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Description

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Diagnosis.—Species of Malacosteus differs from species of all other genera of the family Stomiidae (sensu Fink, 1985) in having one round nostril on each side of head anterior to eye; palatine bones unossified and lacking teeth; margin of anterodorsal portion of neurocranium convex; a median toothplate associated with fourth basibranchial, and a single large (4.2–7.6% SL) tear-shaped accessory light organ (AO) along anteroventral margin of eye (Fig. 1). It differs further in having the following combination of character states: hyoid barbel and skin between mandibular rami absent, with only protractor hyoideus muscle linking mandibular symphysis to hyoid; jaws enormous (UJL 23.4–32.7 %SL, LJL 22.3–30.0% SL), extending far posterior to fleshy orbit; all jaw teeth fixed and not depressible (Type 1 attachment; Fink, 1981); vomerine teeth absent; maxillary teeth minute, closely and regularly set; lateral ethmoid, parietal, mesopterygoid, and posttemporal absent; eyes directed slightly anteriorly; lateral and ventral photophore series clustered, reduced in size, those of IP series in two distinct rows.

Description.—Body elongate, depth tapering slightly from cleithrum (9.6–18.0% SL) to anal-fin origin (5.9–10.7% SL), and more abruptly toward caudal peduncle (1.6–3.0% SL). Head small (7.8–12.3% SL). Eye large (EW 4.3–6.9% SL). Snout blunt and short. Opercular flap long and sloping posteroventrally to posterior tip of jaw. A single round nostril on snout facing anterolaterally. Tip of basihyal with a white, fleshy, finger-like tab. Branchiostegal rays 7–10.

Dentary teeth on left side 18–53, variable in size and decreasing in number with SL; anteriormost tooth minute, next to mandibular symphysis, followed by a large (7.5–19.2 %UJL), barbed, sharply curved, fang-like tooth emanating from dorsolateral margin of mandible; fang-like tooth followed by several large (2.1–7.5% SL), slightly curved, barbed, knife-shaped teeth; middle third of jaw populated by small barbed teeth of subequal size, some directed anterodorsally. Premaxillary teeth less numerous (12–53), small to moderate, slightly curved, often produced in two rows, a ventral row positioned along the ventral margin of the premaxilla and a dorsolateral row positioned along the anterolateral margin of the premaxilla; anteriormost tooth of the dorsolateral row barbed and usually isolated; posterior teeth of the ventral row often directed anteroventrally. Maxillary teeth minute, numerous. Palatine teeth absent. Basibranchial tooth patches placed dorsally in four groups, anteriormost three groups as bilateral patches associated with basibranchials 1, 2, and 3; posteriormost group associated with basibranchial 4, produced either as a pair of patches or as a singular, often laterally offset patch; 1–6 teeth per patch. Pharyngobranchial teeth in two groups on each side.

Pectoral fin moderately elongate (10.3–23.8% SL); pectoral-fin rays [2]3–4[5–6]; pelvic fin moderately elongate (14.5–25.7% SL), placed at about mid-body, prepelvic length 54.0–62.2 % SL; pelvic-fin rays 6. Dorsal and anal fins placed far back on body and nearly opposite, predorsal length 76.2–84.1 % SL, preanal length 74.1–83.2 % SL. Dorsal- and anal-fin bases subequal (11.7–17.4 and 12.2–18.5% SL, respectively). Dorsal-fin rays [16–17]18–20[21], anal-fin rays [18]19–22[23–24]. Caudal fin small and emarginate, lower lobe longer than upper. Total vertebrae 45–51.

Three cephalic photophores near orbit: AO, PO, and SO (Fig. 1). AO large (15.3–27.2% UJL, 4.2–7.6% SL), subequal to orbit in specimens larger than 50 mm (79.1–126.8%), relatively larger than orbit in smaller specimens (110.6–144.7%); teardrop-shaped and positioned at anteroventral edge of fleshy orbit. PO small to moderate (2.5–11.8 % UJL, 0.6–3.1% SL), an ovoid body posterior to fleshy orbit, ventral edge aligned with ventral margin of fleshy orbit, center aligned about one orbit-width posterior to posterior margin of eye; covered by a transparent membrane; size varying with sex and between species, larger in males, smaller in females. SO minute, round, situated on posteroventral margin in an elongate pocket with a dorsomedial opening. Small photophores and unorganized areas of accessory white luminous tissue scattered over head and body, often arranged around lateral photophores or along dark vertical lines composed of minute photophore-like structures. Ventrolateral and opercular photophores glowing blue in fresh specimens.

