The common understanding is that oxygen is essential for animal survival, stemming from our own biological needs and observations. However, Earth’s diverse environments raise a fundamental question: do all animals truly need oxygen to live? This query reveals surprising adaptations and challenges conventional biological assumptions.
The Essential Role of Oxygen in Animal Life
For the vast majority of animals, oxygen is central to generating energy for life functions. This process, aerobic cellular respiration, efficiently converts nutrients into adenosine triphosphate (ATP), the primary energy currency of cells. Oxygen acts as the final electron acceptor in the electron transport chain, a series of reactions producing significant ATP. Without oxygen, this efficient energy production pathway cannot proceed.
ATP from aerobic respiration powers virtually all cellular activities, from muscle contraction and nerve impulses to growth, repair, and reproduction. This constant energy supply is fundamental for maintaining an organism’s complex structure and carrying out the intricate biochemical reactions that define life.
How Animals Acquire Oxygen
Animals have evolved specialized systems to acquire oxygen from their environments. Terrestrial vertebrates, including mammals, birds, and reptiles, use lungs. In lungs, oxygen diffuses across thin membranes into the bloodstream for transport throughout the body.
Aquatic animals, such as fish and many invertebrates, possess gills for efficient gas exchange in water. Water flows over gill surfaces, allowing dissolved oxygen to transfer into the blood, often through a countercurrent exchange mechanism that maximizes absorption. Insects employ a tracheal system, a network of air-filled tubes delivering oxygen directly to tissues without circulatory transport. Simpler organisms, like some amphibians and worms, absorb oxygen directly through their moist skin via diffusion.
Animals That Live Without Oxygen
While most animal life requires oxygen, some exceptions exist. These creatures have evolved metabolic adaptations to thrive in oxygen-deprived or anoxic environments. They generate energy through anaerobic processes, which do not require molecular oxygen.
Certain parasitic worms, such as tapeworms and roundworms, are examples of animals that survive with little to no oxygen. These parasites often inhabit host intestines, an environment with low oxygen levels. They rely on anaerobic respiration or fermentation pathways, which are less efficient at producing ATP than aerobic respiration but suffice for their energy needs. Some parasitic helminths can even switch between aerobic and anaerobic metabolism depending on oxygen availability.
A recently discovered example is Henneguya salminicola, a microscopic cnidarian parasite of salmon. This organism lacks a mitochondrial genome, meaning it lost the cellular machinery responsible for oxygen-dependent energy production. Henneguya salminicola is believed to derive energy directly from its host’s cells or utilize an undetermined oxygen-free metabolic pathway. This discovery altered the understanding of animal life, showing some multicellular animals can live without oxygen.
Certain species of Loriciferans, tiny marine invertebrates, were discovered in deep, anoxic basins of the Mediterranean Sea. These microscopic animals complete their entire life cycle in environments devoid of oxygen. Research suggests they possess specialized organelles called hydrogenosomes, which function similarly to mitochondria but produce energy without oxygen, generating hydrogen as a byproduct. These animals highlight life’s adaptability to extreme conditions.