When animals are pregnant, the foetuses have limited movement... maybe a kick or a small wriggle, but nothing that could be described as "locomotion"... right? Well, recent scientific research has shown us that that is, in fact, not the case for some sharks. Scientists studying a captive tawny nurse shark (Nebrius ferrugineus), found that, not only are the developing foetuses able to move, but they actually swim around inside their mother! So how do these sharks move? And what does it look like?
Reproduction in sharks is absolutely fascinating and bizarre. There are a myriad of different ways that sharks breed and every species is unique... Some sharks lay eggs (known as "oviparity"), but some species give birth to live young. This is known as "viviparity" (meaning 'live-bearing') or "ovoviviparity" (when an egg hatches inside the mother and embryos are nourished by yolk). To learn more, you can check out Don't Put All Your Eggs in One Basket and What Came First, The Shark or the Egg (Ebert et al, 2021).
Tawny nurse sharks reproduce by ovoviviparity (also known as "aplacental viviparity"). This means that the developing young are not connected to their mother via an umbilical chord and placenta, but are nourished by yolk from their egg sac. Instead, after the foetuses have depleted their yolk stores, they are then nourished by eating other, unfertilised eggs which are produced by the mother. This is known as "oophagy".
The mother usually only has two pups with each pregnancy; one per uterus. If two siblings do share a uterus, one is often significantly smaller than the other (Ebert et al, 2021; Teshima et al, 1999).
Sound and Light Show
When you go to your doctor for an ultrasound sappointment, you are probably more interested in checking in on the health of your baby, than thinking about how the science works. But now that you are happily scrolling through the internet, allow me to explain to you how an ultrasound actually works... Ultrasound devices actually use sound to create an image. A transducer emits high-frequency sound waves, which enter the body and bounce back to the transducer in different ways depending whether they have been reflected by body tissues, bones or fluid. This allows us to visualise structures inside the body (Tomita et al, 2019).
In the past, it was difficult to actually see how developing shark embryos look or what they do when they are inside the mother because ultrasound devices were not easily portable. As the technology has been developed, they have become more suitable for studying gravid fish, like sharks. Then a research team in Japan managed to develop a pressure- and water-resistant ultrasound scanner, which could be used underwater (Tomita et al, 2019).
From scanning pregnant tawny nurse sharks, these scientists discovered that growing shark pups are able to move a lot more than we previously thought. In fact, they are not even restricted to their original uterus, but are actually able to swim between the two! The first time the researchers scanned a female shark, she had a pup in each uterus, but the next time, one pup had moved in to share with it's sibling. In another female, there were four pups moving between the two uteri. Then, they even managed to capture footage of the tiny shark moving between the uteri (Tomita et al, 2019).
It is seriously THE coolest video ever made!
As the pups grew, the researchers noticed that they were often very active, moving repeatedly between the uteri and even poking their heads out through the cervix to take a peak at the world outside. This is undeniable evidence that these sharks are capable of locomotion before they are even born! These types of movement in utero have not been observed in any species of shark before!
To learn about how other sharks move around before they are even born, check out He Can't See Us If We Don't Move!
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Teshima K, Toda M, Kamei Y, Uchida S & Tamaki M (1999). Reproductive mode of the tawny nurse shark, Nebrius ferrugineus (Elasmobranchii: Ginglymostomatidae) in Okinawa waters, with comments on individuals lacking the second dorsal fin. Proceedings of the 5th Indo-Pacific Fish Conference, 329-333. Access online.
Tomita T, Kiyomi Murakumo K, Ueda K, Ashida H, Furuyama R (2019). Locomotion is not a privilege after birth: Ultrasound images of viviparous shark embryos swimming from one uterus to the other. Behavioural Note, 125, 122–126. Access online.