Hemerythrin pigment is formed in:

Question

Accepted Answer

The correct option is 1: Brachiopods.

Hemerythrin is a distinctive, iron-containing protein that serves as a respiratory pigment for oxygen transport in a select and small group of marine invertebrates. Crucially, students must understand that unlike Hemoglobin, which has a heme group, Hemerythrin's iron atoms are bound directly to the protein structure in a non-heme arrangement. This difference gives the pigment its characteristic color change: when oxygen is bound, the pigment appears a violet-pink, and it becomes colorless when deoxygenated. Its presence is highly confined to three main phyla: Sipunculids (peanut worms), certain Annelids (marine worms), and the **Brachiopods** (or lampshells). Brachiopods, which are ancient filter-feeding marine animals, circulate this pigment within specialized coelomic cells to ensure the efficient supply of oxygen to their metabolically active feeding structure, the lophophore. The fact that its distribution is so limited makes it a key feature for identifying these specific animal groups.

The incorrect option is 2: Cephalopods.
Cephalopods, a class of the phylum Mollusca that includes octopuses and squids, do not form Hemerythrin. Instead, they rely on a different respiratory pigment known as **Hemocyanin**. Hemocyanin is a large, copper-containing protein that is dissolved directly in the blood (hemolymph), giving their oxygenated blood a characteristic blue color. The cephalopod's circulatory system, while more advanced than other mollusks, is fundamentally built around the Hemocyanin-based oxygen transport, which is highly efficient in the cold, low-oxygen marine environment where many of these animals live. This copper-based system is entirely separate in composition and function from the iron-based Hemerythrin.

The incorrect option is 3: Gastropods.
Gastropods, which include all snails and slugs, are another class within the phylum Mollusca. The predominant respiratory pigment used across the phylum Mollusca is **Hemocyanin**, making it the most likely oxygen carrier in gastropods, especially marine and freshwater species. Some terrestrial gastropods and a few aquatic ones, however, have evolved to use the iron-heme-based **Hemoglobin**. Regardless of the variation, the use of Hemerythrin is not a feature of the class Gastropoda. Thus, while members of this group may have blue blood (Hemocyanin) or red blood (Hemoglobin), they are defined by a biochemical pathway that does not synthesize Hemerythrin.

The incorrect option is 4: Ophionoids.
Ophionoids, which are the brittle stars and basket stars, belong to the phylum Echinodermata. A defining feature of this phylum is that its members typically **lack a dedicated circulatory system and, consequently, a respiratory pigment** for systemic gas transport. Oxygen transport in Ophiuroids occurs primarily through simple diffusion across the body surface and is then distributed throughout the body cavity by the circulation of the coelomic fluid. Since they do not have a specialized blood protein to carry oxygen, they have no biological need for, nor do they possess, Hemerythrin.