For a long time in the geological history, sharks were not the fast silhouettes that we see today swimming across open waters. A new study on Ecology and Evolution, lead by Joel Gayford and based on 45O actual species, rebuilds the history and shows they spent nearly 200 years glued to the seafloor, discrete and somehow flattened, before this breed change into a much deeper and symmetrical bodies, capable of moving through open waters. The image we have of the White Shark and the Tiger Shark is put in doubt, those famous big hunters of the sea, turns out, are the new outcome of a very long story.
The study explains the transition from the bottom to the top of the sea wasn’t a sudden migration, but more of a anatomical transformation that created an opportunity to new habitats. Most of the species alive today don’t fit the pelagic stereotype. Their history is closer to the darkness of the bottom of the sea than to the lights of the surface.
From the seafloor to the surface
The first sharks were benthonics: more flattened bodies, asymmetrical tails, and a more shy lifestyles, discrete at the very bottom of the sea. The pelagic body plan came after, with a deeper trunk, a prominent dorsal fin and a more symmetrical tail. And this happened at least 4 times in different occasions. So different lineages became a similar solution.
The interesting part is the body change happened before the ecosystem change; the anatomy transformation allowed the evolutive preparation to conquer the surface with speed and range. This transitions align with periods where there was a planetary reorganization, like very high sea levels that increased the size of continental shores, fragmentations of land masses, warming of waters and real changes in the marine food chains.
Far form been just a noise at the background, this events created steps between the seafloor and the surface, making it possible for sharks to go out of the shades.
Competitors, prays and real diversity
Why did all of this happened? In the Jurassic and the Cretaceous sea level peaked, expanding the “in between” ecosystem, at the same time, sea reptiles like ichthyosaurs declined, leaving some free space there. The bony fish (actinopterygians) diversified in open waters, becoming new prays, and competition. In this scenario, sharks were able to go further and get more resources, creating strategies to hunt in the surface. Those who didn’t change stayed at the seafloor.
The pelagic sharks are only fraction off the total. From more that 500 species alive, the bigger part are bentonic or benthopelagic, and their linked to the bottom or some zones in the middle. There are still some species leaving at the bottom the sea. The morphological and ecological diversity is way bigger and richer than what the iconic pictures show.
Lesson from the ocean and it’s history
This new perspective not only describes the sharks narrative, but it also highlights how changing the the ocean ecosystems has change, and the possibility of newer transformations. It’s understanding what pushed the colonisation of the open waters in the past helps frame questions facing the pression of climate change, the acidification, and the intense fishing. History has made clear that the change is constant… Are they still transforming at this very moment? What will they look like 100 years form now?
It’s a beautiful story of adaptation, transformation and survival. For 2000 millions of years, sharks were seafloor creatures, and only after many lines transformed themselves, they dominated the blue surface. It dismantles the myths about this animals, which far from being “monsters” or machines, are a very important piece of the puzzle to understand complex sea evolution.