An Overview of
Lahille's Dolphins
If you are familiar with the research done by the Learning from Whales team, you may already know that we have identified three separate populations of bottlenose dolphins off the East Coast: one offshore, one inshore, and one on the continental shelf. But did you know that this kind of genetic separation is a pattern we see in other bottlenose dolphin populations across the world?
In the Southwestern Atlantic Ocean along the coast of South America, a new subspecies has emerged that is genetically and morphologically distinct from its offshore counterparts. This inshore population has even divided into two separate groups, very similar to what we have discovered off the East Coast. The inshore dolphins living along the coast of South America are a separate subspecies called Lahille’s bottlenose dolphins.
It is important to understand the difference between genetic and morphological analysis. Genetic analysis involves looking at the DNA of the dolphins, such as by examining how the chromosomes are alike or different between individuals. Morphological analysis involves examining the physical characteristics of the dolphins, such as skull shape, color, or body size.
I created the figure to the left to help visualize where Lahille’s dolphins are located. The red and blue colors both represent Lahille’s dolphins, with one subpopulation along the coast of Brazil and Uruguay and the other along the coast of Argentina. The yellow color represents the offshore population of dolphins that are genetically and morphologically distinct from Lahille’s dolphins.
Genetic Differences
Lahille’s bottlenose dolphins, Tursiops truncatus gephyreus, are inshore dolphins that live along the east coast of South America. They are genetically different from the offshore common bottlenose dolphins, Tursiops truncatus. A number of research papers written on these dolphins show that the genetic diversity of inshore populations are much lower than offshore populations, indicating that Lahille’s dolphins are confined to a specific region with limited gene flow and have therefore developed into a separate subspecies with a separate evolutionary trajectory.
Remember the two subpopulations of Lahill's dolphins you saw in the map above? Genetic evidence shows that these two groups are also diverging. The subpopulation along the coast of Argentina appears to have more genetic diversity than the subpopulation along the coast of Brazil and Uruguay.
A 2015 paper used biopsy and tissue samples from bottlenose dolphins to analyze genetic population structure. Figure 1, right, shows that offshore dolphins (cluster 3) have a higher mitochondrial haplotype diversity and nucleotide diversity than inshore dolphins (clusters 1 and 2), which supports the conclusion that Lahille’s dolphins are genetically distinct from their offshore counterparts.
This table also supports the claim that the two clusters represent genetically distinct subpopulations. Since there are more different haplotypes for cluster 1 than cluster 2, there is a higher haplotype diversity and nucleotide diversity in the population along the coast of Argentina than in the population along the coasts of Brazil and Uruguay. The two different levels of haplotype diversity indicate two genetically distinct subpopulations.
The image to the left is a haplotype network from the same paper. A haplotype is a group of DNA variants that are inherited together on one chromosome. The haplotype network shows how frequently dolphins of each cluster displayed a certain haplotype. The hash marks represent nucleotide differences between haplotypes, so circles that are closer together have fewer differences between them and are therefore more closely related. This haplotype network shows that clusters 1 and 2, the inshore groups, are more closely related to each other than they are to cluster 3, the offshore group. This supports the claim that Lahille’s dolphins are their own genetically distinct subspecies. It’s also important to note that although the circles for cluster 1 and cluster 2 are closer together than they are to cluster 3, they are still two distinct circles separated by some significant nucleotide differences. This gives us more evidence that the dolphins on the coast of Argentina are genetically different than the dolphins along the coasts of Brazil and Uruguay!
Morphological Differences
There are also physical differences between Lahille’s dolphins and the offshore dolphins. Several research projects have looked into the differences in their skulls and skeletons and found that there are consistent differences in skull morphology between the inshore and offshore dolphin populations in the Southwestern Atlantic ocean. These findings show that Lahille’s dolphins have evolved and adapted to have different body shapes and sizes. But why did they develop these different skulls? Is it an adaptation to their coastal environment? Could it be because of the way they forage for food? This is something I hope to explore further through more literature research.
Another paper, published in 2016, examined skull differences between inshore and offshore dolphins. The image on the right depicts four skulls - A and C are from offshore dolphins while B and D are from inshore dolphins, also known as Lahille’s dolphins. The top half of the image points out the skull vertex, or the roof of the skull. You can see from the black arrows that the offshore dolphin’s skull vertex (left) is longer than the inshore dolphin’s skull vertex (right). The bottom half of the image compares the angles of the pterygoid hamulus, which is an extension of the pterygoid bone in the skull. The white arrows show that the offshore skull’s pterygoid hamulus (left) is flattened at the bottom, while in the inshore skull (right) it has more of a sharper angle. These two differences were key characteristics that could reliably distinguish Lahille’s dolphins from offshore dolphins.
A third paper, also published in 2016, contained this image of an offshore dolphin skull on the right and an inshore dolphin skull on the left. At a glance, it is possible to see some noticeable differences between the two skulls! The inshore dolphin skull has a longer rostrum, or beak, and the two bone plates that make up the rostrum are joined together, while they are separated in the offshore skull. The skulls have different shapes and a number of other differences, which were largely consistent between the two ecotypes. The fact that skull morphology is consistently different between Lahille’s dolphins and offshore dolphins supports the claim that Lahille’s are a distinct subspecies.
How it all ties together
My research on Lahille’s dolphins has improved my understanding of bottlenose dolphin populations on the U.S. East Coast. I learned a lot about the genetic and morphological distinctions between inshore and offshore populations, the divergence between inshore populations, and the techniques of genetic analysis that these studies use. I hope to put this new knowledge to use by continuing our subteam’s exploration of the population structure of U.S. East Coast bottlenose dolphins. We know that there are genetic distinctions between the three different populations, but what about morphological distinctions? Are there ways to tell which population the dolphins belong to just by looking at them? This is something I plan to explore by examining the photo IDs of dolphins along the East Coast. The more we know about our dolphin populations, the better we can study and protect these amazing animals.
SOURCES
Costa, A.P.B., Fruet, P.F., Daura-Jorge, F.G., Simões-Lopes, P.C., Ott, P.H., Valiati, V.H., Oliveira, L.R. 2015. Bottlenose dolphin communities from the southern Brazilian coast: do they exchange genes or are they just neighbours? Marine and Freshwater Research 66: 1201-1210.
Costa, A.P.B., Rosel, P.E., Daura‐Jorge, F.G. and Simões‐Lopes, P.C. 2016. Offshore and coastal common bottlenose dolphins of the western South Atlantic face‐to‐face: What the skull and the spine can tell us. Marine Mammal Science 32: 1433-1457.
Fruet, P. F., Secchi, E. R., Di Tullio, J. C., Simões-Lopes, P. C., Daura-Jorge, F., Costa, A. P. B., Vermeulen, E., Flores, P. A. C., Genoves, R. C., Laporta, P., Beheregaray, L. B., & Möller, L. M. (2017). Genetic divergence between two phenotypically distinct bottlenose dolphin ecotypes suggests separate evolutionary trajectories. Ecology and evolution, 7(21), 9131–9143. https://doi.org/10.1002/ece3.3335
Vermeulen, E., Fruet, P., Costa, A., Coscarella, M. & Laporta, P. 2019. Tursiops truncatus ssp. gephyreus. The IUCN Red List of Threatened Species 2019: e.T134822416A135190824. http://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T134822416A135190824.en
Wickert, J.C., von Eye, S.M., Oliveira, L.R., Moreno, I.B. 2016. Revalidation of Tursiops gephyreus Lahille, 1908 (Cetartiodactyla: Delphinidae) from the southwestern Atlantic Ocean. Journal of Mammalogy 97(6): 1728-1737.