<P> The gas / tissue interface at the swim bladder produces a strong reflection of sound, which is used in sonar equipment to find fish . </P> <P> Cartilaginous fish, such as sharks and rays, do not have swim bladders . Some of them can control their depth only by swimming (using dynamic lift); others store fats or oils with density less than that of seawater to produce a neutral or near neutral buoyancy, which does not change with depth . </P> <P> The swim bladder normally consists of two gas - filled sacs located in the dorsal portion of the fish, although in a few primitive species, there is only a single sac . It has flexible walls that contract or expand according to the ambient pressure . The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases . By adjusting the gas pressurising organ using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths . Due to the dorsal position it gives the fish lateral stability . </P> <P> In physostomous swim bladders, a connection is retained between the swim bladder and the gut, the pneumatic duct, allowing the fish to fill up the swim bladder by "gulping" air . Excess gas can be removed in a similar manner . </P>

Where is the swim bladder located in a fish