What is the primary respiratory pigment in crustaceans and arachnids?

• A. Hemocyanin, a copper-based protein that turns blue when oxygenated.
• B. Hemoglobin, the iron-containing pigment in blood.
• C. Melanin, a pigment involved in coloration.
• D. Chlorocruorin, a pigment found in some invertebrates.

Answer: (A)

The main idea is how oxygen is carried in the blood (hemolymph) of arthropods like crustaceans and arachnids. They use a copper-containing protein called hemocyanin to transport oxygen dissolved in the hemolymph. When hemocyanin binds oxygen, it turns blue, so the blood can appear blue in many of these animals. This copper-based pigment is well suited to their open circulatory systems, where gases move through body fluids rather than being tightly bound to cells as in vertebrate blood.

Hemoglobin, by contrast, is iron-containing and red when oxygenated, and it’s the dominant respiratory pigment in vertebrates and some other invertebrates but not the primary pigment in crustaceans or arachnids. Melanin isn’t involved in oxygen transport at all; it’s a pigment related to coloration and protection. Chlorocruorin is another pigment found in a subset of invertebrates, but it isn’t the principal respiratory pigment in crustaceans or arachnids.