the pivot axle 38 alternatively the pins may be fixed in the blades in which case the pins would slide in slots in the rotatable disc the rotation of which will cause the blades to move inward toward the closed position when the disc is rotated in one direction and to move outward toward the open position when the disc is rotated in the opposite direction the movable ring may be driven by hydraulic cylinders or the ring may incorporate gear teeth on the outer periphery which are driven by a hydraulic motor or manually by a shaft that is rotated by a remotely operated underwater vehicle while the method of closing the valve described above is preferred alternative methods are also feasible among the alternatives the one most preferred is the use of hydraulic cylinders to position each blade individually various cam and lever arrangements are also obvious alternatives an alternative to the iris valve is a clamshell type closure 50 shown in fig5 in this embodiment the inner cylinder 16 a is stationary and fixed to the base ring and the outer cylinders 16 b and 16 c slide up over one another during deployment attached to the outer cylinder 16 c are a set of shell like ellipsoidally shaped steel surfaces 54 in fig5 only one shell 54 is.