Stomiiformes

(Dragonfishes and relatives)

Class Actinopterygii

Order Stomiiformes

Number of families 4

Evolution and systematics

The fossil record of the stomiiform fishes is scant, as is the case with most groups of thin-boned deep-sea pelagic fishes. The oldest known stomiiform-like fossils (Paravinciguerria) date back to the Upper Cretaceous, some 70 million years ago, though critical evaluation has shed some doubt as to whether it is related. Fossil evidence suggests that the hatchetfish family Sternoptychidae arose during the early Tertiary and reached its present evolutionary grade by the mid-Miocene, 16 million years ago. Several other fossils from the Miocene have been placed in modern stomiiform genera. In general, the fossils that have been described and verified as stomiiform-like differed little from recent forms. As such, they have not proven highly useful in determining the origin of stomiiform fishes. In a 1984 treatise on the origin of pelagic fishes, Nikolay Parin suggested that the Stomiiformes, an order of entirely pelagic, open-ocean fishes, arose from fishes living near the bottom of the deep sea. In Interrelationships of Higher Euteleostean Fishes, published in 1973, Donn Rosen stated that the ancestral form of the Stomiiformes is thought to be allied with the salmon-like fishes.

Among living fishes, the Stomiiformes are thought to be most closely related to the "jellynose fishes" of the order Ateleopodiformes. (Ateleopodiformes is a closely related order that has not been resolved completely [Olney et al. 1993], and is not discussed here.) These two orders comprise the Stenopterygii, which is the basal taxon of the Neoteleostei ("new bony fishes"). With respect to the entire spectrum of fishes, the stomiiforms can be thought of as more advanced than eels, herrings, minnows, catfishes, and trout, but more primitive than lanternfishes, cod, bass, and flounders.

Currently, four families are recognized: Gonostomatidae (bristlemouths), Phosichthyidae (lightfishes), Sternoptychidae (marine hatchetfishes), and Stomiidae (dragonfishes and relatives). The classification of families within this order was radically revised in 1985. At that time William Fink combined the 230 or so species of "barbeled stomiiforms," previously recognized in six families, into one large family, the Stomiidae. At the close of the twentieth century, approximately 321 species of stomiiform fishes were described, grouped in two suborders, four families, nine subfamilies, and 51 genera. The majority of the species occur within the Stomiidae, and almost half of these species belong to a single dragonfish genus, Eustomias.

Physical characteristics

The stomiiform fishes are small to moderate in size, ranging from 0.6 in (15 mm) to 20 in (508 mm). The high species diversity of the Stomiiformes is matched only by its morphological diversity. Some species are extremely elongate and slender, while others are deep-bodied and laterally compressed. In one group there are radical differences in the form of males and females (called sexual dimorphism; see Idiacanthus fasciola species account). The order Stomiiformes includes some of the most morphologically specialized bony fishes. Much of this specialization relates to their deep-sea milieu:

• Both food and potential mates are scarce in the vast expanse of the deep sea.
• The interior of the sea is dimly lit or completely dark.
• The structureless pelagic environment provides no refuge from predation.

One of the trademark physical features of many stomiiform fishes is their fearsome dentition; their large mouths are filled with enormous fang-like teeth. This allows them to efficiently capture relatively large prey that are infrequently

encountered. On average, fishes possessing these huge fangs take prey about a third their own size. This would be equivalent to an adult human eating more than 100 hamburgers in a single sitting—and such an equivalent would be an average meal for these fishes! In some cases, the weight of their prey has been known to exceed that of the predator. These fishes have a suite of other adaptations for this type of diet: long, sac-like stomachs; reduced ossification of the anterior vertebrae, allowing the mouth to expand dramatically; and a lack of gill rakers in adults.

Another feature possessed by all but one stomiiform species is the ability to create light (known as bioluminescence) with specialized organs called photophores. Bioluminescence, while rare on land, is quite common in the middle depths of the deep ocean's interior. Bioluminescence results from an oxidative reaction in which an organic molecule, called a luciferin, is raised to a chemically excited state in the presence of the enzyme luciferase. The excited luciferin then decays to the stable state by releasing light (most oxidations release heat). This energy can also be transferred to a fluorescent molecule, which releases light of its own color. Some fishes (e.g., deep-sea anglerfishes) rely on bacterial symbionts for light production. The stomiiforms, however, rely on self-generated, self-regulated luciferin/luciferase reactions occurring within their photophores for light production. The bodies of most stomiiform species bear two serial rows of photophores along each flank. The majority of stomiiform species also bear barbels on their chin. At the end of these barbels are often elaborate, bulb-like bioluminescent organs, thought to serve as "fishing lures." These chin-barbels range in size from less than the head length to as much as 10 times the length of the fish. Many species, particularly those of the genus Eustomias, are differentiated solely on the form of the barbel.

