Deep Sea: the Twilight Zone and Beyond
A smallish, bioluminescent bottom-dweller of upper-middle slopes, the Velvetbelly Lanternshark (Etmopterus spinax) represents yet another strategy for earning a living in the deep-sea. Typically inhabiting depths of 650 to 2,450 feet (200 to 750 metres), this species is fantastically abundant in the western Mediterranean and eastern North Atlantic, from subarctic Norway to tropical Gabon. The Velvetbelly’s geographic range and abundance attests to the successfulness of its adaptation to deep-sea life.
Just the Facts:
Habitat: Rocky Reefs, Deep Sea, Polar Sea
Depth: 230-6,560 ft (70-2,000 m)
Distribution: Arctic, Eastern North Atlantic/Mediterranean, West African, Southern African
Although originally described by Linnaeus in 1758, little is known about the Velvetbelly Lanternshark and much must be inferred from other members of its genus. The genus Etmopterus consists of at least 28 described species, including the Dwarf Lanternshark (E. perryi) from the Caribbean, which — maturing at a length of 6 to 7.5 inches (16 to 19 centimetres) — is probably the world’s smallest shark. The lanternsharks also include the Brown Lanternshark (E. unicolor), — a Japanese species which exhibits a high incidence (23%) of hermaphrodism (possibly facilitating mating any time two consenting adult Brown Lanternsharks happen to meet) — and the Green Lanternshark (E. virens) of the Gulf of Mexico and Caribbean — which hunts in packs, attacking and consuming squids many times larger than themselves. Unfortunately, no one knows whether Velvetbellies employ hermaphrodism or cooperative hunting, but what little we do know about them reveals much about the challenges imposed by the deep-sea environment.
One of the harshest physical challenges imposed by the deep-sea environment is relentless cold. Like other sharks, the Velvetbelly relies on a wide variety of protein catalysts called “enzymes” to help carry out certain vital metabolic processes. Most enzymes are rather large, clunky molecules whose stability and functionality are restricted to very narrow temperature ranges. In the Velvetbelly Lanternshark, however, an enzyme important in the splitting of dietary fats and oils has been modified so it remains stable and functional over a wide range of temperatures. Other enzymes, such as those involved in mediating contraction of swimming muscles, are undoubtedly also modified to withstand the insults of extreme cold.
The deep-sea is not only cold but also dark, making it difficult to remain with others of one’s own kind. Like many other deep-sea sharks, the Velvetbelly is bioluminescent. This “cold light” is produced by a 98% efficient chemical reaction between a protein (luciferin) and an enzyme (luciferase) in the presence of oxygen. This reaction takes place within special light-producing organs called “photophores”. To a greater or lesser extent, each species of Etmopterus has its own, species-specific pattern of photophores. For example, the Velvetbelly Lanternshark’s photophores are arranged in a distinct pattern: running along the lateral line of each flank, scattered under the snout and chin (but not around the mouth), and distributed over the belly; they are especially densely distributed above and below the pelvic fins and on the ventral surface of the tail stalk. No other species of Etmopterus has a photophore pattern exactly like that of the Velvetbelly. The main function of this species-specific pattern of photophores seems to be visual coordination of schools, so that Velvetbelly’s can find mates, evade predators, or attack prey as a group.
The Velvetbelly Lanternshark feeds on small animals on or near the sea bed. Its diet has been most extensively studied in the Rockall Trough, west of Ireland and Scotland, and in the western Mediterranean. The Velvetbelly’s most important prey types include deep-sea krill (Euphausiidae), shrimps (Pasiphaeidae), cuttlefishes (Sepiolidae), squids (Ommastrephidae and others), and a wide range of small teleosts, including shads (Sternoptychidae), barracudinas (Paralepidididae), lanternfishes (Myctophidae), and pouts (Gadidae).
