That's an excellent question. Members of Class Chondrichthyes (sharks, skates, and rays) do not have bones; they only have cartilage. Bony fish (Class Osteichthyes) have bone marrow that they use to make blood.
The skeleton of a bony fish gives structure, provides protection, assists in leverage, and is a site of red blood cell production.
A couple of interesting side notes: Our spleen is used for maturation of white blood cells and storage of red blood cells.
Birds (Class Aves) use the cavities in their bones for air sacs, so their bed blood cells have nuclei (unlike ours) which allow them to hang around longer.
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