Most shark species are still poorly understood by scientists. Access to their habitats is rare, and observing their developing young is even more so. The images of hammerhead shark embryos (Sphyrna tiburo) captured by this team of biologists are thus exceptional, allowing for the first time the study of the curious development of their characteristic heads.
Developmental biology is a discipline that focuses on the mechanisms of organism growth to understand their diversity. One technique involves studying real-time development, often practiced on species that lay eggs, like some shark species. But how do you study the embryonic development of a fascinating shark species like the hammerhead, which does not lay eggs? A team of American researchers has succeeded in this, unveiling the mystery of the formation of their strange heads and sharing their unseen images.
“No sharks were sacrificed for our study”
Hammerhead sharks (Sphyrna tiburo) do not lay eggs; they are viviparousviviparous like humans. Each of the 16 embryosembryos grows in utero, connected to the mother by an umbilical cordumbilical cord. The newborns, averaging 40 centimeters, are fully autonomous and ready to survive on their own. But how do you observe a hammerhead shark embryo if it develops inside its mother’s womb?
A team of researchers from Florida obtained embryos from adult female sharks captured as part of population studies off the coasts of the Gulf and Atlantic of Florida. These females were among the few individuals that did not survive the marking process, typically preceding their release. They were intentionally preserved, along with their embryos – which would not have survived regardless – for scientific purposes. Gareth Fraser, the developmental biologist leading this study, states: “No shark was sacrificed for our study.” The researchers were able to study a total of 177 embryos and created a visual growth chart: a first for hammerhead sharks. These efforts allowed them to study crucial developmental stages, particularly the precise moment when the embryo develops the characteristic head shape.
Why This Hammer-Shaped Head?
This small shark from the western Atlantic is known as a “bonnethead” due to its relatively small and rounded “hammer,” butBelonging to a group of eight species, the hammerhead shark is a part of the Carcharhiniformes order, all sharing this distinct hammer-shaped head. Contrary to common belief, this shape is highly hydrodynamic, enabling these large predators to swiftly maneuver and pin down their prey. Scientists suggest that their head evolved into a wide, flattened shape to enhance their sensitivity: the positioning of their eyes on each side widens their field of vision, and the spread-out nasal capsules improve their olfactory abilities. Moreover, their “sixth sense” is also heightened, with the ampullae of Lorenzini – which allow them to detect the electromagnetic field – being distributed across the width of their head.
These groundbreaking studies once again demonstrate the ocean’s abundance of wonders, each adaptation more fascinating than the last. However, the opportunities to document them are at times very rare. Hence the famous saying “no science without patience”…