Sexual dimorphism in the gills of the spawning river lamprey, Lampetra fluviatilis L.

Two types of mitochondria-rich cells were found in the interplatelet areas of the gills of sexually mature, male river lampreys. Type 1 cells (previolsy referred to as "male glandular cells") showed some ultrastructural characteristics in common with ion-transporting cells but were readily distinguished by large lipid structures with electron-lucent centres. Type 2 cells were found to be identical to the presumed ion-uptake cells during other stenohaline freshwater phases of the animal's life history. In the sexually mature female, only Type 2 cells were positively identified. This sexual dimorphism in gill structure is discussed in relation to the possible functions of the lamprey gill, with particular reference to ionic regulation.