Cryptic species
Two or more species that look so alike they were long taken for one, often told apart only by DNA.
Also: species complex
Cryptic species look the same to the eye but are separate species that do not interbreed. DNA studies keep revealing them among fish. The reef manta ray and giant manta ray were split in 2009, and work on the mahseers of India has shown that several names had been mixed up for a century, which matters because conservation depends on knowing which fish is which. A group of very similar species that is still being sorted out is called a species complex.