I only learnt recently that fish account for between 97% and 99.99% of all vertebrates on Earth. The three percent uncertainty is because there is a lot of uncertainty about the number of reptiles. As an aside, I don’t even know how to count the number of lizards in my house.

That would seem to say that the most common vertebrate is a fish. But which fish?

The most common fish is any of the species of a deepwater fish sometimes called a “bristle mouth.” The fish is about the size of a small minnow. It is caught at 500 meters or deeper (wikipedia says 1000 metres or deeper) all over the world. https://thefisheriesblog.com/2017/07/17/the-most-common-fish/

It’s 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.

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. Cyclothone signata

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.

https://trawlersonthequay.co.uk/the-three-most-common-fish-in-the-sea/
The Cyclothone Bristlemouth is not only the most common fish in the world, but probably the most common vertebrate animal of any kind too. They’re not particularly edible, due to the phosphorescent sacs that line their upper bodies.

Even if you took the conservative estimate of the number of rats in the world and then added that to the human population, you’d still fall about 10 million times short of how many Cyclothones there are. And that’s just one species of Bristlemouth, of which there are about 10 – although Cyclothones are the most common.

Brislemouths feed mostly on zooplankton and small crustaceans. Their diet is 92-98% Crustacea. A minor part of their diet is made up an opportunistic encounter with smaller fish. Brislemouths that consume fish prey are found in individuals ranging from 70 mm to 75 mm. Bristlemouths do not have seasonal trends when it comes to their feeding habits. Bristlemouths are vertical migrators, migrating closer to the surface waters in the nighttime in order to find more food.

Protandrous sex reversal has been observed in these fish.

The fishes with the greatest commercial harvest are different. See https://www.worldatlas.com/articles/the-world-s-most-common-types-of-fish.html for a list.

mollwollfumble thinks it would be fascinating to tag and track these. Do they exist separately or in vast schools? Which fish and squid species eat them? Do they have migration routes?

mollwollfumble said:

I only learnt recently that fish account for between 97% and 99.99% of all vertebrates on Earth.

Should be noted that “fish” is a very broad category, not even a clade (it’s a paraphyletic group), basically meaning “all groups of vertebrates that are not tetrapods”.

‘The Nobel Prize winning biologist Stephen Jay Gould concluded after a lifetime’s studying of fish that there is no such thing as a fish. He reasoned that while there are many things that live in the sea, most of them are not related to each other. For example, a salmon is more closely related to a camel than to a hagfish. A similar argument is that there are lots of things that fly like bees, vultures and flying lizards, but they are not all insects, birds or reptiles.’

https://www.comedy.co.uk/tv/qi/episodes/8/3/

dv said:

mollwollfumble said:

I only learnt recently that fish account for between 97% and 99.99% of all vertebrates on Earth.

Should be noted that “fish” is a very broad category, not even a clade (it’s a paraphyletic group), basically meaning “all groups of vertebrates that are not tetrapods”.

So there is no need for a fish to live in the sea?

The Rev Dodgson said:

dv said:

mollwollfumble said:

I only learnt recently that fish account for between 97% and 99.99% of all vertebrates on Earth.

Should be noted that “fish” is a very broad category, not even a clade (it’s a paraphyletic group), basically meaning “all groups of vertebrates that are not tetrapods”.

So there is no need for a fish to live in the sea?

Certainly not. They can live in lakes, rivers, ponds …

dv said:

The Rev Dodgson said:

dv said:

Should be noted that “fish” is a very broad category, not even a clade (it’s a paraphyletic group), basically meaning “all groups of vertebrates that are not tetrapods”.

So there is no need for a fish to live in the sea?

Certainly not. They can live in lakes, rivers, ponds …

OK, fair point, let me re-phrase:

So there is no need for a fish to live predominantly under water?

dv said:

The Rev Dodgson said:

dv said:

Should be noted that “fish” is a very broad category, not even a clade (it’s a paraphyletic group), basically meaning “all groups of vertebrates that are not tetrapods”.

So there is no need for a fish to live in the sea?

Certainly not. They can live in lakes, rivers, ponds …

and fish tanks?

The Rev Dodgson said:

dv said:

The Rev Dodgson said:

So there is no need for a fish to live in the sea?

Certainly not. They can live in lakes, rivers, ponds …

OK, fair point, let me re-phrase:

So there is no need for a fish to live predominantly under water?

Let’s put it this way: you can’t define “fish” as “water dwelling vertebrate” because there are marine mammals, marine snakes, some amphibians who spend almost all their time in the water etc.

I won’t speculate about what would happen to the word “fish” in the future if some non-tetrapod vertebrate managed to spend more time a-land than a-sea. (There are of course fish that spend at least part of their time out of the water. African lungfish spend months out of the water, for instance.)

Looking at the use of the term now, and what it is applied to, Fish = Vertebrata minus Tetrapoda.

dv said:

The Rev Dodgson said:

dv said:

Certainly not. They can live in lakes, rivers, ponds …

OK, fair point, let me re-phrase:

So there is no need for a fish to live predominantly under water?

Let’s put it this way: you can’t define “fish” as “water dwelling vertebrate” because there are marine mammals, marine snakes, some amphibians who spend almost all their time in the water etc.

I won’t speculate about what would happen to the word “fish” in the future if some non-tetrapod vertebrate managed to spend more time a-land than a-sea. (There are of course fish that spend at least part of their time out of the water. African lungfish spend months out of the water, for instance.)

Looking at the use of the term now, and what it is applied to, Fish = Vertebrata minus Tetrapoda.

Thanks to Rev’s interrogations I am now familiar with the word “olm”.

https://en.wikipedia.org/wiki/Olm

The olm or proteus (Proteus anguinus) is an aquatic salamander in the family Proteidae, the only exclusively cave-dwelling chordate species found in Europe. In contrast to most amphibians, it is entirely aquatic; it eats, sleeps, and breeds underwater. Living in caves found in the Dinaric Alps, it is endemic to the waters that flow underground through the extensive limestone bedrock of the karst of Central and Southeastern Europe, specifically southern Slovenia, the basin of the Soča River (Italian: Isonzo) near Trieste, Italy, southwestern Croatia, and Bosnia and Herzegovina. Introduced populations are found near Vicenza, Italy, and Kranj, Slovenia.

> Stephen Jay Gould

Stephen Jay Gould made a number of mistakes.

But that’s got nothing to do with Cyclothones.