hemerythrin

Hemerythrin is a specialized respiratory protein found in specific marine invertebrates. It serves a similar physiological role to hemoglobin in vertebrates or hemocyanin in mollusks, but it is chemically distinct because it binds oxygen without the use of a heme group.

As a technical term in biochemistry and zoology, it is used almost exclusively in scientific literature. It is an uncountable noun because it refers to a chemical substance or a class of proteins, meaning it does not typically take a plural form in standard scientific discourse.