By Abigail Lynch

The most common fish in the sea is likely a fish that you’ve never seen or maybe even heard of.

You may be scratching your head and wondering how that is possible but consider the scale of the ocean.  Approximately 70% of the earth’s surface is covered by ocean with an average depth of 2.3 miles.  For reference, that’s about the same distance as the average altitude for sky divers.  So, all that air between sky divers and the earth is the same as all that water between the ocean surface and ocean floor.  And, once beyond the continental shelf into the deeper depth zones, the habitat in the ocean is relatively homogeneous; many of the species that inhabit it are globally panmictic, meaning that the same species is found anywhere around the world.

All things considered,  it may not be so surprising that the most common fish in the sea lives in this uniform habitat, at 1000 feet or deeper, and can be found anywhere around the world.  What is this elusive, ubiquitous fish?  Bristlemouths.

And what is a bristlemouth?  It is a modestly-sized fish, reaching only around 3 inches in length, belonging to the genus Cyclothone.  They are also known as “minnows of the deep.”  These bristlemouths are characterized by bristle-like teeth and bioluminescent photophores, or light producing organs.  There are trillions, maybe even quadrillions, of them swimming in the ocean depths.

All added up, Cyclothone is believed to be more abundant and have more biomass than any other vertebrate genus, not only in the sea, but in the entire world.  Yet, until recently very little is known about the behavior and ecology of this most common of fish because of its inhospitable (to humans) habitat in the ocean’s depths.  New advances in technology, such as remotely operated vehicles with cameras, have given humans more of a window into their shadowy, foreign world.

Still, it’s not common to encounter the most common fish in the sea…unless you have a habit of swimming 1000 ft down!