Most chromosomes in an animal
Atlas blue butterfly, Polyommatus/Plebicula atlanticus
Morocco ()

The animal with the most chromosomes per cell among diploid, non-polyploid eukaryote species is the Atlas blue butterfly (Polyommatus/Plebicula atlanticus). Native to Morocco and Algeria, it possesses 448–452 chromosomes (i.e. 224–26 pairs) per cell. This was confirmed in 2015 via a study conducted by Russian researcher Vladimir A Lukhtanov.

Prior to that discovery, the animal species traditionally deemed to hold this record was the hermit crab (Pagurus ochotensis), which possesses 254 chromosomes (117 pairs) per cell. However, nowadays it does not even hold the record for most chromosomes in a crustacean, as this is now held by crayfishes of the genus Astacus, with 376 chromosomes (188 pairs) per cell.

The organism with the most chromosomes is the protist Oxytricha trifallax (aka Sterkiella histriomuscorum), which is thought to have around 15,600 chromosomes (albeit nanochromosomes).

By comparison, humans possess just 46 chromosomes (23 pairs per cell).