The constant movement of substances, known as circulation, is necessary for life in complex organisms. Every cell requires a steady supply of oxygen and nutrients while simultaneously needing the removal of metabolic waste products. This universal need for efficient transport has driven the evolution of diverse internal systems across the animal kingdom. While many animals use a familiar red fluid, numerous life forms have developed survival strategies that operate without what we commonly recognize as blood.
What Defines Blood in the Animal Kingdom
True blood, particularly in vertebrates, is a specialized connective tissue contained within a closed circulatory network. Its characteristic red color comes from the iron-containing protein hemoglobin, packaged inside red blood cells. The primary function of this fluid is the efficient transport of respiratory gases, binding to oxygen for delivery to tissues and carrying carbon dioxide back for expulsion. Beyond gas exchange, blood plasma distributes nutrients absorbed from the digestive system and collects nitrogenous wastes for processing by excretory organs. The system is considered closed because the fluid continuously cycles through arteries, capillaries, and veins, never leaving the vessels.
Organisms That Rely on Direct Diffusion
Many of the simplest multicellular animals lack any internal circulatory system, relying instead on the direct movement of substances across their body surface. This mechanism, called direct diffusion, is only feasible for organisms with a specific body architecture. Sponges (Porifera) are sessile filter feeders whose porous bodies ensure that nearly every cell is exposed to the surrounding water. Cnidarians, such as jellyfish, employ a thin, non-living layer called the mesoglea, which keeps living cells close to the water-filled gastrovascular cavity or the external environment.
This reliance on diffusion is also the defining feature of flatworms (Platyhelminthes), which have a flattened body shape. Their ribbon-like structure provides a high surface-area-to-volume ratio, drastically shortening the distance for gases and nutrients to travel inward. Oxygen simply diffuses from the surrounding water directly into the cells, and carbon dioxide diffuses out. For these simple body plans, a dedicated transport system is metabolically inefficient and unnecessary.
The Function of Hemolymph in Open Circulatory Systems
Many invertebrates, including insects, spiders, and most mollusks (Arthropods), possess a circulating fluid known as hemolymph, which differs from true blood. Hemolymph is often colorless or pale green/yellow because it lacks the high concentration of respiratory pigments found in vertebrate blood. This fluid operates within an open circulatory system, where a heart pumps the hemolymph through short vessels that empty into a central body cavity called the hemocoel.
In this open system, the hemolymph directly bathes the internal organs and tissues before collecting and re-entering the heart through small openings called ostia. For insects, the hemolymph’s primary role is not oxygen transport; that is handled by the tracheal system, a separate network of air tubes. Instead, hemolymph functions as a medium for distributing nutrients and hormones, collecting metabolic wastes, and playing a defensive role through circulating immune cells.
Specialized Survival Tactics in Bloodless Vertebrates
The most striking example of a complex animal surviving without traditional blood components is the Antarctic Icefish (family Channichthyidae), the only known vertebrate to lack hemoglobin. This loss resulted from a gene deletion that rendered the oxygen-carrying protein obsolete. The icefish’s survival relies heavily on the unique conditions of its environment, the frigid Southern Ocean.
Oxygen is significantly more soluble in cold water, providing the icefish with a sufficient supply of dissolved oxygen transported directly in the plasma. To compensate for the lack of hemoglobin, the icefish possesses specialized cardiovascular features, including a higher volume of blood, a larger heart, and wider blood vessels. These features reduce resistance and increase the flow rate of the less viscous, watery blood. Additionally, antifreeze glycoproteins in their body fluids prevent ice crystals from forming in their tissues, an adaptation necessary for life in sub-zero temperatures.