In general, stomiiform fishes fall into three major body plans. Plan A, exhibited by the dragonfishes (Stomiidae), is characterized by an elongate body, usually black or dark brown, with dorsal and anal fins placed far back on the body, the previously mentioned chin-barbel, and large teeth that are hinged to accommodate the passage of large prey items. Plan B, exhibited by the bristlemouths (Gonostomatidae), the lightfishes (Phosichthyidae), and maurolicines (Sternoptychidae: Maurolicinae), is characterized by a moderately elongate, minnow-like body, with dorsal and anal fins near mid-body, and large mouths studded with bristle-like teeth. Plan C, exhibited by most marine hatchetfishes (Sternoptychidae: Sternoptychinae), is characterized by a short, deep, highly compressed body with an abdominal keel that gives the body a hatchet-like shape, silvery flanks with a dark back, and a mouth nearly vertical in position. Some species of the latter body plan have tubular eyes directed upwards.

Distribution

Stomiiform fishes are found over great depths throughout the world's oceans, including the Antarctic seas (but absent in the Arctic Ocean). Highest abundance and diversity is found in tropical seas. Distribution near landmasses is dictated by water depth. Stomiiform fishes are rarely found in waters shallower than 1,650 ft (500 m).

Habitat

Most species are mesopelagic (residing between 660 and 3,300 ft [200 and 1,000 m] deep), while some others are bathypelagic (residing below 3,300 ft [1,000 m] deep). Stomiiform fishes have been recorded to a depth of more than 14,000 ft (4,270 m). Most species undertake a daily vertical migration, swimming from a daytime depth of 1,600 to 3,300 ft (490 to 1,000 m) to near the surface at night, and then back down again before sunrise. This upward migration is thought to be mainly one for feeding—food in the form of plankton, fish, and other invertebrates is much more plentiful near the surface. Daytime depths, on the other hand, provide refuge from visual predators as well as an energy savings in the colder deep water. This vertical migration, undertaken by most mesopelagic fishes, shrimp, and squid, is the largest animal migration on Earth, and it happens every day in the deep sea.

Behavior

Very little is known of the behavior of stomiiform fishes, mainly because the vast majority of species have never been seen alive. Most specimens have been observed only after being caught in trawls, and the combination of physical trauma,

pressure trauma, and temperature shock is usually lethal. The best-known behavior is that of diel vertical migration, and that is indirectly inferred from differences in abundance as a function of depth and time of day. As of 2001, no stomiiform fishes have been kept alive in aquaria for any length of time.

Feeding ecology and diet

For their size, the stomiiforms are voracious predators. Their feeding ecology falls into two main categories. First is predation on other fishes, shrimps, and squid. Within this category, predation on fishes, particularly lanternfishes (see Myctophidae, this volume), overwhelmingly predominates. Most of the larger stomiiform species feed in this manner (e.g., dragonfishes, loosejaws, viperfishes, and snaggletooths of the family Stomiidae). The second category is planktivory (feeding on microscopic organisms). Within this pattern, predation on copepods (small crustaceans less than 1/8 of an inch [3 mm]) predominates. Most of the smaller stomiiform species feed in this manner (e.g., bristlemouths, lightfishes, and hatchetfishes). As a general rule, planktivorous fishes are always more abundant than higher-level predators. It should be noted that even the higher-level predators are planktivorous during their early life stages.

The manner of feeding of planktivorous stomiiforms is similar to that of all planktivorous fishes—these fishes search small volumes of water, snapping up prey within an appropriate size range as they are encountered. The predacious dragonfishes, however, use a novel means of capturing prey. Though it has not been directly observed, it is thought that the barbeled forms use their luminescent lures to attract their prey, thus conserving their own energy in a food-limited environment. The posterior fin position of these fishes resembles that of "lie-in-wait" predators, such as pike (Esocidae). That, combined with the muscular control of the barbel, and the otherwise weak body musculature of these fishes, suggests such a strategy. This strategy is most likely the mechanism that has allowed the stomiids to become the top predators of the mesopelagic zone of the world ocean.

