|Evolution in the News - September 2014|
|by Do-While Jones|
Have scientists really discovered how fish evolved into land animals?
One reader alerted us to this shocking statement in The Australian.
THE emergence of fish on to land was a pivotal transition in evolution. Now scientists have attempted to re-create it in a remarkable experiment, in which 100 fish were reared from birth on land. The study used an African fish, the polypterus, that has lungs and can “walk” between pools, but normally spends its entire life in water. The creatures survived “happily” out of water for nearly a year, suggesting ancient marine creatures may have made the leap to land more readily than previously thought. 1
Another reader told us about the news report in Science Daily about the same study.
About 400 million years ago a group of fish began exploring land and evolved into tetrapods – today's amphibians, reptiles, birds, and mammals. But just how these ancient fish used their fishy bodies and fins in a terrestrial environment and what evolutionary processes were at play remain scientific mysteries.
The fish showed significant anatomical and behavioural changes. The terrestrialized fish 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 became 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," says Trina Du, a McGill Ph.D. student and study collaborator.
"Because many of the anatomical changes mirror the fossil record, we can hypothesize that the behavioural changes we see also reflect what may have occurred when fossil fish first walked with their fins on land," says Hans Larsson, Canada Research Chair in Macroevolution at McGill and an Associate Professor at the Redpath Museum. 2
These two stories seem to indicate that experiments had been done in which fish evolved the ability to live on land, so we had to check out the peer-reviewed study in the professional technical literature ourselves. We discovered that the news reports weren’t exactly accurate.
To be politically correct, the study assures us that “Animals were acquired through the pet trade (Mirdo Importations),” and “All experiments were conducted under Carleton University animal care protocol B09-28 and McGill University animal care protocol #6000.” Now that we are comfortable knowing that the poor little fish were humanely studied, let’s read what the scientists did to them.
Because this study addresses how environment influences growth, animals were acquired as young as possible, but after gill absorption to ensure survival during shipping. All animals possessed juvenile markings on arrival, suggesting that they were less than 70 days old. 3
Before arrival, all fish had been raised in fully aquatic environments (n = 149). Although most fish still exhibited strong juvenile markings upon arrival, any fish that had fainter stripes were assumed to be older and were left in the aquatic control group, as they had been in an aquatic environment from their beginning.
All fish were kept in a 300-gallon recirculating aquarium system that provided the control and treatment groups with identical water quality conditions over time. The water was kept at 78 ± 2 °F and cleaned with an active bio-filter. Fish were fed a high protein diet, and both groups received the same amount of food daily. Control animals were kept in an aquatic environment with a water depth maintained at 210 mm [about 8 inches]. Treatment group animals were raised in a terrestrial environment (water depth 3 mm [about 1/8 inch]). Water misters provided a continual mist in the terrestrial environment to prevent desiccation [drying out]. The terrestrial environments also had a mesh flooring scattered with pebbles to stimulate climbing and navigating non-uniform surfaces and to provide habitat complexity, reducing negative fish interactions. The terrestrial and aquatic environments had plants to provide habitat complexity. 4
So, in the “remarkable experiment, in which 100 fish were reared from birth on land” (according to The Australian), all 149 fish used in the study were actually born and raised in the water for less than 70 days before being purchased for the study. During the study (which began after they had grown sufficiently to breathe air), 111 of them were raised in water too shallow to swim in (“on land”), and 38 were raised in deeper water.
It is important to realize that the fish they purchased were the offspring of fish that were already able to breathe air and move (with difficulty) on land (if the terrain isn’t too challenging). The experiment did not cause the fish to evolve the ability to breathe air or walk on land. Their ancestors had been doing that for generations. No evolution took place. If they had taken ordinary goldfish and tried to raise them in such a shallow aquarium that they could not swim, and had learned how to walk and breathe air, that would have been newsworthy!
Getting back to the article, they purchased fish and let them grow for an unspecified period of time, 111 in very shallow water, and 38 in deeper water. (If I had been a peer reviewer, I would have asked them to specify the length of time they were allowed to grow in the two different environments before testing them. That’s an important fact that was omitted from the report.)
