Caecilians are an enigmatic group of amphibians, often mistaken for large earthworms or small snakes due to their elongated, limbless bodies and subterranean or aquatic lifestyles. The question of how these burrowing creatures breathe involves exploring the diverse respiratory adaptations found within this ancient amphibian order, Gymnophiona. The answer to whether caecilians possess lungs is not a simple yes or no. This article clarifies the standard anatomy of caecilian respiration and highlights the rare exceptions that thrive without conventional lungs.
The Standard Caecilian Respiratory System
Most caecilian species, particularly terrestrial and semi-aquatic ones, rely on lungs for a significant portion of their gas exchange. Their pulmonary anatomy is highly specialized and reflects their long, slender body shape, resulting in pronounced asymmetry in their lung structure.
The right lung is typically dominant, becoming greatly elongated to extend down a substantial portion of the body cavity, sometimes reaching 50 to 75 percent of the animal’s total body length. Conversely, the left lung is often reduced to a vestigial stump or is completely absent, an adaptation common in other elongated vertebrates like snakes. Some aquatic species, such as those in the family Typhlonectidae, exhibit less extreme asymmetry. Furthermore, some caecilians possess a modified structure called a “tracheal lung,” a vascularized section of the trachea that assists in air breathing.
To inflate these lungs, caecilians use a mechanism known as buccal pumping, similar to other amphibians, where they use muscles in the mouth and throat to force air down the trachea. This method can involve a series of rapid buccal cycles in a single breath to maximize air intake. The elongated body of terrestrial caecilians requires them to generate high internal pressures, sometimes three to ten times greater than other amphibians, which helps maintain their body form and aids in burrowing locomotion.
The Unique Case of Lungless Caecilians
While the majority of species possess lungs, the most striking aspect of caecilian biology is the existence of rare exceptions that have completely lost this organ. The most famous example is the genus Atretochoana, which contains the largest known lungless tetrapod in the world. This highly aquatic species can grow to lengths exceeding 80 centimeters.
The absence of lungs in Atretochoana is accompanied by morphological changes supporting its unique respiratory strategy. It possesses sealed internal nostrils (choanae) and lacks pulmonary arteries, consistent with relying on non-pulmonary gas exchange. Although scientists initially hypothesized it was adapted to cold, oxygen-rich water, its rediscovery in the warmer, turbid Madeira River in the Brazilian Amazon makes its lunglessness unusual.
A second, smaller example of lunglessness was discovered in a terrestrial species from Guyana, demonstrating the evolutionary flexibility within the order. This small, burrowing caecilian also lacks external nares and lungs, relying entirely on other methods for oxygen uptake. The loss of lungs is an extreme adaptation, requiring the skin and associated circulatory systems to take over the full burden of gas exchange.
Breathing Without Lungs: Cutaneous and Aquatic Respiration
All caecilians, regardless of whether they possess lungs, rely on non-pulmonary methods to supplement their breathing, showcasing the fundamental amphibian trait of bimodal respiration. This gas exchange through the skin is known as cutaneous respiration and is a constant process. The skin is highly vascularized, meaning it is richly supplied with capillaries that lie close to the surface.
This thin, permeable skin must remain moist to allow oxygen to diffuse into the bloodstream and carbon dioxide to be released. Studies show that even in lunged species, cutaneous respiration is the primary method for eliminating carbon dioxide, accounting for over 50 percent of the total \(\text{CO}_2\) output. This continuous skin breathing supports the caecilian’s low resting metabolic rate, an adaptation to their often inactive, subterranean lifestyle.
For species that begin their lives in water, respiration often involves external gills during the larval stage. These gills allow the young amphibian to efficiently extract dissolved oxygen from the water. Like frogs and salamanders, these gills are lost during metamorphosis, at which point the adult form transitions to a respiratory system dominated by lungs and skin. The combination of cutaneous and, when present, pulmonary respiration allows caecilians to thrive in their diverse and often oxygen-poor habitats.