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LANTERNFISHES IN GENERAL
Lanternfishes are the most widely distributed, the most species diverse and the most abundant of all deep-sea pelagic fishes. As such, they not only play an important role in the cycling of energy in the oceanic food-web, but also represent a potential resource for commercial exploitation. Their allies, the neoscopelids, are less abundant. They occur over the continental and island slope regions in tropical and subtropical waters.
So-called because of the kidney-shaped light organs (photophores) which occur on their heads and bodies, lanternfishes are small fishes with large eyes, a single dorsal fin, an adipose dorsal fin and with pectoral fins situated well in front of the ventral fins. A rudimentary spine precedes the dorsal, anal and ventral fin rays. The mouth is terminal (subterminal in some genera) and the jaws extend behind the posterior margin of the eye. The upper and lower jaws are formed by the premaxillae and dentaries respectively. The jaws are armed with bands of small, closely-set teeth, the inner series of which may be enlarged in some species (Diaphus fragilis). Cycloid or ctenoid scales cover the body, and freshly-caught specimens of heavily-scaled species may be iridescent blue, green or silver in colour. Deeper-living species with deciduous scales tend to be grey or dark brown.
Photophores are absent in Taaningichthys paurolychnus. They are present in all other lanternfishes and are arranged in distinct groups. This not only provides scientists with a reliable character for species identification, but suggests that the unique pattern in each species may serve as an identifying signal in shoaling behaviour. However, the arrangement of an almost continuous ventral series of photophore groups also suggests that they play a role in camouflaging the fish's silhouette against ambient light from above. In the genus Diaphus, two pairs of light organs situated immediately in front of the eyes are well-developed and presumably act as headlights for illuminating prey. Other light-producing structures may include minute "secondary" photophores which cover the head and body, and more-diffuse patches of luminous tissue at the bases of the dorsal, anal and ventral fins. In addition, luminous glands may be found on the dorsal and ventral surfaces of the caudal peduncle. The structure of these glands varies in complexity, ranging from a single (sometimes black-bordered) organ to a series of overlapping luminous scales. Typically males possess a supracaudal and females an infracaudal gland, although in some species (Lampanyctus, Lampadena spp) males and females possess both glands. It has been postulated that the flashing of these caudal glands during an escape reaction may confuse a predator. Blue, green or yellow light is emitted from the light organs and photophores, following nervous stimulation of a chemical reaction within the light-producing cells (photocytes) of these structures. Each photophore is overlain by a modified scale which acts as a lens to focus the light.
Planktonic crustaceans (copepods, euphausiids, amphipods, ostracods and small decapods) are the main food items of lanternfishes. In temperate latitudes where there is seasonal variation in food abundance, energy reserves are laid down in the form of fats (lipids and wax esters) in the body tissue and swim bladder. The reserves may be used for egg production during the late winter-early spring spawning period, as well as providing an additional buoyancy mechanism to the swim bladder. Lanternfishes are preyed on by tuna, bonito, albacore, dolphin fishes, hakes, horse mackerel, kingklip, snoek and large squids. Many species of sea-birds (particularly penguins), seals and cetaceans take myctophids opportunistically in addition to their staple diet.
Myctophids are essentially an oceanic pelagic family, occurring from Arctic to Antarctic waters. Species distributions can be correlated with both ocean currents and the physical-biological characteristics of the sea water. By day, lanternfishes live in depths between 150 and 2 000 metres depending on their size and sexual maturity . Mesopelagic species migrate vertically at night to feed in the upper 50 metres; some may be dip-netted at the surface of the sea on moonless nights. However, this daily migration is not undertaken by all fishes. Even within a single species, the behaviour may vary with latitude, season, sex and stage in the life-cycle. Bathypelagic species apparently do not migrate vertically.
As adults, some larger species (Notoscopelus kroeyerii, Gymnoscopelus piabilis, Gymnoscopelus bolini) live a few metres above the bottom, especially over continental slope regions. Here, their shoreward penetration appears to be limited by both water depth and water temperature, although their planktonic larvae can be found further inshore. A few species, like Lampanyctodes hectoris, that have been derived from oceanic ancestors, occur above the continental shelf itself and are termed pseudoceanic species. Their close proximity to the coast, coupled with their shoaling behaviour, makes them suitable targets for commercial fishing.
Lanternfishes comprise 60-70% of all mesopelagic fishes and estimates of their global stock size vary between 550-million and 660-million tonnes. Acoustic surveys near Shag Rocks (western South Atlantic) suggest a stock of 1.2-million tonnes of Electrona carlsbergi, while a value of 100-million tonnes has been obtained for Benthosema pterotum in the northern and western Arabian Gulf. Recent acoustic investigations off the west coast of South Africa have demonstrated the existence of large shoals of subantarctic species along the shelf break, with biomass values up to 33 tonnes.km-2 for Symbolophorus boops and 7 tonnes.km-2 for Diaphus hudsoni. A single haul off Argentina yielded 30 tonnes of Diaphus dumerilii in one hour.
Limited commercial exploitation occurs off South Africa, where annual purse-seine landings (mainly Lampanyctodes hectoris) have fluctuated between 100 and 42 400 tonnes. The lanternfishes are reduced to fish meal and fish oil. Because of their high oil-content, reduction plants are forced to mix lanternfish with other species to prevent clogging of the machinery. Around South Georgia and Shag Rocks, experimental fishing on Electrona carlsbergi (mainly juveniles) averaged about 20 000 tonnes per year between 1988 and 1990, but increased to 78 488 tonnes in 1991. The Commission for the Conservation of Antarctic Marine Living Resources has therefore introduced a 200 000 tonne TAC (total allowable catch) for the 1992 season. Specimens of Gymnoscopelus bolini and Gymnoscopelus nicholsi, caught as a by-catch in the Antarctic marbled rockcod fishery that was operational in the 1970's, were smoked for human consumption.P Alexander Hulley
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