by Matthew Cobb
This video of a bamboo shark (Hemiscyllium halmahera) strolling across some coral was posted on Twitter by Kyle Hill (@sci_phile):
This oviparous species, which is about 70 cm long, has just been described by Dr Gerald Allen of the Western Australian Museum. For the shark-heads out there, the abstract of the paper says:
Hemiscyllium halmahera new species is described from two specimens, 656-681 mm TL, collected at Ternate, Halmahera, Indonesia. The new species is clearly differentiated on the basis of colour pattern. Its features include a general brown colouration with numerous clusters of mainly 2-3 dark polygonal spots, widely scattered white spots in the matrix between dark clusters, relatively few (< 10), large dark spots on the interorbital/snout region, a pair of large dark marks on the ventral surface of the head, and a fragmented post-cephalic mark consisting of a large U-shaped dark spot with a more or less continuous white margin on the lower half, followed by a vertical row of three, smaller clusters of 2-3 polygonal dark marks. The new species is most similar in general appearance to H. galei from Cenderawasih Bay, West Papua, which differs in having 7-8 large, horizontally elongate dark spots on the lower side between the abdomen and caudal-fin base, a cluster of solid dark post-cephalic spots, and usually about 25 dark spots on the upper surface of the head.
But the really interesting thing to me is the gait the fish is using – this is apparently typical of Hemiscyllium sharks, which prowl across coral looking for food. It is using the classic alternate movement of a tetrapod.
We’ve previously talked about the evolution of the tetrapod gait in lungfish. Now you are more closely related to Tiktaalik, the famous fish/tetrapod intermediate form, than Tiktaalik was to a shark (because you and I and Tiktaalik all have bones; sharks do not have bones, only cartilage).
So this suggests that the neuronal control of the way that you run (your right arm moves with your left leg, and your left arm moves with your right leg – try it) goes waaaayyyy back even beyond our fishy ancestors, to the time before the evolution of bone.
Another alternative is that this is convergent evolution – if you are going to ‘walk’, the alternate gait is the best way of doing it. Today’s question: How could we test between these two hypotheses?
I may be exaggerating the importance of this gait in sharks – are there any locomotion experts out there who can comment?
The film was made near Ternate, on the Malaku islands in Indonesia, by Mark Erdmann of wedaresort.