Without Bones, How Do Sharks
Make Red Blood Cells?
How do animals, such as sharks, produce red blood cells if they have no bone marrow?
Since sharks have no bone tissue, they also lack red bone marrow — which, as you point out, produce red blood cells in most vertebrates. In elasmobranchs (sharks and rays), the job of manufacturing red blood cells is performed by the spleen, special tissue around the gonads (reproductive organs), and — in at least some species — by a special structure called "Leydig's Organ" (named after the German histologist who first described it in 1857).
In those shark species in which it occurs, Leydig's organ is nestled along the top and bottom of the esophagus. Leydig's organ can be quite large — a 1.6-kilogram (3.5-pound) one was reported from a 1.8-metre (6-foot) long Bluntnose Sixgill Shark (Hexanchus griseus). Leydig's organ is absolutely unique to elasmobranchs, and — as more and more fisheries and marine biologists become aware of it — this interesting structure will undoubtedly be reported in an ever greater number of shark species.