Distribution of nerve fibers in the barbels of sea catfish Plotosus lineatus

Abstract

The sea catfish Plotosus lineatus has four pairs of barbels of almost the same length as major taste and tactile organs. The patterns of innervation in the barbels of the sea catfish were studied to reveal peripheral neural organization for taste and mechanosensory systems. Taste buds are located in the epidermis of each barbel throughout the length of barbel. The density of taste buds is low at the proximal and increases distally and at the transverse levels, highest at the rostral, fairly high at the caudal and very low in the intermediate sides throughout the longitudinal axis of the barbel. Observations of cross-sections of the barbels show that the taste buds are usually located as pairs innervated by two different nerve strands originating from the same nerve bundle. The carbocyanine dye, Dil, was applied to the nerve stump in dissected fixed barbel specimens to trace the peripheral course of nerve fibers. The barbel nerves enter the caudal region at the base of the barbel as a trunk. The trunk sends various sizes of bundles to the rostral side coursing toward the tip. These bundles move in turn to the rostral and caudal epidermis and ramify repeatedly to make networks under the epidermis, but few networks were found in the intermediate sides. The networks usually make hexagons in shape. Their size is smaller in the rostral than in the caudal side and becomes smaller toward the apical region in both sides. The maximum and minimum diameters for the hexagons range 240-400 μm and 100-250 μm, respectively. Nerve strands appear in pairs from each network to innervate taste buds. The number of strands originating from one network ranged 20-50. Each strand divides into two substrands, each of which enters the same bud laterobasally opposite one another. Perigemmal and extragemmal fibers, which end around taste buds and in the epidermis, respectively, were also frequently found. These results show the unique organization of nerve fibers in the barbel taste and tactile systems of the sea catfish.