Little is known of the predators of stomiiform fishes. Some of the smaller, planktivorous species, such as Vinciguerria and Maurolicus, are important prey of both pelagic species (e.g., tunas) and outer continental shelf demersal fishes (e.g., Acadian redfish). It is likely that the vast numbers of stomiiform larvae (e.g., Cyclothone, Vinciguerria) are an important food source in the early life stages of many oceanic fishes. Larger stomiiform species (e.g., dragonfishes) have been found in the diets of benthopelagic fishes (e.g., rattails, roughies) as well as epipelagic fishes (e.g., swordfish) and mammals (e.g., Fraser's dolphin).

Reproductive biology

As with behavior, very little is known of the reproduction of stomiiform fishes. Because of the gear used to capture specimens, few sexually mature individuals of most species have been available for study. It has been theorized that larger, sexually mature individuals are more able to avoid midwater trawls than smaller individuals. That which is known suggests that stomiiforms exhibit a wide range of reproductive modes, mirroring their taxonomic and morphological diversity. It is thought that most stomiiforms spawn at their deeper daytime depths. Higher-latitude species appear to have more discrete spawning seasons than do tropical species, which spawn year-round. Some species spawn several times, while others spawn once and die. Some species have separate sexes, while others mature into males, produce sperm to fertilize eggs, and then later develop into females, producing eggs that are fertilized by younger males. In species with separate sexes, males often have greatly developed olfactory (smell) organs to help in locating females.

Conservation status

Degradation of the marine environment is a global phenomenon, but as of 2000 no stomiiform fishes are known to be endangered. No species are on the IUCN Red List.

Significance to humans

Due to their bizarre and fearsome appearance, stomiiform fishes have been depicted in myth, literature, and art. Often the forms are exaggerated for effect, but the similarity to extant species suggests that they are often the inspiration.

No direct fishery exists for stomiiform species, though they are an important component of marine food webs. The main value of the stomiiforms is that they are key trophic mediators in the overall economy of the sea. A few isolated ecosystems notwithstanding, life in the ocean begins near the surface, where microscopic plants turn sunlight and carbon dioxide into organic carbon. The consumption of this "primary production" by small planktic animals (zooplankton) serves as the primary means of transforming plant life into animal life. As stated previously, many stomiiform species consume zooplankton directly, while others are the key predators of fishes that consume zooplankton. Thus, through their own consumption, stomiiform fishes are a key link in the transfer of microscopic organic matter to higher trophic levels. For example, the lightfish Vinciguerria lucetia is one of the most abundant pelagic fishes in the North Pacific and in turn is an important prey item of commercially fished tunas. To emphasize their importance, it has been claimed that the stomiiform fishes are the most abundant vertebrates on Earth.

Species accounts

List of Species

Brauer's bristlemouth

Cyclothone braueri

family

Gonostomatidae

taxonomy

Cyclothone braueri Jesperson and Tåning, 1926, Mediterranean Sea.

other common names

English: Bristlemouth

physical characteristics

Females to 1.5 in (38.1 mm), males to 1 in (25.4 mm); elongate body, with dorsal and anal fins near midbody; scaleless; nearly transparent, save a few scattered black spots on head; very small eyes; relative to its size it has one of the largest gapes (mouth opening) of any fish; teeth small, bristle-like; males have well-developed olfactory organs to locate females.

distribution

Species occurs throughout Atlantic, but confined to a band at 15–50°S in Pacific and Indian Oceans.

habitat

Oceanic and mesopelagic. Depth distribution is age-specific: larvae occur from 30 to 160 ft (9 to 50 m). Adults occur from 825 to 3,000 ft (250 to 900 m) and juveniles in between. This is one of the shallowest living species in an otherwise deep-living genus.

behavior

This is one of the few stomiiforms that does not vertically migrate.

feeding ecology and diet

Small crustaceans, mainly copepods, are the primary prey.

reproductive biology

Thought to spawn once and die; spawning occurs from spring to fall.

conservation status

Not threatened.

significance to humans

There is no commercial significance. It is claimed to be the most abundant vertebrate on Earth.

Silver hatchetfish

Argyropelecus aculeatus

family

Sternoptychidae

taxonomy

Argyropelecus aculeatus Valenciennes, in Cuvier and Valenciennes, 1830, Azores.

other common names

One of many species called "marine hatchetfish."

physical characteristics

Females to 3.3 in (83.8 mm), males to 2.4 in (61 mm); body very deep, laterally compressed, hatchet-like shape derived from abdominal keel-like structure; abdomen bearing well-developed, ventrally directed photophores (these photophores actually account for 21% of the animal's surface area and 15% of its volume); silvery flanks with black back; eyes are tubular and directed upwards; mouth nearly vertical; teeth large, recurved, with two very enlarged canine teeth in lower jaws; scales present, but easily lost.

