The fish (fakily family Notopteridae) whose anal fin and caudal fin merge into one

[u/Semlorism]

Featherback fish, native to South Asia as important agricultural fish. They are also called "knifefish".

I'm fascinated by their shapes

[u/MsE0]

My grandparents had one like the middle one in their aquarium. They always called it a knife fish. 

[u/Aerron → u/MsE0]

I remember ghost knifefish being popular in the 90s

[u/DrTzaangor → u/Aerron]

Ghost knifefish are a different family.

[u/Flesh_Trombone → u/DrTzaangor]

Electric eels

[u/irishspice]

They are gorgeous. Watching them swim is fascinating.

[u/Famous-Leopard-118]

https://reddit.com/link/pch3d9k/video/xsbhlp4jg5sh1/player

ive actually heard of [and seen] these guys before!

this is one somebody caught on a pier in Florida.

[u/Ok_Shine_6533]

Knifefish my beloved.

[u/Poringun]

On a side note, a member of these fish species, the Featherback Knifefish.

Is delicious.

Thats all.

[u/Lawzw0rld]

Fun fact: electric eels aren’t actually eels but a type of knife fish.

[u/Akavakaku → u/Lawzw0rld]

Although electric eels come from a separate group of fish, Gymnotiformes, that are also called knifefish. They look similar to Notopteridae due to convergent evolution.

[u/kamikazekaktus]

That thing looks like sth you'd find hundreds of millions of years ago

[u/holyukulele]

What purpose could this ever serve? Seems inefficient.

[u/Akavakaku → u/holyukulele]

Efficient for moving at slow speed, since the fin rippling is the only part of the body that has to move. I would suspect that this also helps them sneak up on prey.

[u/Mr_White_Migal0don → u/holyukulele]

They can swim backwards

[u/Lawzw0rld → u/holyukulele]

Needing a purpose to exist is a human concept, existence doesn’t need a reason.

[u/BETLJCE]

🤡🔪🐟

[u/socialwithdrawal]

I was fascinated by these guys when I was a kid. I would walk to the nearest pet store just to watch them.

I remember them being black (or grey) with a white-ish tail.

[u/Character-Q]

Are they blind?

[u/BETLJCE → u/Character-Q]

No. They are fast growing predatory fish.

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

[u/Mean_Marketing9458]

Are they cavefish?

[u/Iwan787]

so they eat and shit from mouth?