Swimming in the Womb
Updated: Apr 27
When animals are pregnant (also known as "gravid"), 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!
Reproduction in sharks is absolutely fascinating and bizarre. 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). Mature females have one functional ovary (the organ which produces eggs), but have two uteri (where the offspring grow before birth).
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. Interestingly, in this species, after the foetuses have depleted their yolk stores, they are also nourished by eating other, unfertilised eggs which are produced by the mother. This is known as "oophagy". These sharks usually only have two pups with each pregnancy; one per uterus. If two siblings do share a uterus, one is often significantly smaller than the other (Teshima et al, 1999).
Well, that's what we thought... until a research team in Japan developed a pressure- and water-resistant ultrasound scanner, which could be used underwater. This allowed them to study the pups developing inside a shark more closely (Tomita et al, 2019).
In the past ultrasound devices were not easily portable, but as the technology has been developed, they have become more suitable for studying gravid fish, like sharks. Ultrasound devices (just like the ones your doctor will use to show you your baby), use sound waves 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).
From scanning three pregnant tawny nurse sharks, they discovered that the growing pups are not restricted to their original uterus, but are actually able to swim between the two! The first time they scanned one female, 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).
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!
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.