Frilled sharks are generally thought to represent plesiomorphic, or primitive/prehistoric, traits of sharks. This particular study focuses on Frilled Shark embryos at different stages of development.
Why study this?
Researchers want to gain more insight into plesiomorphic (primitive) and amorphic (derived or evolved) body structure to look at the possible evolutionary paths of the species. The study also serves as a reference point for future studies on many sharks, rays and skates.
No sharks were harmed for this study! It was conducted on embryos preserved in 70% ethanol from the 1980s.
Four Stages of Development of The Frilled Shark Mouth
Factors Observed
- The shape of the caudal fin (tail fin) did not change much throughout the stages. It remains unknown if this changes later in adulthood or if it has adapted this shape due to living in a deep-sea habitat.
- The shape of the shark’s mouth changes late in development; it was thought that the mouth shape of the adult Frilled Shark was primitive, but the embryos show that it actually begins like most modern sharks. This is significant because it shows that evolutionarily, it could be derived/evolved. (see 4 stages above)
- As pups, the size of males and females are not noticeably different. In adulthood, females are larger than their male counterparts.
Summary
The changes seen early in the pup’s formation are quite varied which makes the scientists believe these anatomical changes slow as it reaches the last stage of development. Scientists found that their lips start unevenly like most modern sharks, then even out as they grow. It is intriguing to scientists that they start out “normal” in their development when compared with other sharks and then veer off in a different direction.
Source: J Fish Biol. 2020 May 19;97(1):212–224. doi: 10.1111/jfb.14352
Study Conclusion
This study was significant in that it was the first time a study comparing Frilled Shark embryos through various stages in prenatal development had been undertaken. The conclusion suggests that the terminal position of the jaws may be a derived (evolutionary) trait rather than a plesiomorphic (ancestral) condition. When compared with earlier sources, this study shows that Frilled Sharks might not have remained unchanged for 80 million years as previously thought.
Summary of the scientific article written by Medea Ramnath-Christiansen, B.S. in Marine Science
Scientific Article:López-Romero, F. A., Klimpfinger, C., Tanaka, S., & Kriwet, J. (2020). Growth trajectories of prenatal embryos of the deep-sea shark Chlamydoselachus anguineus (Chondrichthyes). Journal of fish biology, 97(1), 212–224. https://doi.org/10.1111/jfb.14352