The researchers describe the report as the first reliable evidence of “active embryonic locomotion” in live‐bearing vertebrates, contradicting the generally accepted concept of mammalian embryos being sedentary.
The water- and pressure-resistant type of ultrasound device used in the study was similar to the type used recently by scuba divers to check the state of pregnancy in free-swimming whale sharks in Galapagos, as reported on Divernet .
The Okinawa researchers captured ultrasound clips from three pregnant aquarium sharks over several years.
A total of 24 migrations were observed during one mother’s pregnancy, with one of the scans succeeding in capturing a moment of transfer, which occurred at a far-from-sedentary 8cm a minute.
One shark began her pregnancy with two pups in each uterus, and by the time she gave birth only one remained.
The research is published in Ethology.