In the dark waters of the deep sea, lurk many curious and enigmatic animals that few have seen. Frilled sharks (Chlamydoselachus anguineus), for instance, have been known about for more than a century, but much of their lives remain mysterious.
We know what they look like from the occasional specimens that are hauled up in deep sea fishing nets around the world.
Most of their 2m-long (6.5ft) brown or grey body is a smooth, finless tube, with a dorsal fin far down on the back, near the trailing, ribbon-like tail.
The strangest features are up at the head end, however. Frilled sharks have six large gill slits – one more than most sharks.
In fact, there are only five other living species in the frilled shark’s taxonomic order, the Hexanchiformes, which includes sixgill and sevengill sharks.
Frilled sharks get their common name from the uniquely ruffled edges of their gill slits, where the extended tips of their gill filaments stick out.
The first pair of gill slits connect together under their throats, making it look as if they’re wearing lacy collars.
They also have large mouths, more like lizards than regular sharks. Inside, lie rows of three-pointed teeth, resembling miniature tridents.
By inspecting the stomach contents of rare specimens, scientists have discovered that frilled sharks use their three-pronged teeth to snag soft-bodied squid.
They’re also known to sometimes eat fish and other sharks. Pregnant female frilled sharks don’t lay eggs as some sharks do, but the eggs hatch inside their bodies – a process known as ovoviparity.
A shark pup starts life as an embryo attached to the egg yolk, which it uses as a source of nutrition while it grows.
One of the mysteries that has yet to be solved is exactly how long their gestation lasts. If
other deep-sea sharks are anything to go by, then it’s likely to be at least several years before the frilled shark pups emerge into the ocean.
Frilled sharks are often misleadingly labelled as literal ‘living fossils’. In 2022, after a viral video showed rare footage of a living frilled shark in Japan, news outlets around the world had to step in to set the record straight, informing audiences that this wasn’t an 80-million-year-old prehistoric shark.
These animals probably live for no longer than a few decades.
It’s true, however, that the oldest known fossils of frilled sharks date back to the Late Cretaceous period, around 80 million years ago. And, sure enough, they look a lot like the living species, which haven’t changed a great deal since then.
Fossil remains also suggest that frilled shark ancestors shared a similar habit of swimming in the deep sea.
So, we can be sure that unusual-looking sharks were slithering their snake-like bodies through the dark depths tens of millions of years ago, and their descendants are still there today, even if they’re very rarely seen.
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