distribution

This species exhibits an antitropical distribution in the Atlantic, Pacific, and Indian Oceans (i.e., it occurs on each side of the equator, but not directly along the equator). It is much more abundant in the western Pacific than in the east.

habitat

Oceanic, mesopelagic; most abundant between 1,000 and 2,000 ft (300 and 600 m) during daylight, 330 and 1,000 ft (100 and 300 m) at night.

behavior

This species is a strong vertical migrator. It is known to form discrete, well-defined layers within its vertical range, but it does not form schools.

feeding ecology and diet

Feeds mainly at dusk; young fish eat small crustaceans, mainly ostracods, while older fish take larger crustaceans such as euphausiids and decapod shrimp.

reproductive biology

Sexes are separate; thought to spawn once and die; spawning takes place year-round, peaking in summer.

conservation status

Not threatened.

significance to humans

No commercial significance; conspicuous component of the "deep scattering layer" registered by the depth sounders of ocean-going ships.

Viperfish

Chauliodus sloani

family

Stomiidae

taxonomy

Chauliodus sloani Bloch and Schneider, 1801, Gibraltar.

other common names

English: Sloane's viperfish.

physical characteristics

Length can be up to 13.8 in (350.5 mm). The body is very elongate and compressed with a large head. The body is enclosed in a gelatinous sheath. The dorsal fin is well forward on the body, the second ray of which is elongated and thought to serve as a fishing lure. The anal fin is well back, near the caudal fin. There are adipose dorsal and anal fins. The teeth are rigid and large to enormous (so large, in fact, so as not to fit within the confines of the mouth, giving this fish a unique perspective—it sees the world through its teeth). There is iridescent yellowish to blue green on the flanks, with a dark back. There are more than 1,500 photophores on the body of this fish. There are five rows of large scales on each side of the body. Juveniles have a short barbel, which degenerates in adults.

distribution

Worldwide in tropical to temperate seas, including the western Mediterranean; absent in the northern Indian Ocean.

habitat

Oceanic, meso-to bathypelagic; most abundant between 1,650 and 9,200 ft (500 and 2,800 m) during daylight, with some of the population migrating up to between 70 and 660 ft (20 and 200 m) at night. The rest (non-hungry component) stay at daylight depths.

behavior

Nothing known.

feeding ecology and diet

Juveniles eat small crustaceans, mainly euphausiids, while adults eat fishes, mainly lanternfishes, and occasionally decapod shrimp.

reproductive biology

Little is known; spawning takes place year-round with a peak in late winter/early spring.

conservation status

Not threatened.

significance to humans

No commercial significance. Fearsome appearance has inspired depiction in myth, literature, and art.

Scaleless dragonfish

Eustomias schmidti

family

Stomiidae

taxonomy

Eustomias schmidti Regan and Trewavas, 1930, North Atlantic. One of more than 100 species called "scaleless dragonfish."

other common names

None known.

physical characteristics

It is recorded to 8 in (203.2 mm). It is very elongate and slender with a long, protrusible snout, as well as a scaleless body uniformly black to dark brown. There are dorsal and anal fins placed far back on the body, with an anal fin that is twice as long as the dorsal. There are fang-like, depressible teeth. The pectoral fins are encased in a black membrane. The belly bears a deep but short groove. The chin-barbel is present and is about half as long as the head, with a main stem and three branches. The main stem bears a bulbous light organ constricted near the tip.

distribution

Worldwide in tropical to subtropical seas; not known from the northern Indian Ocean.

habitat

It is oceanic, meso-to bathypelagic. It is most abundant between 2,000 and 3,300 ft (600 and 1,000 m) during daylight, with some of the population migrating up to between 330 and 500 ft (100 and 150 m) at night. The rest (non-hungry component?) stay at daylight depths.

behavior

Nothing known.

feeding ecology and diet

Preys on fishes, primarily lanternfishes.

reproductive biology

Nothing known.

conservation status

Not threatened.

significance to humans

No commercial significance, but of particular interest to deep-sea taxonomists. The true number of valid species in this genus may exceed 100.

