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A Fish That Walks


In the journal Scientific Reports, a new paper reports the discovery of a blind cavefish that walks up the walls of waterfalls. The fish is reported to show changes to its spine and pelvic girdle that would support a walking movement like that of salamanders. Its fins also project laterally, presumably to help it walk. The discovery, if true, is a big deal, because it adds to the story of how fish may have evolved in tetrapods (animals walking on four feet).

Karl Zimmer has picked up the tale at the New York Times, telling the traditional evolutionary story of how fish could have turned into tetrapods:

The early land vertebrates, known as tetrapods, evolved adaptations that enabled them to move efficiently over solid ground. A pelvis joined their hind limbs to their spines, for example. Their vertebrae grew flanges so that they interlocked, helping the spine hold itself stiff and straight even when being pulled down by gravity.

These adaptations led tetrapods to walk in a distinctive fashion, moving their forelegs and hind legs together in a cycle. Early tetrapods probably walked much the way salamanders do today, bending their trunk from side to side as they traveled.

But this is just a story, a tale told to fill in the gaps based on the before and after snapshots of living and fossil species. Maybe it's true, maybe not. But that's why some scientists are so excited.

The fish in question, Cryptotora thamicola, is rare and protected, so a living specimen had to be imaged at a dental lab and returned to the wild. The image of its skeleton was compared to that of a goldfish and amphibian, with significant differences reported.

A short gif showing the walking movement is available at the New York Times article. I could not tell if the fish was using its fins to push itself forward, because the view was from the top, and all I could see was a back and forth swaying. Perhaps the additional stiffness of the spine and pelvic girdle made the movement more effective. I don't know.

Some of the skeletal differences reflect those expected for a water to land transition. I'd like to compare it to other fish that move on land, like mudskippers. Does the modified pelvis resemble that of any primitive amphibian or putative transitional fossils like Tiktaalik? What about the structure of the fins? Tiktaalik is claimed to have a wrist, but see here. There is no sign of wrist or digit formation in Cryptotora, yet it has been believed that digit formation in the fins preceded the development of the pelvis.

The authors say:

A number of researchers have shown that it is possible to use the biomechanical knowledge of extant vertebrates to attempt to better understand the functional constraints that shaped the origin of tetrapod limbs. Even though fish bone is anosteocytic, it has been shown to remodel and reinforce skeletal structures in response to mechanical stress. This process is known to cause the formation of intermuscular bones in fishes and to increase pectoral girdle shape and thickness in polypterus, which walks on land using its pectoral fins8. Thus, the enlarged neuropophyses, ribs, and pelvic girdle of Cryptotora may have evolved as a result of mechanical stressors from climbing and walking. Developmental plasticity plays an important role in the appearance of complex heritable phenotypic traits and extreme environments, like those in caves, are catalysts for the evolution of novel traits. Standen et al. hypothesized that environmentally induced phenotypic plasticity may facilitate macroevolutionary change; however, our current data does not allow us discriminate between selection for a robust pelvic girdle specifically or for the plastic response to extreme environmental conditions.

Translation: the spine, ribs and pelvic girdle may have been an adaptation as a result of the stresses of climbing and walking. But wait. I thought the phenotypic change had to come first in order to be adaptive. Random mutation, leading to the development of the spine and pelvic girdle, would then fortuitously allowed the fish to walk, and allow for its selection as a beneficial trait or traits.

It's an intriguing find. Does this fish disprove intelligent design? No. There is nothing in the theory of intelligent design that necessarily rules out common descent, so this fish could be a relic of the transition to land, and it would not matter. That's if the finding holds up to scrutiny. What matters for intelligent design is that it has not been demonstrated that there is a stepwise selectable path from a fish ancestor to this, each step showing benefit and leading to this walking behavior and morphological change. Would a fish with only a stiff spine have any advantage? What about just an incipient pelvis? What about the fins that stick out? Or can anyone demonstrate that all these changes can be the result of one or two non-adaptive mutations, followed by adaptive ones?

Maybe it happened that way. Maybe not. Too soon to tell. Let's just not make it a "Big Fish" story.

Image credit: New Jersey Institute of Technology.