Short Communication Filament Position in Fish Gills Is Influenced by a Smooth Muscle Innervated by Adrenergic Nerves

The two rows of filaments on fish gill arches are arranged in the respiratory water current to maintain a continuous 'gill curtain'. The position of the filaments is controlled mainly by striated adductor muscles, which can draw together the filaments, and also by the elasticity of the cartilaginous filamental skeleton (gill rods) which tends to keep the filaments extended in water. In addition, the striated abductor muscles (which link the filamental rods and the gill arch bone) are arranged to increase the angle between the two rows of filaments (Riess, 1881; Bijtel, 1949; Hughes, 1984; Laurent, 1984) (see also Fig. 1). The gill arches of teleost fish are innervated by the branchial branches of the glossopharyngeal (IX, 1st pair of gill arches) and vagus nerves (X, 2nd-4th pairs of gill arches). Although most nerve fibres in the branchial nerve trunks are believed to be sensory, motor fibres to the striated muscles and autonomic nerve fibres which control the branchial vasculature are known to be present (Nilsson, 1984). This study was started as a fluorescence histochemical investigation of the distribution of adrenergic fibres in effector tissues of the gills of the Atlantic cod, Gadus morhua. It was observed that the greatest density of adrenergic nerve terminals occurred in the tissue connecting the bases of the filamental cartilage rods within the gill arch. This tissue has previously been regarded as a connective tissue ligament (transverse lamina) (Riess, 1881; Bijtel, 1949; Dornesco & Miscalenco, 1967; Hughes, 1984), but recent electron microscopical studies in the perch, Perca fluviatilis, have shown that this ligament is in fact a smooth muscle innervated by a-type nerve profiles (Dunel-Erb & Laurent cited in Laurent, 1984). This paper describes the function of the adrenergically-innervated smooth abductor muscle of the cod gills. The function of another recently described adrenergically-innervated smooth muscle in the gills of some teleosts, previously also