Fish breathe and swim in water. Everybody but the lungfish knows this. To the naked eye, the lungfish is like any ordinary fish until it takes a big gulp of air and “walks” instead of swims. With six species in Africa, Australia, and South America, the lungfish can grow to seven feet and weigh twenty-two pounds. Due to its primitive respiratory and limb system, the lungfish represents an evolutionary link that bridges life on land and in water.
In Season 1, Episode 2 of Life in Cold Blood, Attenborough details the muscular, limb-like pelvic fins of the lungfish. Unlike ray-finned fish, lungfish can support themselves on the sediment with a “foot” created by their pelvic fins [1]. By moving their pelvic fins and body in an alternating, rhythmic pattern, lungfish can physically lift themselves up and “walk” on a solid surface [2]. This creates a bipedal gait similar to modern tetrapods like lizards and salamanders [2]. It is this primitive limb and locomotion system that allowed later vertebrates to transition from water to land.
In addition to a bipedal walk, lungfish are defined by a dual respiratory system that can breathe in both water and air. Appearing as paired structures behind the gills, lungs likely evolved within lungfish as a means of accessing atmospheric oxygen in low oxygen, aquatic environments [3]. While most fish swim to the surface and breathe through their mouths, the lungfish breathe through their nasal openings [4]. Their lungs also serve as a modified swim bladder that provide buoyancy. This added buoyancy allows lungfish to support themselves in water and makes it easier for them to walk on their fins [2]. Despite their ability to breathe atmospheric oxygen, the lungfish is still dependent on water and can use both gills and lungs interchangeably [5]. Oxygen is primarily absorbed through the lungs while carbon dioxide is excreted by the gills and skin [6]. During dry spells, lungfish swim to the water’s surface where they breathe air for long periods of time. When drought conditions are extreme, lungfish can burrow themselves into the mud and hibernate until water levels return to normal conditions [5].
Lungfish represent a hypocritical duality in nature. Lungfish can drown if they don’t breathe air and they can’t “exhale” unless they are placed in water. While lungfish have intermediate forms for life in water and on land, they don’t suit either particularly well. The lungfish could be considered nature’s astronaut as it took a small step for fish and a big step for evolution, pioneering the path for life on land.
by Jimmy Nguyen and Joseph Lee
Life in Cold Blood Season 1, Episode 2 (Land Invaders), starting at 2:48
References
- King, HM & ME Hale. 2014. Musculoskeletal morphology of the pelvis and pelvic fins in the lungfish Protopterus annectens. Journal of Morphology 275(4): 431-41
- King HM, Shubin NH, Coates MI, & ME Hale. 2011. Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. Proceedings of the National Academy of Sciences of the United States of America 108(52): 21146-21151
- Daniels CB, Orgeig S, Sullivan LC, Ling N, Bennett MB, Schurch S, Val Al, & CJ Brauner. 2004. The origin and evolution of the surfactant system in fish: insights into the evolution of lungs and swim bladders. Physiological and Biochemical Zoology 77(5): 732-749
- Nakamuta S, Nakamuta N, Taniguchi K, & K Taniguchi. 2013. Localization of the primordial vomeronasal organ and its relationship to the associated gland in lungfish. Journal of Anatomy 222(4): 481-485
- Tota B, Amelio D, Cerra MC, & F Garofalo. 2018. The morphological and functional significance of the NOS/NO system in the respiratory, osmoregulatory, and contractile organs of the African lungfish. Acta Histochemica 120(7):654-666
- Johansen K & C Lenfant. 1968. Respiration in the African lungfish Protopterus Aethiopicus. Journal of Experimental Biology 49: 437-452
The Evolution of Planet Earth 