Scaling Down: Myctophid morphology is typified by a slender, compressed body covered in small, silvery deciduous cycloid scales, a large bluntly rounded head, large elliptical to round lateral eyes, and a large terminal mouth with jaws closely set with rows of small teeth. The fins are generally small, with a single high dorsal fin, an adipose fin, and an anal fin—supprted by a cartilaginous plate at its base.
Light my way: In all but one species (Taaningichthys paurolychnus), a number of photophores (light-producing organs) are present; these are paired and concentrated in ventrolateral rows on the body and head. Some may also possess specialised photophores on the caudal peduncle, in proximity to the eyes. The photophores—which emit a weak blue, green, or yellow light—are known to be arranged in species-specific patterns; some may also be sexually dimorphic in nature.
Lanternfish range in size from 3–30 centimetres standard length. Shallow-living species are an iridescent blue to green or silver, while deeper-living species are dark brown to black.
Migration: Lanternfish are well-known for their daily migrations: During daylight hours most species remain within the gloomy bathypelagic zone, between 300–1,200 metres depth; but towards sundown the fish begin to rise upwards into the epipelagic zone, between 10—100 metres depth. The lanternfish are thought to do this in order to avoid predation, and because they are following the daily migrations of zooplankton upon which the lanternfish feed. After a night spent feeding in the surface layers of the water column, the lanternfish begin to descend back into the lightless depths and are gone by daybreak. Most species remain within proximity to the coast, schooling over the continental slope. Different species are known to segregate themselves by depth, forming dense, discrete conspecific layers—this is believed to be a means of avoiding interspecies competition. Due to the lanternfishes' gas bladders, these layers are visible on sonar scans and give the impression of a "false bottom": This is the so-called deep-scattering layer that so perplexed early oceanographers.
It should be noted that there is great variability in migration patterns within the family. Some deeper-living species may not migrate at all, while others may do so only sporadically. Migration patterns may also be dependent on life history stage, sex, latitude, and season.
See the Light: The arrangements of lanternfish photophores are different for each species, so it is assumed that their bioluminescence plays a role in intraspecies communication, specifically in shoaling and courtship behaviour. The concentration of the photophores on the flanks of the fish also indicate the light's use as camouflage: In a strategy termed counterillumination, the lanternfish regulate the brightness of the bluish light emitted by their photophores to match the ambient light level above, effectively masking the lanternfishes' silhouette when viewed from below.
Chain of Foods: A major source of food for many marine animals, lanternfish are an important link in the food chain of many local ecosystems, being heavily preyed upon by cetaceans, including whales and dolphins; large pelagic fish such as tuna and sharks; grenadiers and other deep-sea fish (including other lanternfish); pinnipeds; sea birds, notably penguins; and large squid such as the Jumbo Squid (Dosidicus gigas).
All lanternfish are non-guarding pelagic spawners: That is, they release eggs and milt en masse into the water column, where fertilization takes place. The tiny eggs are made buoyant by lipid droplets; the eggs, and later the hatched larvae (~2.0 millimetres), drift at the mercy of the currents until they have developed. Spawning may continue year-round in some species, but there is a peak during the winter to early spring season.
Study Time: The study of lanternfish embryos is hampered by their extreme fragility: the chorion (outer membrane) often ruptures during sampling. On the other hand, lanternfish are among the best-studied as larvae. Species' larvae can be identified confidently by morphological characteristics (e.g., head, gut, and body shape), and with the exception of Diaphus species, from pigment patterns. Most larvae possess branchiostegal photophores while some species have a fuller compliment. Some species' larvae have eyes situated on stalks, for example the spotted lanternfish.
The Shallow End of the Pool: Lanternfish larvae are generally found in shallower waters than are adults. Larger species, such as the glacier lanternfish (Benthosema glaciale), may live for up to eight years (reaching maturity at 2-3 years). The smaller species, such as the Diogenes lanternfish (Diogenichthys laternatus) tend to have higher resiliences, doubling their populations within 15 months. Larger species may require up to five years to do the same. All have low fecundity, with females producing ca. 100-2,000 eggs per spawn, the number dependent on species and age.
The World Over: Sampling via deep trawling indicates that, together with the bristlemouths (Gonostomatidae) and lightfishes (Phosichthyidae), lanternfish account for as much as 90% of all deep sea fish biomass. Indeed, lanternfish are among the most widely distributed, populous, and diverse of all vertebrates, playing an important ecological role as prey for larger organisms. With an estimated global biomass of 550-660 million metric tonnes, lanternfish also account for much of the biomass responsible for the deep scattering layer of the world's oceans. In the Southern Ocean, Myctophids provide an alternative food resource to Krill for predators such as squid and the King Penguin. Although plentiful and prolific, currently only a few commercial lanternfish fisheries exist: These include limited operations off South Africa, in the subantarctic, and in the Gulf of Oman.
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