Animals With Blue or Yellow Blood Instead of Red

Why blood isn't always red

sea cucumber on a tan surface

Mint Images/Frans Lanting/Getty Images

One fun Halloween chemistry project is making edible fake blood recipes. One of these recipes can be used to make blood in any color you like. Why colored blood? Blood comes in different colors, depending on species.

While humans and many other species have red blood, due to the iron in their hemoglobin, other animals have different colored blood. Spiders (as well as horseshoe crabs and certain other arthropods) have blue blood due to the presence of copper-based hemocyanin in their blood.

Some animals, such as the sea cucumbers, even have yellow blood. What could make blood yellow? The yellow coloration is due to a high concentration of the yellow vanadium-based pigment, vanabin. Unlike hemoglobin and hemocyanin, vanabin does not seem to be involved in oxygen transport. In addition to vanabin, sea cucumbers have enough hemocyanin in their blood to sustain their oxygen needs. Actually, the role of vanabin remains a bit of a mystery.

Perhaps it is part of a defense mechanism to make the sea cucumbers unappealing or toxic to parasites and predators. However, sea cucumber is used for cooking in many cultures, where it is prized for its slippery texture and possible health benefits. Vanadium is a controversial dietary supplement, potentially affecting insulin sensitivity and athletic performance.