The coelacanth, a fish rediscovered in 1938 after being thought to have vanished with the dinosaurs, is a “living fossil” offering a glimpse into the distant past. A central point of interest is the coelacanth’s method of breathing. This leads to a fundamental question about its anatomy: does this ancient fish possess lungs for breathing air, or does it rely on gills like most other fish?
The Coelacanth’s Pulmonary Organ
Coelacanths possess a structure that is anatomically related to the lungs of air-breathing vertebrates, often called a “fatty lung.” This single-lobed organ is located in the fish’s body cavity. In adult coelacanths, this pulmonary organ is filled with fat, rendering it non-functional for gas exchange and instead helping to control buoyancy.
This organ is considered vestigial, a remnant of an ancestral past. Studies on coelacanth embryos show the lung is well-developed and potentially functional in the earliest stages of life. As the coelacanth matures, the lung’s growth is arrested and it regresses. This developmental trajectory suggests that the ancestors of modern coelacanths had functional lungs.
The fatty lung can also feature small, hard plates, homologous to the “calcified lung” found in some fossil species. The presence of this structure provides a direct physical link to ancient relatives. The organ’s shift in function is an adaptation to the deep-water environments modern coelacanths inhabit.
Gill-Based Respiration in Coelacanths
While its lung is a non-functional remnant, the coelacanth relies entirely on its gills for respiration. These structures are designed to efficiently extract dissolved oxygen from water. The gills are composed of several gill arches, each supporting filaments and lamellae that create a vast surface area for gas exchange.
The structure of the coelacanth’s gills is well-suited for its lifestyle in deep marine waters, which have stable, low oxygen levels. Coelacanths have a slow metabolism and a correspondingly low demand for oxygen. Their gills are adapted to meet these physiological needs, allowing them to thrive at great depths where they were long hidden from science.
Evolutionary Insights from Coelacanth Respiration
The coelacanth’s respiratory system provides significant insights into vertebrate evolution. The presence of both gills and a vestigial lung shows that lungs appeared very early in the evolution of bony fishes. They existed alongside gills in a common ancestor to both lobe-finned fishes (like the coelacanth) and ray-finned fishes. This suggests lungs first evolved not for leaving the water, but as an accessory organ for fish in low-oxygen aquatic habitats.
As a lobe-finned fish (Sarcopterygii), the coelacanth is related to lungfishes and tetrapods—the four-limbed vertebrates that moved to land. The coelacanth’s fatty lung is evidence for the ancient origin of this organ within this lineage. While tetrapods came to rely solely on lungs and lungfish retained functional lungs, the coelacanth followed a different evolutionary path.
Its lineage adapted to deep water where air-breathing was unnecessary, causing the lung to become a defunct respiratory organ. This contrasts with its fossil ancestors, who lived in shallower waters and likely used their lungs to breathe both air and water. The modern coelacanth is not a direct ancestor to land animals but a unique branch of the family tree, showing how evolutionary pathways can diverge.