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 . It can grow to a maximum total length of .
Distribution and habitat
Maurolicus muelleri is found across the Pacific and Atlantic Oceans from subpolar waters to the equator, as well as in the Mediterranean, however they are absent in the Indian Ocean. M. muelleri is most abundant around bathymetric features such as seamounts and continental shelf breaks, and is scarce in the open ocean. This species is predominantly found at depths of around during the day, but can be found as shallow as during the nighttime. They can be found in depths of at least at maximum. It lives in tropical, subtropical and temperate waters in the deep sea.
Description
Maurolicus muelleri has a fusiform body shape with a moderately sized, subvertical mouth. M. muelleri is countershaded to provide camouflage in the open-ocean, with a dark dorsal surface, silvered flanks and clustered photophores on the ventral surface for counterillumination. In fresh-caught specimens, these photophores are coloured a light pink/purple. They have 9 to 12 dorsal fin rays, 17-19 pectoral fin rays, 7 pelvic fin rays and 22 to 28 anal fin rays. They can grow up to at maximum length, but usually grows up to .
Photophores
Photophores are glandular organs that, in M. muelleri, are made up of several parts. The photogenic chamber, made up of small, spherical light-producing cells, is split into a subspherical tank and conic projector, embedded inside a reflector made of guanine crystals. Ventral to the photogenic chamber is a cellular lens that is itself covered on the ventral surface by a gelatinous dioptric layer.
Eyes
Maurolicus muelleri has large eyes with a retina uniquely adapted to the animal's mesopelagic habitat. M. muelleri inhabits surface waters only during twilight hours, requiring acute mesopic vision which in most vertebrates is achieved through combining dim-light rod cells and bright-light cone cells. 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. These rod-like cone receptors are tuned to the blue-shifted mesopic light conditions dominant in M. muelleri's habitat and are likely a more efficient method of mesopic vision than would be feasible with two improperly-functioning photoreceptor types.
Ecology
Trophic ecology
Mueller's pearlside is a zooplanktivore, with exact diet composition varying geographically and seasonally. For example, Copepods are the main constituent of their diet in the Sea of Japan and in waters surrounding Korea, with the euphausiid species Euphausia pacifica of secondary importance near Japan. Euphausiids and copepods are the dominant prey items year-round off near the eastern continental slope of Tasmania. In Masfjorden, Norway, copepods are most important in the autumn, while earlier in the year Cladocerans are most important. Amphipods and pteropods have also been reported from stomach contents.
Maurolicus muelleri inhabits a tertiary trophic position and, as such, provides a trophic link between zooplankton and larger predators. A wide range of fish species prey on M. muelleri, including commercially-important species such as albacores, skipjack tuna, hake, and blue whiting. They are also predated on by several cephalopods, including the squids Illex coindetii and Todaropsis eblanae and the octopus Enteroctopus magnificus, and marine mammals including common dolphins, sei whales, Bryde's whales, and fin whales.