and 11 pairs are shorter than the body length) of Polynemus multifilis was studied in an aquarium . When P. multflis was swimming, the long filaments were spread out in front of and above the fish and the short filaments were spread downward like a rake being in contact with the bottom or wall. During 10 minute observations, the contact time of the short filaments with the bottom or wall (range, 506.8-599 .1 sec.), was longer than that of long filaments (6.8-95.4 sec.). But the contact time of each type of filament was constant among the three feeding conditions (before feeding, while feeding and after being satiated). The contact rates (contact frequency of long or short filaments/total contact frequency of any filament) of the short filaments with falling defrosted prey (meant SD, 69.3±4.2%) was higher than those of long filaments (30.7±4.2%). The feeding rates (feeding frequency of the prey/contact frequency of filaments with prey) of short filaments (95.7±5.3%) were higher than those of the long filaments (56.41±11.1%). The contact rates of the long filaments with obstacles, which were vinyl pipes placed on the tank bottom and wall, (63.0±10.7%) were higher than those of the short filaments (37.0±10.7%). The avoidance rates (avoidance frequency/contact frequency of filaments with obstacles) of the long filaments (55.0±18.9%) were higher than those of the short filaments (28.8±9.5%). The filaments may have functions to detect prey items, for use to swim along the bottom or wall and to sense obstacles, and the first and second functions were mainly the role of the short filaments and the last function was mainly the role of the long filaments.
