Galapagos Whale Sharks Undergo Ultrasounds For First Time
This isn’t your ultrasound appointment.
For one, the doctors on call weren’t sure if the patient(s) would show up. While the Galápagos Islands are one of many global locations that are visited by whale sharks (Rhincodon typus) during the year, there was no guarantee they would show. But the signs were promising, with the filter-feeder’s usual food plentiful along the rocky outcrops. The second indication this wasn’t your normal ultrasound appointment is the procedure would be taking place underwater.
Not only that, but they were often working in strong currents and, although quite placid in nature, whale sharks are by nature “a lot faster than us, and emerge from the blue with little warning; we only get a short time to work with each animal,” says Marine Megafauna Foundation co-founder Simon J. Pierce, who is author on the paper and stresses the techniques in this research were also quite challenging to apply.
The third giveaway? Scientists weren’t even certain the whale sharks that showed up would be pregnant. But they had hope, as said Alex Hearn, director of conservation science at Turtle Island Restoration Network, and a team of scientists discovered back in 2015 that Darwin Island is the only known place in the world where apparently pregnant whale sharks are regularly seen: “It’s a mystery why they are here, but we believe they may use the Island as a pit stop before moving into the open ocean to give birth.” The Galapagos Whale Shark Project team from the Marine Megafauna Foundation also have huge expertise in this area, so Pierce says it was a great opportunity to test and refine their new techniques here.
An iconic species, there are still large gaps in our knowledge about these spotted gentle giants, with reproduction being one of them. “Whale sharks are a relatively well-studied shark, but ‘relative’ is still a key word here. Only one pregnant female, caught in a Taiwanese fishery way back in 1995, has ever been examined by scientists – and we don’t know where the babies live, either. It’s mostly juvenile male sharks that frequent coastal areas and tourism sites. Females, both juveniles and adults, probably live offshore, where they’re difficult to find, let alone study,” explains Pierce. “There are a few whale sharks in some large aquaria, such as the Okinawa Churaumi Aquarium and Georgia Aquarium, but no adult females. The Okinawa aquarium has a very sexually frustrated adult male whale shark, which matured in their facility, but females probably don’t become adults until they’re over 30 years old, so it might have to continue waiting for a while yet.”
Until that whale shark gets a sexually-mature female whale shark companion to better study reproduction in a captive setting, the team set out to study them in these remote islands. “The ultrasound unit is the size of a large briefcase, so it’s far from hydrodynamic. Rui, the lead author of the study, was using a ‘jetpack’ (actually an underwater propeller mounted on his air tank) to keep up with the sharks and scan them with the ultrasound wand,” says Pierce. “Kiyomi Murakumo has been examining underwater ultrasound imagery in captive sharks and rays in the Okinawa Churaumi Aquarium for years now, monitoring pregnancy in species from nurse sharks to manta rays, and she was able to discern the developing follicles in these greyscale images that prove that these female whale sharks are indeed adults – something that was previously only possible through dissections.” Collecting blood samples was also something that took a lot of trial and error to get right, too, since whale sharks have extremely thick skin. This meant the researchers couldn’t the shark’s actual veins, so they needed to try and draw blood from the vascularized tissue on their fins. “That generally only works on relaxed sharks, and requires a large dose of luck! We also have to avoid contamination from seawater, which has necessitated the development of a two-syringe system, with one creating an initial vacuum, enabling the second to draw pure blood.”
Twenty-two female whale sharks were assessing using the underwater ultrasonography and a novel blood-sampling technique, and found that despite the widely held assumption among researchers that the post-pelvic distention of large females is indicative of pregnancy, they found no evidence of embryos or egg cases using ultrasound. But follicles were confirmed in two female sharks, and six female sharks presented three different steroid hormones (estradiol, testosterone, and progesterone) in their blood plasma – both were in individuals measuring 36 – 39 ft (11-12 m) total length. Based on these results, the team infers that female whale sharks (TL>11 m) in this study were mature but not pregnant.
This is the first time these techniques have been applied in the wild and will revolutionize how researchers can further understand the physiology of this vulnerable marine species. “Shark biology studies used to be based on dissections. For endangered species like the whale shark, where we’re doing everything possible to keep them alive, that’s made us quite creative in the development and application of new techniques, such as underwater ultrasound and in-water blood sampling. Now, we can start investigating their reproduction while they’re in the wild – and it’ll allow for other studies too, such as looking at stress or pollution levels in their populations, which really improves our understanding of their conservation needs.”
Due to their decreasing population numbers, whale sharks are considered ‘endangered’ on the International Union for Conservation of Nature (IUCN) Red List. “Whale sharks are gentle giants, so they’re lovely animals to work with, and have become a bit of a ‘poster species’ for the use of new techniques, like photo-identification and laser photogrammetry, that are then adapted for other endangered sharks and rays,” concludes Pierce. “Other groups are already starting to use underwater ultrasound wands on poles to scan tiger sharks while they swim by. It’s going to unlock a lot of new opportunities for minimally-invasive research, that can give us a wealth of information while avoiding any harm to the animals. It’s been a super exciting project to be part of!”