Fish typically draw oxygen from water using gills, specialized organs that efficiently extract dissolved gases. However, some fish species possess an unexpected biological feature: structures that function similarly to lungs, allowing them to breathe atmospheric air. This adaptation challenges the common perception of fish and their respiratory capabilities. It highlights a remarkable evolutionary pathway that enabled certain fish to thrive in environments where gill-based respiration alone might not be sufficient.
The Origins of Lungs in Fish
The emergence of lungs in fish traces back to ancient aquatic environments, particularly during periods when oxygen levels in water bodies fluctuated dramatically. Scientists propose that lungs evolved from or alongside the swim bladder, a gas-filled organ primarily used for buoyancy control. This organ, located in the abdomen of bony fish, is connected to the digestive tract in some primitive fish.
In environments with low dissolved oxygen, the swim bladder developed a more vascularized structure, enabling it to absorb oxygen directly from gulped air. This adaptation provided an alternative means of gas exchange when water quality deteriorated. These primitive lung-like structures offer insights into the broader evolutionary journey of vertebrates. They represent a branch point in evolution, with some descendants transitioning fully to land, while others remained aquatic but retained their air-breathing capabilities.
Meet the Lunged Fish
Several fish species today showcase the adaptation of air-breathing through lung-like organs, demonstrating diverse strategies for survival. African lungfish, found across various freshwater habitats in Africa, are known for their ability to survive severe droughts. When their homes dry up, they burrow into the mud and secrete a protective cocoon, entering estivation where they can breathe air for months or years. They possess two lungs, similar in structure to those of amphibians, which enable this prolonged air-breathing.
South American lungfish, inhabiting the Amazon, also exhibit air-breathing and can estivate during dry seasons. Like their African counterparts, they rely on their paired lungs to gulp air, supplementing the oxygen they obtain through their gills. This dual respiratory system allows them to persist in oxygen-poor waters or during droughts. The Australian lungfish, a more ancient lineage, primarily uses its single lung as a facultative air-breather. While it can breathe air, it cannot survive prolonged periods out of water and relies heavily on its gills for oxygen when water is available.
Bichirs, predatory fish native to freshwater systems in Africa, possess ventral and paired lungs resembling those of terrestrial vertebrates. They are facultative air-breathers, frequently rising to the surface to gulp air, especially in stagnant or poorly oxygenated water. Their reliance on air-breathing increases with water temperature and decreasing oxygen levels, providing an adaptive advantage in challenging aquatic conditions.