They picked 20 “volunteers” from the shallow tank, and 10 from the deep tank, to compete in a land obstacle course, and the results were filmed.
P. senegalus (control, n = 10; treatment, n = 20) was filmed walking freely across a rough plastic surface (30 cm × 30 cm) to assess the kinematic performance of body and fin motions during terrestrial locomotion. The filming area (~22 cm × 20 cm) was located in the centre of the walking surface. Sequences in which fish walked steadily, demonstrating a minimum of three consecutive fin beats, were selected for analysis (control, n = 6; treatment, n = 12). Fish were randomly removed from their rearing tanks and, to avoid exhaustion artefacts, each fish was only walked once and then returned to a holding tank until the experiment was completed. Cameras were situated above the walking platform, with one directly above the surface and one at an angle, to provide clear views of at least one pectoral fin during walking. 5
The researchers were more concerned about exactly how they moved their fins than which fish completed the obstacle course first—but they did say,
… the terrestrialized Polypterus has a more efficient gait.
These differences in timing between the control and treatment fish groups may indicate that the fish raised on land have a conditioned ‘training’ advantage. 6
These performance differences suggest that walking is energetically more expensive than swimming. 7
In other words, fish find it easier to swim than walk. Who would have guessed that?
Then, they dissected some of the fish from each group to compare their anatomy.
Fish from the treatment and control groups were randomly chosen and sacrificed. 8
The report is filled with tables, charts, photographs, and diagrams comparing the fin motion and anatomy of both groups of fish. The point of all that data seems to be that the fish raised in shallow water were more muscular than the control group that just floated around in the deeper aquarium.
The three simple lessons to be learned from this experiment are:
The fact that strong fish raised in shallow water for a long period of time can move better on land than weak fish that have never tried to do that before is not surprising. In a race, always bet on the strong, experienced competitor rather than the weak novice.
What would have happened if the fish had competed on an aquatic obstacle course? Would the strong fish that hadn’t been in water deep enough to swim in for most of their lives beat the weaker fish that had spent their entire lives swimming? Of course, that single test would not prove anything more than a single baseball game would answer the classic question, “Does good pitching beat good hitting?”—but at least it would have been interesting!
It isn’t surprising that fish raised in shallow water would beat fish raised in deep water on a shallow water obstacle course. But what would have happened if they had to swim against a strong current in a recirculating pool? They didn’t test that. They did, however, do this:
P. senegalus (control, n = 10; treatment, n = 20) was filmed swimming freely through a still water aquarium (1.5 feet wide × 6 feet long × 1 foot deep) to assess the kinematic performance of body and fin motion during steady swimming. 9
It was a pretty easy test for both groups. The result was:
The minimal differences in the kinematic variables between the treatment and control groups during swimming indicate that there was minimal ‘loss’ of swimming function associated with being raised in a terrestrial environment without the ability to ‘practise’ swimming after gill absorption. 10
Apparently, swimming (for fish) is like riding a bicycle (for people)—they never forget how to do it. But, how well do they do it after not doing it for a long time? Because they used still water instead of a strong current, we don’t know.
There were interesting aspects of the experiment. They should not have claimed they had learned something about evolution to make it interesting. It wasn’t about evolution. It was really a valuable study about instinct, learning, and exercise. But, because they focused on evolution, they didn’t learn all they could have.
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Hannah Devlin, The Australian, August 29, 2014, “Fish out of water can quickly find their feet”, http://m.theaustralian.com.au/news/world/fish-out-of-water-can-quickly-find-their-feet/story-fnb64oi6-1227040527711?nk=fad28d9161b24929422b6d27fec9324f#
2 Science Daily, August 27, 2014, “Walking fish reveal how our ancestors evolved onto land”, http://www.sciencedaily.com/releases/2014/08/140827131547.htm
3 Standen, et al., Nature, 4 September 2014, “Developmental plasticity and the origin of tetrapods”, http://www.nature.com/nature/journal/v513/n7516/full/nature13708.html