In the Rockall Trough, Velvetbellies undergo a dietary shift as they increase in size. Individuals between 5 and 11 inches (12 and 27 centimetres) in length feed heavily on a euphausiid krill known as Meganyctiphanes norvegica and a teleost known locally as the Sheppy Argentine (Maurolicus muelleri). But these prey are not eaten by those Sharks between 15 and 21 inches (39 and 53 centimetres). Instead, large Velvetbellies of the Rockall Trough feed primarily on a pasiphaeid shrimp, Pasiphaea tarda, and various deep-sea squids. The euphausiid krill and Sheppy Argentine are not found at depths greater than about 1,600 feet (500 metres) but the pasiphaeid shrimp and deep-sea squids are found both shallower and much deeper. Since larger Velvetbelly Lanternsharks in the Trough are caught primarily at depths greater than 1,600 feet (500 metres), this dietary shift may be a way that adults of this species avoid competing against their own young for food.
Competition with other sharks may limit the depths at which the Velvetbelly Lanternshark feeds. One study conducted in Japanese waters indicated that the Brown Lanternshark, Blackbelly Lanternshark (Etmopterus lucifer), and the Whitefin Dogfish (Centroscyllium ritteri) feed on different prey at different depths, which may reduce the intensity of competition among them. But what if another shark feeds on more-or-less the same kind of prey as the Velvetbelly?
In the depths of the Catalan Sea (in the western Mediterranean), the Velvetbelly is the third most abundant type of shark, after the Portuguese Dogfish (Centroscymnus coelolepis) and the Black-Mouthed Catshark (Galeus melastomus). The diet of the Black-Mouthed Catshark is quite diverse and does not overlap that of the other two sharks. But the diets of large Velvetbellies and the Portuguese Dogfish are both very squid-intensive — indeed, the Portuguese Dogfish eats little else. However, unlike the Velvetbelly Lanternshark, the Portuguese Dogfish is an extremely deep-dwelling species — it is, in fact, the record holder for the greatest known depth attained by any shark, having been caught at an incredible 12,057 feet (3,675 metres). Although competition between species is often portrayed as a kind of head-to-head “may the best competitor win” sort of affair, in nature interspecific competition is often subtler in practice: the competitor who can shift to a different niche does. Therefore, although it feeds on similar prey, the Portuguese Dogfish may reduce competition with the Velvetbelly Lanternshark simply by feeding at greater depths.
But not all deep-sea dangers can be avoided so simply. One of the fiercest predators of Velvetbelly Lanternsharks is the Sharpnose Skate (Dipturus oxyrhynchus). Despite their best efforts at maintaining safety by schooling, significant numbers of Velvetbellies are consumed each year by this pancake-shaped relative. One wonders whether the same pattern of photophores that enables Velvetbelly Lanternsharks to visually coordinate in the deep-sea might also betray them to a hunting skate.
Toxic heavy metal compounds are another danger, occurring in the sea both naturally and due to human activities. Due to concentration in the tissues of prey animals and in bottom sediments, metallic compounds can accumulate to dangerous levels in the tissues of Velvetbelly Lanternsharks. Like some sharks, Velvetbellies have a special lymphoid organ wrapped around the esophagus called “Leydig’s gland”, after the German histologist who first described it in 1857. Leydig’s gland forms part of the Velvetbelly’s immune system, manufacturing T-cells that may ‘flag’ heavy metal contaminants in the bloodstream and thus mark them for destruction or elimination. A recent study also demonstrated that the Velvetbelly possesses special proteins in its liver that may help detoxify such metallic pollutants as cadmium, copper, mercury, and zinc. Many of these metals are known to interfere with shark endocrinology, sperm production and motility. Therefore, the ability of the Velvetbelly Lanternshark to detoxify these pollutants is important to its species’ survival.
The deep-sea is a harsh environment, imposing upon all who live there conditions of intense cold, perpetual dark, inter- and intra-specific competition, risk of predation and pollutants. Yet the little Velvetbelly has evolved adaptations to deal with every one of these environmental challenges. In its own way, the Velvetbelly Lanternshark is every bit as impressive as the bizarre Goblin Shark (Mitsukurina owstoni) or the spectacular and celebrated White Shark (Carcharodon carcharias).