THE MUCOUS COAT ON GILL LAMELLAE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The existence of a layer of mucus covering the gill lamellae of health y rainbow trout (Oncorhynchus mykiss) was investigated. Using cryo-sca nning electron microscopy, a smooth, undulating, thin layer was observ ed which completely covered gill filaments and lamellae, thereby obscu ring epithelial microridges. After processing cryopreserved gill arche s in glutaraldehyde for conventional scanning electron microscopy, the layer was no longer present and epithelial microridges were clearly v isible. The identity of this layer was investigated using cryopreserve d gills which were treated in one of two ways. First, gills were incub ated with a rabbit antiserum to gill mucus, with normal rabbit serum, or with phosphate-buffered saline. Following fixation in glutaraldehyd e and processing, only the gill tissue incubated with the mucus-specif ic antiserum was still covered with the smooth layer. The layer was al so retained on the gills of fish anesthetized in a solution containing mucus-specific antiserum and then processed in glutaraldehyde for con ventional scanning electron microscopy. The tenacious nature of the mu cous layer was demonstrated by its stability following exposure to for malin and a cationic detergent. Second, the presence of this layer was confirmed on gill tissue which was cryopreserved, followed by freeze- substitution and vapor fixation, and then examined by transmission ele ctron microscopy.