Maurolicus muelleri, commonly referred to as Mueller's pearlside, Mueller's bristle-mouth fish (not to be confused with the Gonostomatidae), or the silvery lightfish (not to be confused with the Phosichthyidae), is a marine hatchetfish in the genus Maurolicus, found in deep tropical, subtropical and temperate waters of the Pacific Ocean and the Atlantic Ocean, from the surface to depths of 1,500 metres (4,900 ft).

[2] M. muelleri is most abundant around bathymetric features such as seamounts and continental shelf breaks, and is scarce in the open ocean.

[4] Maurolicus muelleri has large eyes with a retina uniquely adapted to the animal's mesopelagic habitat.

Fish in the genus Maurolicus have developed a unique photoreceptor where a cone opsin and phototransduction cascade is found in cells transmuted into a rod-like morphology.

[6] Euphausiids and copepods are the dominant prey items year-round off near the eastern continental slope of Tasmania.

Individual fish may move between scattering layers, indicating that the comfort zones are broader than suggested in the typical isolume hypothesis.

[24] Here, M. muelleri displayed consistent, typical DVM patterns (i.e. remaining at depth during the day and ascending to the surface at night) during summer months only, with individuals feeding at dawn and dusk.

In years where their Calanus copepod prey, which overwinter at depth, were abundant, adults in the winter delayed their vertical migration until approximately three hours before dawn due potentially to a reduced need to feed at the surface, with some individuals remaining at depth for the entire night, feeding entirely on deep-overwintering prey.

Some individuals undertook a reversed DVM during winter, diving to slightly greater depths during daylight hours, to feed on deep-overwintering prey in optimal light conditions.

[23] In the Benguela system and in the Gulf of Oman, DVM is known to occur, with fish ascending to within 10m of the surface in response to the first light of dawn before diving into deep waters.

[26] Maurolicus muelleri in scattering layers can detect predators at distances of several metres during the day, and respond by diving as far as 50m below their original depths at speeds of 15–20 cm/s.

[22] Individual females can contain as many as 738 ova,[22] and in enclosed spaces eggs can be extremely abundant, reaching numbers as high as 5.8x10^11 in Fensfjorden.