● THE SWIM-BLADDER FISH. Many fish have swim bladders filled with gas. When the fish descends, the pressure of the water compresses the gas and reduces the size of the swim bladder. If the fish rises, the water pressure lessens, the gas expands, and the size of the bladder increases. When the size of the bladder changes, so does the size of the fish. So when it descends, increased pressure decreases its volume, which means its average density increases, and this lessens its buoyancy. When it rises, its volume increases, which lessens its average density, and this increases its buoyancy. Thus the swim bladder functions to keep the density of the fish equal to the density of the seawater around it, enabling the fish to float at any depth. But it is not always that simple. At a depth of 6,500 feet, pressure has squeezed the bladder’s volume down to only 1/200th of its volume at the surface, the gas in it is 200 times denser, and buoyancy has about disappeared. Yet fish hover motionless at twice that depth, the gas in their swim bladders exerting a pressure of more than 7,000 pounds per square inch to withstand the pressure of the sea! Yet how do they retain buoyancy? Very slowly they can add gas to their swim bladders as they go deeper and resorb it as they rise.

But unlike the shell chambers of the nautilus, the cuttlefish’s buoyancy mechanism is made of bone, the cuttlebone. It is located just under the skin along the back of the cuttlefish. It is a soft, chalky structure, having up to a hundred thin plates held apart by pillars, and honeycombed with many separate chambers. It is this bone that serves as the cuttlefish’s buoyancy tank. As the cuttlefish grows and gets heavier, more chambers are added to the cuttlebone to increase its powers of buoyancy. (Incidentally, it is this cuttlebone that is put in the cages of birds.) By a process of osmosis the cuttlefish can pump water out of the cavities of its cuttlebone or allow water to enter. In this way it varies its buoyancy to ascend or descend in the ocean. In principle, the cavities in its cuttlebone are like the water tanks of a submarine. Cuttlefish usually stay from 100 to 250 feet deep but can descend to 600 feet.

It, like the nautilus and the cuttlefish, can adjust to different depths in the sea but does it differently. The upper two thirds of its body is a large cavity, the coelomic cavity. It is filled with a liquid. If this liquid is removed, the squid sinks. The fluid gives the animal its neutral density to seawater. Analysis has shown that it has a very high concentration of ammonia, 1.2 ounces per gallon. Why is this so? Unlike mammals, the squid excretes its nitrogenous wastes as ammonia instead of urea. This ammonia diffuses from the bloodstream into the fluid of the coelomic cavity, where it dissociates into ammonium ions. These ions are lightweight and make the fluid lighter than seawater, imparting buoyancy to the squid. Scientific American magazine compares it with Auguste Piccard’s bathyscaphe that descends into the ocean depths. The bathyscaphe’s large chamber filled with gasoline, which is lighter than seawater, supports the observation chamber suspended below it. Similarly, the coelomic cavity fluid of the deep-sea squid serves as a flotation device. But the squid did it first, because its Creator thought of it first.