Why sharks have no bones: Borrell 2014

Several years ago
Nature published a report by Borrell 2014 titled, “Why sharks have no bones” referencing a report in the same issue by Venkatesh et al. 2014 featuring their work on the ‘elephant shark’, Callorhinchus milii (Fig. 1). This follows recent discussions and comments on cartilage and bone in sharks and bony fish respectively.

Callorhinchus is a chimaera, a ratfish and an elasmobranch,
but note (Fig. 1) the presence of a single gill opening covered by a broad operculum, otherwise found sturgeons, paddlefish and Osteichthys (other bony fish). The upper jaw is fused to the cranium, distinct from basal sharks, paddlefish and sturgeons, convergent with Osteichthys. That’s why they call it a chimaera!

Figure 1. Callorhinchus, the subject of the Venkatesh et al. 2014 study on cartilage in sharks, in vivo.
Figure 1. Callorhinchus, the subject of the Venkatesh et al. 2014 study on cartilage in sharks, in vivo.

According to Borrell,
“The DNA sequence of the elephant shark helps to explain why sharks have a cartilaginous skeleton and how humans and other vertebrates evolved acquired immunity.”

“Although scientists knew what genes were involved in bone formation, it wasn’t clear whether sharks had lost their bone-forming ability or just never had it in the first place. After all, sharks do make bone in their teeth and fin spines.”

“The sequence reveals that members of this group are missing a single gene family that regulates the process of turning cartilage into bone, and that a gene duplication event gave rise to the transformation in bony vertebrates.”

From the Venkatesh et al. 2014 text:
“All gene family members involved in bone formation were present, except the secretory calcium-binding phosphoprotein (SCPP) gene family.”

Other fish experts note in the article, “Antarctic icefish (Notothenioidei (Fig. 2), lost the ability to form bone over the course of evolution.”

Notothenia, the namesake for the icefish clade, has not been added to the LRT yet, but is clearly a relative of Coryphaena, the open seas mahi-mahi, a bony fish not related to chimaeras. We’ll look at that taxon soon.

From the Venkatesh et al. 2014 abstract:
“Our functional studies suggest that the lack of genes encoding secreted calcium-binding phosphoproteins in cartilaginous fishes explains the absence of bone in their endoskeleton.”

Figure 2. Cladogram from Venkatesh et al. 2015. Second frame shows repairs based on the LRT.
Figure 2. Cladogram from Venkatesh et al. 2015. Second frame shows repairs based on the LRT.

All this is interesting news,
but Venkatesh et al. was using a traditional and outdated cladogram (repaired in Fig. 2). Parts of the Venkatesh et al. phylogenetic context were invalid due to taxon exclusion. Paddlefish, sturgeons and other pertinent taxa are missing. With regard to the included taxa:

  1. Heterostracan head shields are either made of bone.
  2. Osteostracan head shields are made of bone.
  3. Placoderms had bony armor, but the vertebrae, braincase, fin supports and gill arches were all made of cartilage. In the LRT, catfish are living relatives and they have bony skeletons and some catfish have bony armor.
  4. Acanthodians had jaw bones are preformed in cartilage then ossified with mesoderm-derived bone. Though the parts of the jaws have a similar origin, the teeth are very different from those of modern fish histologically, lacking enamel and apparently were not replaced, so had to last for the life of the fish.

So sharks pretty much stand alone lacking bone in most of their skeleton.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

Among extinct sharks,
 (Fig. 3), the proximal outgroup to bony fish in the LRT, “seems to have had highly ossified cartilage making it more like solid bone. This has meant that impressions have been quite well preserved revealing the morphology of the living animal.” Reference here.

Traditional workers never linked
the short-snouted shark, Hybodus, to bony fish, but considered the high degree of ossification convergent. By adding taxa, Hybodus nests basal to bony fish, demonstrating homology (rather than convergence) in the re-acquisition of bone, likely by the reactivation of that one bone gene, That’s what we’re looking for: a gradual accumulation of traits modeling actual microevolutionary events.

Borrell B 2014. Why sharks have no bones. Nature online here
Venkatesh B et al. 2014. Elephant shark genome provides unique insights into gnathostome evolution. Nature 505:174–179.


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