ULTRASTRUCTURE AND TRANSPORT-RELATED ENZYMES OF THE GILLS AND COXAL GLAND OF THE HORSESHOE-CRAB LIMULUS-POLYPHEMUS

The horseshoe crab, Limulus polyphemus, may be unique among marine art hropods in that both its book gills and its coral gland may serve as s ites of ion transport. We have therefore examined the ultrastructure o f these organs, as well as the distribution and relative levels of two major transport-related enzymes: the Na+ + K+ ATPase and carbonic anh ydrase (CA). The ventral surface of the central region of each lamella shows the typical ultrastructural specializations for ion transport: 10 mu m cell thickness, an extensive network of tubules originating fr om infoldings of the basal membrane, and a high density of mitochondri a. This region also contains high levels of activity of the Na+ + K+ A TPase and CA. The distribution of ion transporting epithelium and tran sport enzymes is identical in each of the five gill books. The periphe ral region of the lamellae of each gill book is specialized for passiv e gas exchange. The ultrastructural and biochemical profile of the cor al gland is similar to that of the central-ventral region of the gill. Limulus possesses the same general mechanism of ion regulation seen i n euryhaline decapod crustaceans, but the structural and functional co mponents are uniquely distributed.