Diogenes lanternfish

Diogenichthys laternatus is a lanternfish (family Myctophidae) of oceanic waters, and its accepted name and authority are given in the World Register of Marine Species ^[1]. For Peru it is recorded as «linterna de Diógenes» ^[2].

It is an abundant mesopelagic fish of the central and eastern Pacific, and its larvae dominate the fish-larva assemblages of the eastern tropical Pacific off Mexico, Ecuador, Peru and Chile ^[3]. Off Peru it is prey of the jumbo squid Dosidicus gigas ^[4].

It is a fusiform fish; the FishBase record gives 10 to 13 soft rays in the dorsal fin, 15 to 17 in the anal fin and 29 to 32 vertebrae ^[2].

Its larvae are recognised by elliptical eyes, a head of moderate size and one melanophore on the trunk, above the pre-anal arch of the gut ^[3].

It is an oceanic species of tropical and subtropical affinity, very abundant and widely distributed in the eastern Pacific ^[5]. FishBase places it in the Pacific from California and Mexico to Chile, and also in the Atlantic and Indian oceans, with a depth range of 0 to 2091 m ^[2].

A postflexion specimen was recorded in the Clarion-Clipperton Zone, in a sampling with a remotely operated vehicle whose noted depth range was 75 to 1500 m, on 6 December 2021 ^[3]. Off Chile, in 1997, its larvae were studied in an upwelling area at 23° S and 71° W ^[6].

According to FishBase it is oviparous, with planktonic eggs and larvae, and it stays between 0 and 100 m at night while it descends to 600-650 m by day ^[2].

In an upwelling area of northern Chile, the larvae ate during the hours of light and in the layer where microplankton was most abundant; they chose mainly copepods and nauplii, and their diet was more diverse than that of Triphoturus mexicanus (cited as aff. oculeus), another larva in the same study ^[6]. They show hooked teeth and pharyngeal structures, and the authors speak of opportunistic feeding ^[6]. Sampling covered coastal and oceanic areas, in summer and winter of 1997; the diets of the two larvae were more alike where food was more plentiful and diverged where it was scarce ^[6].

In south-east Pacific larvae, compared with those of its congener Diogenichthys atlanticus, the larvae of Diogenichthys laternatus had a narrower trophic niche, with stomach contents usually highly digested, and the two species did not share diet; in this species, a shorter snout and an eye displaced backwards were related to that digested content ^[7].

In the eastern tropical Pacific, this species and Vinciguerria lucetia often dominate the fish larvae caught in plankton nets, and their abundance changes with the seasons ^[3].

In the southern Gulf of California, in July 2010, 5659 fish larvae were identified, of which 704 were Diogenichthys laternatus; the larvae were found mostly below the thermocline, while those of Vinciguerria lucetia were abundant in and above the thermocline ^[5].

Inside a cyclonic eddy about 60 km across, the upwelling coincided with more Diogenichthys laternatus larvae throughout the water column ^[5]. The authors also cite a study by Rojas and Landaeta (2014) in the Humboldt Current, where the larvae of both species were found associated with the thermocline ^[5].

Off Peru, in 2007 and 2008, 937 jumbo squid were examined; Diogenichthys laternatus appeared in their stomachs only in the 2008 samples, in 158 stomachs from June-July and 37 from November-December ^[4].

The IUCN Red List classifies Diogenichthys laternatus globally as Least Concern (LC), in an assessment of 26 June 2018 ^[8],[2].

Its documented relevance here is as prey. In the Humboldt Current system off Peru, the jumbo squid Dosidicus gigas fed mostly on mesopelagic fishes, among them Diogenichthys laternatus, besides Vinciguerria lucetia ^[4].

According to the authors of a study in the Gulf of California, the three-dimensional distribution of its larvae can indicate mesoscale processes; more larvae above the thermocline could point to upwelling or pumping in cyclonic eddies ^[5].