A study on an African fish called the Senegal bichir (Polypterus senegalus) led by Dr Hans Larsson of McGill University shows what might have happened about 400 million years ago, when marine vertebrates first tried to ‘walk’ out of the water.
The Senegal bichir, Polypterus senegalus. Image credit: © Cam Gillman, via cichlids.com.
The Senegal bichir, sometimes called the dinosaur eel or the dragon fish, is a species of freshwater fish found in the marginal swamps and freshwater lagoons of Nile basin and West Africa.
Unlike most fish, this species doesn’t possess a swim bladder – the internal gas-filled sac which aids buoyancy – instead using a primitive set of lungs to gain oxygen. This means it occasionally rises to the river surface to gulp air.
Amazingly, as long as its skin remains moist, the Senegal bichir can remain alive out of water for long periods of time.
Dr Larsson and his colleagues raised juvenile Senegal bichir on land for nearly a year, with an aim to revealing how ancient fish may have used their fins in a terrestrial environment.
According to Dr Larsson’s paper published in the journal Nature, the ‘terrestrialized’ Senegal bichir showed significant anatomical and behavioral changes. It walked more effectively by placing their fins closer to their bodies, lifted their heads higher, and kept their fins from slipping as much as fish that were raised in water.
“Anatomically, their pectoral skeleton changed to become more elongate with stronger attachments across their chest, possibly to increase support during walking, and a reduced contact with the skull to potentially allow greater head/neck motion,” said study co-author Trina Du of McGill University.
Dr Larsson added: “because many of the anatomical changes mirror the fossil record, we can hypothesize that the behavioral changes we see also reflect what may have occurred when fossil fish first walked with their fins on land.”
“This is the first example we know of that demonstrates developmental plasticity may have facilitated a large-scale evolutionary transition, by first accessing new anatomies and behaviors that could later be genetically fixed by natural selection.”
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Emily M. Standen et al. 2014. Developmental plasticity and the origin of tetrapods. Nature, published online August 27, 2014; doi: 10.1038/nature13708
