GILL ARCH OF THE MULLET, MUGIL-CEPHALUS .1. SURFACE ULTRASTRUCTURE

The surface ultrastructure of the gill arches of the mullet, Mugil cephalus, was studied by scanning electron microscopy. Each gill arch supports two rows of gill filaments which extend posterolaterally, and two rows of gill rakers which extend anteromedially. Secondary lamellae (respiratory lamellae) extend from both lateral surfaces of each filament, and numerous secondary projections, either spiny or smooth, extend from the anteromedial aspect of each raker. Raker morphology varies somewhat from one gill arch to the next and from the dorsal to ventral aspects of the same gill arch. All surfaces of the gill arch except the respiratory lamellae and portions of the rakers are covered with a mosaic of ridged epithelial cells. These cells measure 3‐10 μ in diameter, and possess concentrically arranged surface ridges which measure 0.1–0.15 μ in width. In contrast, the respiratory lamellae and the anteromedial surface ridges. Numerous pores, having rounded cellular extensions projecting from their ineriors, are irregularly located between the ridged epithelial cells. Pores are common on the apposing surfaces of the rows of gill filaments and at the bases of the respiratory lamellae, but are rare or absent on other surfaces of the filament. Because the pores likely represent entrances to apical crypts of chloride cells, scanning electron microscopy may be useful in monitoring changes in the number and distribution of chloride cells in gill filaments of euryhaline fish during adaptation to various salinities. Copyright © 1979 Wiley‐Liss, Inc., A Wiley Company