INTERNAL MORPHOLOGY OF THE GILL OF A LORICARIID FISH, HYPOSTOMUS-PLECOSTOMUS - ARTERIO-ARTERIAL VASCULATURE AND MUSCLE ORGANIZATION

The structural organization of the interbranchial septum of the gill a rch of the air-breathing loricariid fish Hypostomus plecostomus was ex amined using light and electron microscopy. In the middle of the inter branchial septum, an extensive interconnection was found between the a fferent primary arteries from successive and opposing primary lamellae . The blood circulates among numerous trabeculae consisting of connect ive tissue, smooth muscle cells, and collagen fibres. A sheet of smoot h muscle cells is localized at the borders of these interconnected pri mary arteries and joins the cartilage rod from one primary lamella to the adjacent one on the same hemibranch. The adductor muscles are rest ricted to the distal end of the interbranchial septum and consist of t ransverse and oblique striated muscle fibres fixed to the cartilage ro d from the primary lamella of opposite hemibranchs. The arrangement of these muscle fibres suggests a double movement of adduction: approxim ation of the tips of the primary lamellae of opposing hemibranchs and reduction of the space between adjacent primary lamellae of the same h emibranch. The action of both smooth and striated muscles reduces the interconnecting vascular septal space between the primary arteries, wh ich may allow fine adjustment of vascular perfusion of the distal part of the filaments as an adaptation for better blood flow under hypoxic conditions.