Lanternfish

Myctophum nitidulum is a small lanternfish, up to 8.3 cm in standard length, that lives in tropical and subtropical waters of almost every ocean ^[1]. It belongs to the myctophids, deep-water fishes that rise toward the surface at night ^[2].

In Peru it is known mainly from its larvae, found on the 2015 surveys of the Instituto del Mar del Perú, and adults have been caught in the southeastern Pacific off northern Chile ^[3],[4].

It has cycloid scales, that is, without a toothed edge, and shows no spines in the dorsal or anal fins ^[1]. The dorsal fin has 12 to 14 soft rays and the anal fin 18 to 21, and the backbone has between 36 and 39 vertebrae ^[1].

Within its genus, the angular, unserrated rear-upper margin of the gill cover separates it from the other Myctophum species with cycloid scales ^[1]. Males carry 5 to 8 luminous spots on the supracaudal gland and females 2 to 6 small ones on the infracaudal gland ^[1].

It is small: the largest specimen of the Chilean survey measured 85 mm in standard length, somewhat above the 75 mm that had been considered the maximum ^[4]. Females mature at about 4.8 cm and males at about 3.5 cm ^[1].

It is a circumglobal species of tropical and subtropical waters. In the Pacific it ranges from about 32° N to about 31° S, and reaches about 40° N with the Kuroshio Current ^[1]. It is recorded between 412 and 1537 m depth, although at night it rises toward shallower water ^[1].

In the southeastern Pacific, a Chilean survey using a midwater trawl caught the species in summer north of 23° S, and with a surface net as far as 26°30' S, between 70°30' and 73° W ^[4]. The author places it among the species that extend southward in summer within the Peru-northern Chile district, whose southern limit lies near 24° S ^[4].

Nearly all myctophid specimens on that survey were caught in night-time sampling ^[4].

It is a vertical migrant that rises into the surface layer at night; in the Kuroshio, specimens of 50.3 to 78.8 mm standard length were taken in night tows of a surface net, less than a metre below the water line ^[2]. In 10 of the 18 years studied (before 1975 and after 1991) it fed mainly on small crustaceans such as euphausiids, copepods and amphipods ^[2].

Its feeding follows a night-time rhythm, with peaks between 20:00 and 21:00 h and between 03:00 and 04:00 h, and a daily ration of between 5.2% and 7.8% of its wet weight was estimated ^[2].

It is oviparous, and its eggs and larvae are planktonic ^[1]. Larvae appear in zooplankton samples taken down to 50 m depth off Peru ^[3].

In the Kuroshio, over the years 1976 to 1990, Japanese sardine larvae dominated its diet in 7 of 8 years studied, between 20.2% and 61.8% of the weight of stomach contents ^[2]. Before 1975 and after 1991 small crustaceans dominated, so its diet shifted with the prey available ^[2].

Surface-migrating myctophids can thus prey on small pelagic fishes in their larval stage ^[2]. Off Peru its larvae made up only 0.2% of total ichthyoplankton abundance in summer-autumn 2015, in a community dominated by the anchoveta ^[3].

In winter-spring 2015 larvae of this species were still among the myctophid larvae recorded, although at densities of only 3 to 6 larvae per m2 ^[5].

The International Union for Conservation of Nature classes it as Least Concern (LC), under an assessment from 2012 ^[6],[1]. No specific threats are known in the sources consulted, and they give no national category for Peru ^[1].

It has no known commercial value in the sources reviewed; its importance is ecological. As both prey and predator of larvae, it forms part of the food web of the epipelagic zone and can affect small pelagic fishes such as the sardine ^[2].

In the Kuroshio, one study estimated that the species consumed 24 to 52 billion sardine larvae per night between January and March of the years 1976 to 1990, an estimate its authors regard as conservative ^[2].

Adults live in open water and come to the surface only at night ^[2], and there are no known viewing sites on the Peruvian coast. The Peruvian data come from plankton hauls: larvae were recorded in 2015 mainly in the northern zone, between Punta Sal and Paita, during the summer-autumn cruise, from samples taken at 50 m depth or less ^[3].

On the August to October cruise of the same year, which covered the sea from Punta Sal to Morro Sama, its larvae were recorded at densities of 3 to 6 larvae per m2 ^[5].