There is a revenant that haunts the deep sea. An elusive creature, one that has existed on this
planet for millions of years. The Chimaera, or Ghost Shark, is a cartilaginous fish that
is related to sharks and rays; however, they exhibit a morphology unlike any other creature.
With seemingly only one gill on either side of their body, upper jaws fused to their skull,
no scales, and eyes that are backed with a reflective tissue layer that makes them
seem to glow in the dark, the Chimaera have an eerie and ghostlike appearance.
They seem to move like ghosts too; as rather than most sharks that generate
propulsion using their tails, ghost sharks use their large, wing-like pectoral fins
to glide through the water - a characteristic more suited to a bird than a fish.
However, these peculiar features have a purpose.
Although some Chimaera inhabit coastal waters, many live at depths of over 8,200
feet (2,500 metres), meaning that Ghost Sharks need to be well-adapted to life in the deep sea.
Their large pectoral and pelvic fins allow them to cover larger distances while using less energy;
large eyes allow them to absorb as much light as possible in the sunless depths,
and a protruding snout, highly sensitive to electric fields and movement,
allows them to search for prey in the sand.
Their behaviour of swimming on the ocean floor leaves them vulnerable to attacks from above,
but this highly-adapted fish has a solution for that, too,
and protect themselves with a large venomous spine that precedes the first dorsal fin.
The stitch-like lines that can be seen around the eyes and along each side of the body,
make up the lateral line system. This allows them to detect vibrations and movement in the water.
Like many creatures of the deep, ghost sharks exhibit the zoological phenomenon that is Deep
Sea Gigantism. A full video explaining gigantism can be found on my channel.
Put simply, it results from environmental factors in the deep sea, including low predation pressure,
food scarcity, and the need for organisms to become more efficient by growing to larger sizes.
It is thought that chimaeras first emerged in the aftermath of the Devonian extinctions
that took place some 360 million years ago,
due to specialisation occurring in shallow-water species that sought refuge in the deep sea.
The earliest fossil specimen of these creatures was discovered during the 1980s in South Africa.
It was a skull, revealed to date back to around 280 million years ago,
which contained a brain case and numerous features that closely resembled those of modern
day chimaeras. Because of this, chimaeras are believed to be among the oldest fish in the ocean.
This is linked to the fact that they have the slowest-evolving genome of all known vertebrates,
and might explain why many of their characteristics seem out of place
among many of today’s oceanic species, such as three pairs of permanent rodent-like teeth,
and the presence of a fleshy covering over the gills called an operculum. In fact,
Chimaera belong to the only group of fish that possess true nostrils. Respiratory water
is drawn in through these and is passed over the gill arches.
Chimaera have adopted a variety of different names. Some are known as ratfish, as their
body tapers into a long, rodent-like tail and their teeth resemble those of a rat. Others are
called elephant fish, due to the presence of a curled snout that looks like a miniature trunk.
These odd features come as no surprise, when considering the greek word Chimera
refers to a monster whose body consisted of parts of different animals joined together. It’s
therefore understandable how these fish got their name, possessing the nostrils of a mammal,
the spine of a porcupine, and fins like the wings of a bird.
They truly are one of the deep sea’s most bizarre residents, and the depths still hold many secrets.
Who knows what other wonderfully mysterious creatures are out there, waiting to be found?