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Walking sharks: evolution in action?

by

29 September 2006

Scientists have recently discovered over 50 new aquatic species in a coral reef off the coast of Indonesian Papua.1  Among the species discovered was a new bottom-dwelling epaulette shark species (Hemiscyllium sp.) that appears to ‘walk’ along the seafloor when searching for food.  Footage of this shark was taken and has been splashed around as evidence of fish evolving into land-dwellers.

The mechanics of walking

Photo: ©iStockphoto.com/Paul Johnson

Epaulette Shark

Hemiscyllium sp.

However, scientists have long known that epaulette sharks have the ability to use their pectoral fins for aquatic ‘walking’ along the sea floor.2 The problem for using this in evolutionary apologetics is that the underwater ‘walking’ seen in these sharks is unlike anything that occurs on land.  Epaulette sharks studied previously have shown little difference in structure to the pectoral fins of other sharks. Other than size differences in the same muscles in the fins, only one other muscle not found in most other sharks has been observed.3

Finding aquatic ‘walking’ sharks is not the issue; the transition to the rigours of walking on land is the real problem.  Recent studies have shown that even creatures that move both on land and underwater using legs have very different styles of locomotion for when they are either in water or on land.4 Because of water’s viscosity and density, it provides a completely different medium for walking as opposed to air.5 In air, walking requires both a method of propulsion and a method for the creature to support itself. However, support is less of an issue in water because of most organisms’ natural buoyancy in water. The water substantially supports even epaulette sharks, which are slightly denser than water.6 This greatly reduces the effect of gravity, and gives them a much smaller effective weight than what they have on land.7  Water is more viscous than air, which therefore gives much more resistance to movement. This tends to provide greater stability when walking in water.7 This requires very different designs for walking on land as opposed to water.

A fish out of water

‘walking’ sharks are not good candidates to benefit the evolutionary fairytale because they’re knights with the wrong kind of armour; cartilaginous ‘armour’ just doesn’t cut it

Another problem is the sort of fish this is: a shark. Sharks are members of the fish class Chondrichthyes, which are cartilaginous fish. Their skeletons are made out of cartilage. Land dwelling vertebrates, however, are supposed to have evolved from a common ancestor with the class Osteichthyes, which are fish with bony skeletons. Therefore, ‘walking’ sharks are not good candidates to benefit the evolutionary fairytale because they’re knights with the wrong kind of armour; cartilaginous ‘armour’ just doesn’t cut it. Lest it be said that there is no connection between one transition to land and one that is now developing in sharks, an associated point is that a bony skeleton would be needed to support the body on limbs on land.

Evolution presents other problems. Considering the amount of new information that would need to be added randomly (filtered by selection) to the shark’s genome for it to be able to walk on land, it borders on the ludicrous.  No randomly occurring, information-gaining mutations such as would be needed to add this level of functional complexity have ever been observed, which is a must for evolution to work. The shark needs them in droves. Of course, this needs to happen more than once independently. Invertebrates had to do it, bony fish had to do it, and now sharks are doing it too? There are so many problems with fish-to-frog evolution already, without the notion that sharks are heading in the same direction. In any case, evolution is not supposed to have any foresight, direction or purpose.

So what could this fin walking be for? Numerous suggestions have been offered, such as giving them an advantage in catching their bottom-dwelling prey (crabs, snails, small fish),7 and allowing for easier traversal of their ‘structurally complex habitat’: coral reefs.8 There is no need to suppose that they evolved to be like this. Rather, they are well designed to suit their specific environment, which is exactly what one would expect starting from the Bible.

Strolling to a solution

This ‘walking’ shark episode comes not long after the recently publicized Tiktaalik fossil was paraded in Nature as the ‘missing link’ of tetrapod evolution.9  However, even Tiktaalik fails to fill the crucial gap between sea and land locomotion (see Tiktaalik—a fishy ‘missing link’). The conclusions of Azizi and Horton on the differences between aquatic ‘walking’ in cartilaginous fish and ‘early’ tetrapods should be well noted:

‘If in fact aquatic walking was the primitive mode of locomotion in the earliest tetrapods, then why are the limb elements and girdles of Acanthostega and Ichthyostega [two of the supposed earliest tetrapods] so robust? The robust limbs and girdles of these early tetrapods seem capable of bearing mechanical loads which far exceed those associated with aquatic walking.’10

Evolutionary storytelling is not needed to explain the existence of this rather peculiar shark, and it adds nothing to our understanding of this shark’s biology. Nor does the observation of such underwater ‘walking’ provide any evidence for fish-to-frog evolution. Even octopuses have been observed ‘walking’ on two of their ‘legs’, but this is hardly the precursor of bipedalism on land! Rather, the Word of God provides a sound basis from which to understand more of the epaulette shark’s design features, which testify to the intelligence and creativity of the God of the Bible.

Related Articles

References

  1. Roach, J., ‘Walking’ sharks among 50 new species found in Indonesia reefs, National Geographic, 18 September 2006. For the actual footage of the ‘walking’ shark, see news.nationalgeographic.com/news/2006/09/060919-shark-walk-video.html. Return to Text
  2. See e.g. Pridmore, P.A., Submerged walking in the epaulette shark Hemiscyllium ocellatum (Hemiscyllidae) and its implications for locomotion in rhipidistian fishes and early tetrapods, Zoology: Analysis of Complex Systems 98, 278–297, 1995. Return to Text
  3. Lucifora, L.O. and Vassallo, A.I., Walking in skates (Chondrichthyes, Rajidae): anatomy, behaviour and analogies to tetrapod locomotion, Biological Journal of the Linnean Society 77:35–41, 2002.Return to Text
  4. Martinez, M.M., Full, R.J. and Koehl, M.A.R., Underwater punting by an intertidal crab: a novel gait revealed by the kinematics of pedestrian locomotion in air versus water, The Journal of Experimental Biology 201:2609–2623, 1998. Return to Text
  5. Azizi, E. and Horton, J.M., Patterns of axial and appendicular movements during aquatic walking in the salamander Siren lacertian, Zoology 107:111–120, 2004. Return to Text
  6. Lucifora, L.O. and Vassallo, ref. 3, p. 35. Return to Text
  7. Sebastian Troeng, director of Regional Marine Strategies at Conservation International, said concerning the sharks: ‘They are bottom-dwellers which feed on crustaceans such as crabs and snails as well as small fish, and being able to walk may give them an advantage in catching them.’ Bhat, D., Shark that ‘walks’ discovered in Papua, The Australian, 19 September 2006. Return to Text
  8. Lucifora, L.O. and Vassallo, ref. 3, p. 40. Return to Text
  9. Shubin, N.H., Daeschler, E.B. and Jenkins, F.A., Jr, The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb, Nature 440(7085):764-771, 6 April 2006. Return to Text
  10. Azizi and Horton, ref. 4, p. 119. Return to Text

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