Hammering Away

Hammerhead sharks (Family Sphyrnidae) get their name, and are easily recognizable, due to their uniquely shaped heads which are laterally expanded and dorsal-ventrally compressed (the cephalofoil) with an eye at each end. The group itself is approximately 20 million years old, and after its initial evolution it underwent divergent evolution producing species of various head and body sizes and head shapes (much of this evolution occuring within the last 6 million years). Body sizes range from as small as 3 feet (just under 1 meter) to the largest, the great hammerhead (Syphyrna mokarran), at 18 feet (about 5.5 meters). The bonnethead shark (Sphyrna tiburo, from Caribbean and tropical eastern Pacific Ocean) has the least laterally expanded head (18% of the body length) while the winghead shark (Eusphyra blochii, from Australia) has the most laterally expanded head (50% of the body length). These two species also have the most divergent cephalofoil shape as well as some of the smallest body sizes at maturity.

There are various hypotheses as to the function of the cephalofoil. One popular hypothesis is that the wing-shaped head provides lift and greater maneuverability in the water. Sharks are also known for their electroreception abilities, and it is speculated that the lateral head extensions enhance this capacity, making prey detection and capture more efficient.

A new paper published in Molecular Phylogenetics and Evolution analyzes mitrochondrial and nuclear DNA to infer the phylogeny for all species in the group. They took DNA from each of the eight hammerhead species to construct a new phylogenetic structure ("family tree" or "gene tree") for the group. The researchers used four mitochondrial genes and three nuclear genes (amounting to 6292 total base pairs in the study). Mitochondrial DNA is maternally inherited and nuclear DNA is from both parents, and these DNA types undergo differing mutation rates. By tracking the mutations in the genes, researchers can look at evolution of a species/group over time. Using this technique, they found that large (>200cm) and small (<150cm)>

Why the evolution of small body sizes? One reason may be in their development, specifically progenesis or neoteny (adults retaining juvenile characteristics). A smaller body size means that the cephalofoils may not provide as much lift in the water, but, that's ok if you develop gain other functions such as enhanced binocular vision, prey capture, and/or maneuverability. Also, all small-bodied sharks are restricted to continental shelf habitats while large-bodied sharks tend to be pelagic and circum-globally distributed. This could reflect their evolutionary origins, as increased size allows for trans-oceanic movement and colonization - either as small sharks evolving big, moving, then evolving some lineages small again or a large widely distributed shark evolved into smaller species.

So what does all this boil down to?

"The new phylogenetic hypothesis does not challenge the existing classification and taxonomy of the family Sphyrnidae. Nonetheless, we note that proposed subgenera remain paraphyletic. Continued recognition of two distinct genera (Eusphyra and Sphyrna) makes sense given the monophyly of the genus Sphyrna and the degree of divergence between Eusphyra and Sphyrna. If there is a need for subgeneric taxonomic categorization, we advocate using the inferred phylogeny as a guide for defining monophyletic subgenera."

That's just how I would have said it :o)


Here's the paper:
Kim, Douglas D., Philip Motta, Kyle Mara, Andrew P. Martin. (2010) Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution: 55(2), 572, (DOI: 10.1016/j.ympev.2010.01.037)

Here's a write up (but reading the above paper is actually easier and more informative):
http://www.sciencedaily.com/releases/2010/05/100518113132.htm


(images from livescience.com and arkive.com via seapics.com respecitvely)