Diaphus pallidus – Pale lanternfish

Diaphus pallidus – Pale lanternfish ©FAO/M.Lipperi

The Pale lanternfish is a small, bioluminescent fish found in the mesopelagic zone of the Gulf of Aden. It is characterized by its light grey coloration and unique arrangement of the photophores (light-producing organs) along the head and body.

The discovery

During two cruises by the research vessel Dr. Fridtjof Nansen in the Gulf of Aden (August 1979 and February 1981), a large number of myctophids were collected using a large trawl designed to catch krill, small, shrimp-like crustaceans found in oceans around the world which serve as a primary food source for many larger animals.

Among them, some Diaphus specimens were sent to the Flødevigen Biological Station in Arendal, Norway, for further analysis. About ten years later, Dr Jakob Gjøsaeter, a specialist in mesopelagic fish taxonomy, revised the collection from the surveys, and noticed that several specimens did not match any of the known species of Diaphus.

In 1989, he formally described the new species as Diaphus pallidus, referring to its pale, light-grey colour.

Features and characteristics

Diaphus species are differentiated based on several key features, including body measurements, the shape of the opercle, and meristic characters (the count of specific physical features – such as fin rays, scales or vertebrae – which are used to identify and classify species). However, one of the most important distinguishing characteristics is the arrangement and size of the photophores (light-producing organs) along the body. These photophores vary between species in terms of number and position, and taxonomists have developed a nomenclature to describe their specific arrangement and pattern, which aids in species identification.

Reaching up to 12 cm in length, the pale lanternfish is characterized by a light grey body, a high number of dorsal-fin rays, and a low number of gill rakers compared to species occurring in the same area, such as D. coeruleus and D. lucidus. These two species are frequently found in the Gulf of Aden and were taken in the same trawls.

D. pallidus lives in the mesopelagic zone of the ocean, typically found at depths between 200 m and 1 000 m. Along with other mesopelagic fish, it plays a crucial role in the deep-sea food web, serving as prey for larger predators such as squid, fish and marine mammals.

They are primarily planktonic feeders, consuming small zooplankton like copepods and krill. Diaphus species exhibit vertical migration, moving up to shallower waters at night to feed and retreating to deeper waters during the day to avoid predators. However, the pale lanternfish was not caught during the night in shallow waters. This suggests that this myctophid usually does not migrate to the surface layer at night, unlike most of the other Diaphus species caught in the area.

Reproductively, Diaphus species typically have external fertilization, with females releasing eggs that are fertilized by males in the surrounding water. Their life cycle and growth rates are influenced by environmental factors like temperature and food availability. However, little is known about the full range of their reproductive behaviours, as they primarily live in deep, less accessible waters.

Mesopelagic fish, including species like Diaphus, have garnered increasing attention for their potential commercial importance. These species are a significant source of biomass in the ocean, with estimates suggesting they collectively represent one of the largest untapped marine resources. While currently underutilized, mesopelagic fish are being considered as a sustainable source of fishmeal and oil for aquaculture. Their high nutritional value, particularly their rich omega-3 fatty acid content, has potential applications in global food security. However, challenges such as the deep-water harvesting techniques required, the ecological implications of large-scale exploitation, and gaps in knowledge about their life history and population dynamics have thus far limited their widespread commercial use. Sustainable management strategies will be critical if mesopelagic fish are to become a significant contributor to global fisheries.

The EAF-Nansen Programme has produced two comprehensive guides on mesopelagic fish, enhancing knowledge and identification of these deep-sea species.

The EAF-Nansen Programme identification guides to mesopelagic fishes of the southeast Atlantic and western Indian Ocean.

Geographical distribution

This species has been recorded in the Gulf of Aden, but its range may also extend throughout the western Indian Ocean.