Heart chambers

[SamirD]

What group of vertebrate animals has the lowest number of heart chambers?

[fiveworlds]

Anthropods can have one heart chamber

[imatfaal]

What group of vertebrate animals has the lowest number of heart chambers?

 

Anthropods can have one heart chamber

 

But are invertebrate which kinda rules them out of contention.

In broadest terms (and there are many exceptions) mammals, lizards, and birds have four chambered hearts (with some lizards having a mixing foramen which remains slightly patent in adulthood and controllable sphincters between heart and lungs), amphibians have three chambers (double circulatory system but with a mixing chamber), and fish can have a two chambered heart (single circulatory system)

[SamirD]

The answer is Fish