Breathing is an essential biological process for most living organisms. However, there are some notable exceptions in the animal kingdom of creatures that do not require breathing in the traditional sense that we think of it. In this article, we will explore some of the fascinating animals that have evolved alternative methods of respiration and oxygen supply.
Most animals on Earth are aerobic organisms, meaning they require oxygen to survive. The vast majority rely on lungs or gills to extract oxygen from the air or water and expel carbon dioxide. However, there are some incredible exceptions to this rule. These include:
- Jellyfish
- Flatworms
- Tapeworms
- Roundworms
- Tardigrades
These creatures have developed the ability to live without traditional respiratory systems for gas exchange. Some can survive in anaerobic environments completely devoid of oxygen, while others have evolved alternative respiration methods. We will examine how each accomplishes its oxygen needs in more detail.
Jellyfish
Jellyfish are one of the more complex creatures that do not actively breathe. They have no need for gills, lungs or other specialized respiratory structures. Instead, they fulfill their oxygen requirements through simple diffusion across their cell membranes.
A jellyfish’s body is made up of three simple layers- an outer epidermis, an inner gastrodermis separated by a gelatinous substance called mesoglea. Their soft, sac-like bodies have large surface area to volume ratios. This allows for direct diffusion of dissolved oxygen from the surrounding water into their cells and expulsion of carbon dioxide.
While efficient at fulfilling basic metabolic needs, this simplistic diffusion process limits a jellyfish’s ability to engage in active, oxygen-consuming behaviors. Most jellyfish simply drift passively through the water filtering nutrients. The few specialized species with ability to actively swim do so in short bursts before returning to their energy efficient passive mode.
Advantages of Diffusion-Based Respiration
- Requires no specialized anatomical structures
- Allows jellyfish to be free-floating and drift with currents
- Enables a relatively simple body plan
Disadvantages of Diffusion-Based Respiration
- Limits ability to engage in sustained, active energy expensive behaviors
- Constrains jellyfish to low-oxygen environments
- Restricts potential body size
While limited in some capacities, for the relatively sedentary jellyfish, simple diffusion meets metabolic demands without the added complexity of breathing apparatus. Their unique needs allowed them to evolve this efficient diffusion-based strategy.
Flatworms
Flatworms such as planarians and parasitic tapeworms also employ diffusion to acquire the oxygen they need. However, their respiratory processes operate a bit differently than jellyfish.
While jellyfish perform direct diffusion across the outer membranes, flatworms take advantage of their flattened bodies and complex branched network of gas exchange vessels to maximize diffusion. Oxygen is absorbed across the outer membranes into these vessels and carried deeper into tissues.
In addition, some flatworms such as parasitic worms have anaerobic tissues that produce energy in absence of oxygen through fermentation and anaerobic cellular pathways. This allows them to survive for long periods without breathing.
Advantages of Flatworm Diffusion Strategy
- Enables survival in low oxygen environments
- Allows them to live in constrained spaces within hosts
- Provides energy through anaerobic pathways when oxygen is insufficient
These adaptations allow flatworms to thrive in aquatic sediments, within hosts, and other environments where free oxygen may be limited.
Roundworms
Roundworms or nematodes include free-living species and parasites. They have evolved unique strategies to meet their oxygen needs in challenging environments. While they do not have lungs or breathe like us, they have developed alternative mechanisms.
Some marine roundworms perform simple diffusion like jellyfish. But many terrestrial and parasitic species have taken different evolutionary routes. Some utilize their outer cuticles to absorb oxygen from the air or surrounding fluids. Certain intestinal roundworms can even extract oxygen from their host’s blood supply.
However, the most common and interesting respiratory mechanism seen in roundworms is anaerobic fermentation. Roundworms have specialized anaerobic cells allowing energy production without oxygen through alcohol fermentation, releasing ethanol as a byproduct. This allows them to thrive in diverse low-oxygen settings.
Benefits of Roundworm Respiration Strategies
- Allows survival in intestines and tissues of hosts
- Enables life in soils and sediments lacking oxygen
- Provides access to nutrients from decaying matter
The use of fermentation and anaerobic metabolism give roundworms flexibility to inhabit low oxygen environments unavailable to organisms requiring aerobic respiration.
Tardigrades
Tardigrades, also known as water bears or moss piglets, are near-microscopic animals renowned for their ability to survive extreme conditions. This includes total oxygen deprivation for extended periods.
In active state with oxygen available, tardigrades breathe through tracheal systems or simple diffusion across cell membranes. However, when exposed to anaerobic environments, tardigrades can enter dormant states where they don’t breathe at all. These include cryptobiosis and anhydrobiosis.
In cryptobiosis, the tardigrade greatly reduces metabolic activity and initiates anaerobic pathways for energy production. Anhydrobiosis involves the animal losing almost all body water content, shriveling into a dry state and suspending metabolism.
Tardigrades have been shown to survive up to 10 years in anhydrobiosis without oxygen. When rehydrated, they return to normal active state. This ability makes tardigrades the most resilient known animal on Earth.
Perks of Tardigrade Survival Strategies
- Enables survival in total absence of oxygen for many years
- Allows revival after reintroduction to aqueous environments
- Permits inhabitation of environments lacking free oxygen
The cryptobiotic and anhydrobiotic states available to tardigrades provide protection from asphyxiation, allowing them to persist where most other animals cannot.
Gastrovascular Cavity Creature – Coral
Coral polyps meet their oxygen needs in a unique fashion. As coral polyps form coral colonies, each polyp is essentially a single animal. Corals do not have specialized respiratory organs. Instead, they use their gastrovascular cavity both for digestion of food and gas exchange.
The cells lining the gastrovascular cavity absorb dissolved oxygen from the surrounding water into their fluids and tissues. At the same time, carbon dioxide waste produced through respiration is expelled into the cavity and released out through the mouth opening of the polyp.
Corals use tentacles to capture prey. Digestion begins externally before the food is passed into the central cavity. Cilia within the gastrovascular chamber circulate fluids and gases to facilitate distribution of oxygen and digestion products.
Advantages of Gastrovascular Respiration
- Eliminates need for dedicated respiratory organs
- Allows efficient distribution of oxygen to tissues
- Enables relatively large size of coral colonies
By consolidating gastric and respiratory functions, coral polyps achieve effective gas exchange while maximizing use of space within their small cylindrical bodies.
| Animal | Respiratory Adaptation | Benefits |
|---|---|---|
| Jellyfish | Diffusion across outer cell membrane | Simple body plan, energy efficiency |
| Flatworms | Diffusion supplemented by fermentation | Survive low oxygen environments |
| Roundworms | Anaerobic fermentation | Inhabit intestinal tracts |
| Tardigrades | Cryptobiosis and anhydrobiosis | Survive total oxygen deprivation |
| Coral | Gastrovascular cavity gas exchange | Efficient use of space |
Conclusion
While most animals require oxygen and use lungs or gills to acquire it, some fascinating exceptions exist. Jellyfish, flatworms, roundworms, tardigrades and coral have all developed innovative strategies that allow survival without breathing in the traditional sense.
For some like tardigrades, this involves entering dormant states. For others such as roundworms it requires use of anaerobic pathways. The diversity of adaptations allow these organisms to inhabit environments prohibitive to animals dependent on aerobic respiration.
However, the limitations of these alternate respiratory mechanisms prevent the evolution of more complex body forms and high energy activities. So while a select few unique organisms survive sans breathing, most animals on Earth will continue to rely on specialized organs for optimal oxygen intake.