Argyropelecus affinis is a species of ray-finned fish in the family Sternoptychidae, described by Garman in 1899, found in the tropical and subtropical Atlantic, Indian and Pacific Oceans.

It inhabits the upper mesopelagic zone between 350 and 600m during the day, and between 100m and 350m during the night and is either non-migratory or performs short daily vertical migrations.

The analysis of morphological traits suggests that A. affinis and A. gigas retain some primitive features relative to other members of the genus, placing them as a basal in the phylogeny of Argyropelecus.

Derived characteristics present in other Argyropelecus species are absent in A. affinis, supporting its placement in a separate, more primitive clade.

[7] The placement of A. affinis and A. gigas as a distinct clade within the genus Argyropelecus is supported by five derived traits, though some aspects of their evolutionary relationships remain ambiguous.

Baird’s (1972) assessment suggested that A. affinis was one of the more primitive members of the genus, a hypothesis that has been further refined in subsequent phylogenetic studies.

The lychnus complex is considered more derived and is characterized by modifications such as reduced number of hypurals and distinct photophore arrangements.

In contrast, A. affinis retains traits like three hypurals in the lower caudal lobe and a continuous series of posterior ventral photophores, suggesting a closer resemblance to ancestral forms within the genus.

[13] Eye lens pigmentation in the deep-sea hatchetfish is characterized by 1) an abrupt onset and steady increase in concentration during the life history of the fish; 2) restriction to the outer layers of the lenses of adult fishes; 3) a complex absorption profile in the near UV and blue-violet wavelengths; and 4) a chromophore that is tightly bound to a single, specific soluble lens protein, alpha crystallin.

[14] Adult Argyropelecus affinis feed on planktonic organisms, salps, krill, arrow worms, copepods and ostracods.

[15] Smaller fish consume mainly copepods and ostracods, and their adult arrangement of photophores develop when they are about 14 mm (0.6 in) long.

[17] It is not clear what precise function is served by the pigments, but with some of the incident illumination being absorbed, the fish's absolute sensitivity to light must be reduced.