In the year 2000, Dr. William Driggers, now of the National Marine Fisheries Service’s Southeast Fisheries Science Center in Mississippi, was sampling for sharks in South Carolina. Dr Driggers recalls that “at the time I was collecting samples from various species of sharks for life history studies and also collecting tissues for Dr. [Joseph] Quattro’s genetics work.” Dr. Quattro, a professor at the Marine Science Program and Department of Biological Sciences at the University of South Carolina, had been working on a project to characterize the population genetics of fish in South Carolina by “working my way down river systems to the coast,” he told me. “Even sturgeons were showing population differentiation, so I thought the next animal would be marine, but estuarine dependent – sharks.” Analysis of the samples Dr. Driggers collected led to a surprising result.
“I was asked “what are the chances that I would misidentify a ‘scalloped hammerhead’ and answered that there was no chance as they are very morphologically distinctive (looks like I was wrong),” Dr. Driggers told me. “I was then informed that genetic sequences indicated that some of the specimens I had labeled as ‘scalloped hammerhead’ were distinctly different from known S. lewini sequences. At Dr. Quattro’s request, I began bringing back whole specimens so they could be archived and morphometric analyses conducted. The first whole specimen that was vouchered and shown to be the new species was collected in Bulls Bay in July of 2001.”
In 2006, Dr. Quattro and his team published a paper entitled “Genetic evidence of cryptic speciation within hammerhead sharks,” showing that there may be a previously-unknown species hiding within scalloped hammerheads. When genetic samples of scalloped hammerheads, great hammerheads, and bonnethead sharks were phylogenetically mapped, the team found an unexpected result. Dr. Quattro, told me that “while doing the population genetics of this animals, we found two divergent genetic lineages within what were morphologically scalloped hammerheads. We gathered sequences and specimens from other known species and didn’t find a match – that’s what got us on the whole cryptic species [defined by Bickford et al. 2007 as “two or more distinct species erroneously classified and hidden under one species name”] thing.”
Figure 1 from Quattro et al. 2006. This tree shows genetic differences between great hammerheads, bonnetheads, Atlantic scalloped hammerheads, and Indo-Pacific scalloped hammerheads. It also shows a potential cryptic species hiding within Western Atlantic (WA) scalloped hammerhead populations
Last week, the first full description of the new species was published in ZooTaxa. It will now be called the Carolina hammerhead, Sphryna gilberti, named after Florida Museum of Natural History curator Carter Gilbert, who first noted an unusual scalloped hammerhead specimen, now believed to be a Carolina hammerhead, in 1967. Dr. Quattro thinks “it’s cool that he would have mentioned that.”
The holotype of S. gilberti, collected in Bulls Bay, South Carolina
The Carolina hammerhead is extremely difficult to distinguish from S. lewini, the scalloped hammerhead shark. Dr. Quattro notes that “as of now, x-rays of the vertebral column are the most diagnostic feature.” Dr. Driggers aggrees, noting that “currently, the only difference between scalloped and Carolina hammerheads that we have been able to discern is in the number of precaudal vertebrae. In all, we examined 67 external characters (e.g. snout length, dorsal fin height, etc.) and ranges of measurements overlapped in all cases between the two species. ”
An x-ray image of the vertebral column of a Carolina hammerhead, courtesy Dr. Joseph Quattro
The discovery of a cryptic species within a species already known to be Endangered raises conservation concerns. “If cryptic diversity is widespread, then take known species biomass and start doing the division,” said Dr. Quattro. “If threatened or endangered status is an issue now, and there are cryptic species within those taxa, than by simple math, things are in far worse shape.”
Sonja Fordham, President of Shark Advocates International agrees:
“The fact that the new hammerhead is not readily distinguishable from others in the family poses some particular challenges for ongoing conservation efforts under US domestic population assessment and fisheries management, as well as under the ESA and CITES. These will needed to be worked out over time through the various associated deliberative processes. In the meantime, from a practical perspective, the Carolina hammerhead should be benefiting from new, stricter commercial and recreational fishing limits, specific to large hammerheads, that were imposed over the summer. As hammerheads are exceptionally sensitive and prone to die from the stress of capture, additional safeguards – such as fishing modifications that improve the chances of post-release survival – are essential for this species, as they are for its close kin.”
Dr. Driggers was delighted to have played a part in the discovery of a new species of hammerhead shark. “It was pretty exciting. I was pretty blown away that these sharks had been literally “right under our noses” during all the years I spent sampling in South Carolina,” he said. “We have just scratched the surface and know little else about the species other than it exists. It will be exciting to watch information on the distribution, life history and other aspects of the biology of Carolina hammerheads begin to take shape. If there is any truth to the competitive exclusion principle, we are in for some surprises.”
David, in regards to this shark, how did the scientists determine that this was a separate species rather than a morphology change due to genetics, like all those different morphological found in fruit flies?
The short answer is that the morphological and genetic differences are significant and consistent enough to be defined as separate species and not just plasticity within an existing species. The long answer is very math and theory heavy but basically the same.
@The Saipan The new species is found in the same place as one population of the species it looks like (S. lewini). However, if you look at the genetic variation of S. lewini vs the new species one thing stands out – all S. lewini from all over the world (including the Indo-Pacific) are more closely related to each other than any of them are to this group of individuals with a unique vertebral count. For a group of individuals to maintain a unique set of genetic and morphological traits compared to things they are found co-existing with is that they are not mating. In other words, if the new species was not a new species but simply a group of goofy looking S. lewini we would expect them to be mating with the S. lewini with which they co-exist. These matings would mean they would share genes. That isn’t happening. And just to be clear, this is usually done with housekeeping genes that have nothing to do with morphological traits. The tree above is labeled “CR” and likely indicates it is taken from the “control region” – a ubiquitous area of mitochondrial DNA that definitely has nothing to do with the number of vertebrae. I am sure there is another tree based on “nuclear DNA” – it would not be wise to base a new species claim on only mitochondrial DNA since it is only passed down from mum to offspring.