One could organize a global kindergarten contest for children to come up with the most imaginative body plan for a shark, and yet, they will never come up with anything closer to the unique design mother nature had prepared for the scalloped hammerhead shark (Sphyrna lewini). This almost ‘Picassian’ creation, not only gives them the look of a swimming piece of art, but after >400 million years of evolution towards underwater perfection (yes, sharks are older than trees!), scalloped hammerheads are one of the most evolved of the sharkies.
Scalloped Hammerhead Sharks: A perfect Picassian design of nature. Photo credit: Pelayo Salinas.
And it’s not just a fancy design with no functionality: the shape of their cephalophoil – the technical term for the hammer shaped head from the Greek cephalo (head) and foil (wing/foil) – it is believed not only to improve maneuverability, but also to maximize their electroreception capabilities (aka the sixth sense of sharks). This ‘superpower’ allows them to detect the faintest electrical pulse – a very useful skill, when for example chasing squid for dinner in total darkness at depths of >1000 m – but also for the reading of earth’s magnetic field. And it is believed that hammerheads are capable of map-like use of earth’s magnetic field to guide their migrations across thousands of kilometers of big blue ocean.
Close up of a Scalloped Hammerhead cephalofoil. The Ampullae of Lorenzini, electroreceptors able to detect electric fields, are prominent around the outer edge of their hammer shaped head. Photo credit: Pelayo Salinas.
Despite their awesomeness, scalloped hammerheads are listed as Critically Endangered on the IUCN Red List, due to a >80% global population decline in just three generation lengths, fishing being the main cause of their decline. Given their extreme sensory capabilities, scalloped hammerheads are one of the most sensitive and shyest of sharks. Believe me! This is a big problem when you’re trying to study them - to fill in some of the most basic knowledge gaps on their life history. We know pretty much nothing for most of the >500 species of sharks described globally, including the most basic questions on their reproductive biology. This is key knowledge to have to inform policies aimed at recovering global scalloped hammerhead populations.
The shyness of hammerheads towards noise, and the curtain of bubbles generated by SCUBA dive equipment in particular, do not help when you need to get close to deploy a towed satellite tag – sort of fancy (and very expensive) underwater Airtag - designed to track shark movements in (almost) real time as they conduct long-range movements.
Here is where the stealthiness of close circuit rebreathers (CCR), another fancy piece of dive gear that recycles the oxygen our metabolism does not consume on each inspiration, and filters out the CO2 we exhale in a closed loop, so no bubbles are exhaled. This allows us to approach these extremely shy scalloped hammerheads as stealthily as possible. For tagging, we combine the use of rebreathers with specially modified spear guns to externally attach satellite tags using titanium darts. This procedure has become key for us over the past five years to track the movements of these oceanic wonderers. Patience during long-dives waiting for the right individual – we aim for large (>2,5 m in length) and ‘round’ females in their latest stages of pregnancy – and the invaluable collaboration of cleaning fishes, are another two key components of the scalloped hammerhead satellite tagging equation.
The waiting game: Dr. Pelayo Salinas preparing to deploy a satellite tag on a scalloped hammerhead shark. Photo credit: Javier Mahauad/Galapagos Rebreathers.
For satellite tagging dives, we literally ‘park’ ourselves for 4-5 hours nearby cleaning stations - where hammerheads stop by to request a fish spa treatment. When the little reef fish – such as the Mexican Hogfish, the King Angelfish or the Black Nosed Butterflyfish - jump on the sharky to remove parasites and old skin, scalloped hammerheads enter sort of a trance state where they almost stall, giving us the short window of time we have been waiting for, to closely approach and deploy the satellite tag before the spa treatment is over and they return to their normal hammerhead lives.
A school of Scalloped Hammerhead Sharks visiting a cleaning station run by Black Nosed Butterflyfish. Photo credit: Javier Mahauad/Galapagos Rebreathers.
More than five years of satellite tracking data on pregnant hammerheads has allowed us to document for the first time scalloped hammerhead birthing migrations from aggregation sites around the oceanic islands of the Galapagos in Ecuador and Coco in Costa Rica to nursery areas located on the Pacific coast of central and south America.
Satellite track of pregnant hammerhead shark ‘Alicia’ connecting the Galapagos Islands where she was tagged, with coastal nurseries in Panama and international waters to the west of the Galapagos. Obtained from Salinas-de-León, P., Vaudo, J., Suarez-Moncada, J. and Shivji, M., 2025. Long-distance movements of a Scalloped Hammerhead Shark connect the Galapagos Islands with coastal areas and international waters of the Tropical Eastern Pacific. Environmental Biology of Fish, 108(8), pp.1287-1293.
During our long dives to deploy satellite tags safety is vital, so being able to keep an eye on the upcoming hammerhead traffic towards the cleaning station, while at the same time keeping another eye on the partial pressure of oxygen in my rebretaher loop and other dive parameters through my Shearwater NERD 2 (Near Eye Remote Display, a heads up display on the edge of my dive mask) is extremely valuable. The Shearwater NERD2, and brothers Javier and Eduardo Mahahuad of Galapagos Rebreathers, are my trusty ‘guardian angels’ during our stealthy dives to help protect one of the most myterious and shyest of the sharkies.
Shearwater NERD 2 head up display. Photo credit: Javier Mahauad/Galapagos Rebreathers.
---
By Dr. Pelayo Salinas de León
Dr. Pelayo Salinas de León is a marine biologist, explorer, and underwater photographer focused on shark conservation and ocean protection. He holds a PhD in marine biology and a Master’s in environmental management. With over 80 scientific publications, his work combines research, expeditions, and science communication to support the protection of remote and biodiverse marine ecosystems.
Instagram: https://www.instagram.com/pelayosalinas/
Google Scholar: https://scholar.google.com/citations?user=Xy9Cf6IAAAAJ&hl=en
Researchgate: https://www.researchgate.net/profile/Pelayo-Salinas-De-Leon
