Though most vertebrates do have red blood cells, there is one known exception: the antarctic icefish. It's the only known vertebrate that lacks both red blood cells and hemoglobin, the protein that allows iron to attach to red blood cells. Though this should make circulation easier for them since their blood is so thin, they also have abnormally large blood vessels and more blood vessels than other fish, so they have to use about 22% of their energy just for pumping blood.
More about icefish and blood in vertebrates:
- Fish that live in cold water generally have fewer red blood cells and less hemoglobin than those who live in warm water, and some fish that live in areas where the water gets cooler in the winter also produce fewer red blood cells in the winter than in the summer.
- Humans have about 20 to 30 trillion red blood cells.
- Mammals are the only animals that have platelets, which are essential for blood clotting. Other vertebrates have longer, thinner-shaped cells that serve the same purpose.