Black dragonfish

Idiacanthus fasciola

family

Stomiidae

taxonomy

Idiacanthus fasciola Peters, 1877, Pacific Ocean north of New Guinea. One of several species called "black dragonfish."

other common names

None known.

physical characteristics

This is among the most extreme cases of sexual dimorphism known. Females are up to 15 in (381 mm); they are extremely elongate and slender, with a long dorsal fin originating before mid-body and an anal fin about half the length of the dorsal. Pectoral fins are present in the larvae and are absent in adults. Pelvic fins are present; there are fanglike, depressible teeth. A chin-barbel is present with an elaborate leaf-like structure at the tip. Males up are to 2 in (50.8 mm). The body resembles postlarval females. Pectoral fins are present in the larvae and absent in adults. The eyes are larger than those of females. Males lack teeth, internal organs, and pelvic fins. Both sexes are uniformly black to dark brown. Females have three longitudinal rows of white tissue on the flanks. There is extraordinary larval development—the eyes of the larvae are borne on long stalks that retreat as the fish grows until the eyes are nested in sockets.

distribution

There is worldwide but disjunct distribution. It is known from the tropical/subtropical Atlantic and Pacific, the eastern South Atlantic, and the eastern Indian Ocean/Indo-Pacific. It is apparently absent or uncommon in the central Pacific.

habitat

It is oceanic, meso-to bathypelagic. Females occur between 1,800 and 6,600 ft (550 and 2,000 m) during daylight, while migrating up to as shallow as 165 ft (50 m) at night. Males apparently stay between 3,300 and 6,600 ft (1,000 and 2,000 m).

behavior

Females vertically migrate, while males stay deep.

feeding ecology and diet

Females prey on fishes, primarily lanternfishes. Males have no teeth or digestive organs—apparently they do not feed after larval transformation, and may live only a few weeks.

reproductive biology

Little is known; it may spawn in the summer, probably at great depths. Males have enlarged light organs behind the eyes, apparently to attract females.

conservation status

Not threatened.

significance to humans

None known.

Rat-trap fish

Malacosteus niger

family

Stomiidae

taxonomy

Malacosteus niger Ayres, 1848, southeast of Nova Scotia, 42°N–50°W.

other common names

English: Loosejaw.

physical characteristics

It is recorded to 9.5 in (241.3 mm). It is somewhat elongate but deeper bodied than dragonfishes, with dorsal and anal fins well back near the caudal fin. The body is uniformly black. It has an enormous mouth, much longer than the skull. There are four pairs of large fangs in the lower jaw. Like other members of this subfamily, this species has no floor in its mouth (i.e., no ethmoid membrane), hence the common name "loosejaw." The body photophores are very small. The eyes are very large, with a large, deep-red photophore under each eye. There is no chin-barbel.

distribution

Worldwide in tropical to temperate seas; has been taken as far north as Iceland in the North Atlantic, but generally occurs south of 40°N.

habitat

Oceanic, mesopelagic; occurs between 1,650 and 3,000 ft (500 and 900 m) day and night. This species is the only known nonvertically migrating stomiid.

behavior

This is one of the very few animals that can create red light via the large eye photophores, and even more amazing is that it has the ability to see red light (most deep-sea fishes can see only blue/green wavelengths). This confers the ability to search for prey without making itself visible to potential predators. It has been suggested that this fish acquires its red-sensitive visual pigment from the chlorophyll (the green photosynthetic pigment in plants) packaged in the guts of its copepod prey.

feeding ecology and diet

Despite its fearsome appearance, this species mainly eats small crustaceans (copepods), while occasionally taking fish and decapod shrimp.

reproductive biology

Nothing known.

conservation status

Not threatened.

significance to humans

There is no commercial significance, but the protein responsible for its red light production has been investigated for possible medical uses. This protein, if it could be synthesized (this species is rare throughout its range) and attached to an antibody, would provide a means of locating and treating cancerous tumors within a human body without the need for invasive surgery.

Resources

Books

Gartner, J., R. Crabtree, and K. Sulak. "Feeding at Depth." In Deep-Sea Fishes, edited by David Randall and Anthony Farrell. Boston: Academic Press, 1997.

Hoyt, E. Creatures of the Deep: In Search of the Sea's "Monsters" and the World They Live In. Buffalo, NY: Firefly Books, Ltd., 2001.

Periodicals

Fink, W.L. "Phylogenetic Interrelationships of the Stomiid Fishes (Teleostei: Stomiiformes)." Miscellaneous Publications Museum of Zoology, The University of Michigan no. 171(1985): 1–127.

Olney, J.E., G.D. Johnson, and C.C. Baldwin. "Phylogeny of Lampridiform Fishes." Bulletin of Marine Science 52 (1993): 137–169.

Sutton, T.T. and T.L. Hopkins. "Trophic Ecology of the Stomiid (Pisces: Stomiidae) Fish Assemblage of the Eastern Gulf of Mexico: Strategies, Selectivity, and Impact of a Top Mesopelagic Predator Group." Marine Biology 127 (1996): 179–192.

Tracey T. Sutton, PhD