Monacoa is a genus of fish in the family Opisthoproctidae[1] found in Atlantic and Pacific Oceans.

[2] They are also known as long-nosed mirrorbellies[2] or simply mirrorbellies, in reference to the bioluminescent organ in their intestines.

[3] The largest species, Monacoa grimaldii, can grow to 8 cm (3.1 in) standard length.

The M. griseus and M. niger are distinct from the M. grimaldii because of their differences in pigmentation.

An examination of their complete mitochondrial genomes further suggests the species are distinct.