Evidence of Embryonic Locomotion in Tawny Nurse Sharks (Nebrius ferrugineus)

When we think of an embryo developing inside a mother’s womb, we don’t generally think of the embryo being about to travel around as it so pleases, and even snack on some of mom’s unfertilized eggs. (Sorry for any potential nightmares moms!) But that is exactly what a team of scientists studying pregnant tawny nurse sharks (Nebrius ferrugineus) have observed over the last few years. A study published in the journal Ethology has revealed that these aplacental vivparous sharks rely heavily on their ability to migrate between the two uteri in order to consume unfertilized embryos (Tomita, Murakumo, Ueda, Ashida,  & Furuyama, 2018).

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Figure 3. Evidence of embryonic movement from one uterus to the other. (a) Serial ultrasound images for female 1, showing embryonic movement from the left to the right uterus. The embryo passed near the cervix (#), which is located at the junction between the right and left uterus, in the following order: head (i), trunk (ii), and tail (iii) of the embryo. (b) Schematic illustrations of the embryonic posture in uterus, which correspond to (i) and (iii) in (a).

 

In this study, three pregnant females were monitored with an underwater ultrasound throughout their pregnancies. The team discovered discovered that the number of embryos present in each uterus kept changing, sometimes even as they were actively monitoring the embryos (Tomita et al., 2018). It appears that the tawny nurse sharks pups have developed the ability to migrate between the uteri in order to supplement their growing bodies with other embryos. This process of embryos feeding on eggs is referred to as oophagy and it is actually relatively common in elasmobranchs. Just recently, the Sulikowski Shark and Fish Research Lab went viral with their Facebook post about the porbeagle sharks (Lamna nasus), a relative of the great white (Carcharodon carcharias) and mako shark (Isurus oxyrinchus), also being oophagous.

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Tomita et al., 2018. (Authors). Figure 1 (a). Underwater ultra sound developed in this study. (b) Underwater observation for the pregnant tawny nurse shark at the depth of 10 m in the exhibition tank of Okinawa Churaumi Aquarium.

 

So why is this study so ground breaking?! This is the first evidence we have of embryonic locomotion (that is the traveling of the embryo from one uterus to the other) under natural conditions. In 1993, there was one documented case for Discovery Channel on a sand tiger shark (Carcharias taurus), however, an incision was made in the side of the shark and a camera was inserted in order to view the embryonic movement. So this is the first noninvasive evidence we have of this behavior in any elasmobranch species. And it occurred in three separate females over a three year times period (2015 – 2018) (Tomita et al, 2018). Both the tawny nurse shark and the sand tiger shark share the same aplacental vivparous reproduction that rely on oophagy to supplement their growing pups. It is possible that other species share these similar embryonic locomotive behaviors as well and it is worth investigating further.

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a. Embryonic movement between right and left uterus was observed. b. Three embryos in right and one embryo in lefter uterus at 10:20 am. Two embryos in each uterus at 10:40 am. c. Three embryos in right and one in left uterus at 10:37 am. One embryo in right and three embryos in left uterus at 10:55 am. d. Three embryos in right uterus at 10:27 am. Two embryos in right and one embryos in left uterus at 10?:29 am. e. Embryonic number was changed from three to two in right and from zero to one in left uterus during 1 min. f. Embryo was located across right and left uterus. One pup was born.

We’ve come a long way in the study of shark science in the last 100 years. But for all that we know about them, there is still so much more that we do not know. What will we discover tomorrow?

The new Ocean For Sharks Shop is open! There’s handmade ocean inspired plush animals, canvas paintings, and of course my children’s book, Winifred the Wondrous Whale Shark, available in print and PDF. Be sure to stop by. Remember proceeds benefit shark research and conservation with a donation to Project AWARE!

Conservation legislation needs public support in order to become law and help protect the environment and wildlife. Tell your representatives that you care about environmental and wildlife conservation.  It only takes a moment to make a change that will last a lifetime. Until next time finatics!

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Featured Image Source

Andrews, S. (Author). (2015). Tawny Nurse Shark Over Reef in Chagos [Digital Image]. Retrieved from https://www.livingoceansfoundation.org/tawny-nurse-sharks/

Literature Cited

Tomita, T., Murakumo, K., Ueda, K., Ashida, H., & Furuyama, R. (2018). Locomotion is not a privilege after birth: Ultrasound images of viviparous shark embryos swimming from one uterus to the other. Ethology, eth.12828.

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