Lateral and ventral photophore series present, arranged in discrete groups (e.g., IP, PV, VAV, etc.); each group often containing one to several clusters of photophores. Lateral series low on body extending posteriorly from a position just anterior to level of posterior angle of jaw (when closed) to just anterior to origin of anal fin; ventral series extending posteriorly from anterior tip of isthmus along ventral midline to insertion of anal fin. IPp [3]4–7[8], IPc 3–7 clusters, arranged in off set rows; PVp [3]4–7[8–9], PVc 2–6 ; VAVp [2–3]4– 6[7], VAVc 2–4. OVp [3–4]5–9[10–11], OVc 3–6, arched anteroventrally; VALp 2–6, VALc 1–4; ACp [0]1–3[4], ACc 1–4, rarely absent, with several subcutaneous photophores anterior to visible group or clusters. A single small photophore placed approximately at mid-point of opercular flap on posterior margin of operculum.

Skin thin, scaleless; color of fresh specimens black, fading to dark brown after lengthy preservation. Size moderate, up to 253 mm.

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Stoplight loosejaw

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Northern stoplight loosejaw, Malacosteus niger, caught off Newfoundland

The stoplight loosejaws are small, deep-sea dragonfishes of the genus Malacosteus, classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of the Arctic and Subantarctic, in the mesopelagic zone below a depth of 500 meters (1,600 feet). This genus once contained three nominal species: M. niger (the type), M. choristodactylus, and M. danae, with the validity of the latter two species being challenged by different authors at various times. In 2007, Kenaley examined over 450 stoplight loosejaw specimens and revised the genus to contain two species, M. niger and the new M. australis.[1]

Malacosteus and the related genera Aristostomias, Chirostomias and Pachystomias are the only fishes that produce red bioluminescence. As most of their prey organisms are not capable of perceiving light at those wavelengths, this allows Malacosteus to hunt with an essentially invisible beam of light. Furthermore, Malacosteus is unique amongst animals in using a chlorophyll derivative to perceive red light.[1] The name Malacosteus is derived from the Greek malakos meaning "soft" and osteon meaning "bone".[2] Another common name for these fishes is "rat-trap fish", from the unusual open structure of their jaws.[3]

Species

There are currently two recognized species in this genus:[4]

Distribution and habitat

These fishes have a wide distribution in all oceans: M. niger is found between 66° N and 33° S, except for the Mediterranean Sea, while M. australis is found in the southern transition zone between 25° and 45° S, where it is bound by the Antarctic Circumpolar Current. M. niger appears to be replaced by M. australis south of 30° S, while M. australis does not occur north of that latitude outside of the Indian Ocean and the Indo-Australian Archipelago. Both species are usually found below a depth of 500 meters (1,600 feet) in midwater. They are the only known stomiids that do not seem to conduct significant diel vertical migrations.[1]

Description

Closeup of jaw

Malacosteus has an elongated body with short, blunt snouts and large eyes that face forward, granting binocular vision. Unlike other stomiids, it has a single round nostril on each side in front of the eye. Relative to its size, Malacosteus has one of the widest gapes of any fish, with a lower jaw measuring one-quarter of the fish's length. The lower jaw has no ethmoid membrane (floor) and is attached only by the hinge and a modified tongue bone. There are several large, fang-like teeth in the front of the jaws, followed by many small barbed teeth. There are several groups of pharyngeal teeth that serve to direct food down the esophagus.[1][5]

The pectoral and pelvic fins are moderately long, containing 3–4 and 6 fin rays respectively. The dorsal and anal fins are placed far back on the body and contain 18–20 and 19–22 rays respectively. The caudal fin is small, with the lower lobe larger than the upper. There are three bioluminescent photophores near the eyes: beneath the eye is a large, teardrop-shaped suborbital photophore that emits red light. Behind it is an ovoid postorbital photophore that emits green light; this photophore is larger in males than females. These red and green photophores are evocative of traffic lights, hence the fish's common name. The third is tiny and round, located between the eye and the large red photophore. Several rows and clusters of blue photophores are present on the sides and belly. In addition, there are small photophores and accessory areas of white luminous tissue scattered over the head and body. The skin is thin and scaleless; the coloration is black.[1]

Biology and ecology

Malacosteus is thought to use its suborbital photophores like searchlights to find prey.

As long wavelengths of light (i.e. red) do not reach the deep sea from the surface, many deep-sea organisms are insensitive to red wavelengths, and so to these creatures, red-colored objects appear black. The red photophore of Malacosteus thus allows it to illuminate prey without being detected. These fishes exhibit a number of adaptations for feeding on large prey. The "open" structure of its jaws grants the fish the ability to swing its entire head forward like an arm to grab food in addition to reducing water resistance, allowing them to be snapped shut more quickly, while large recurved teeth and powerful jaw muscles assure a secure hold on prey items. The connection between the head and the body is reduced, with unossified vertebrae, allowing the cranium to be tilted back and the jaws thrust forward for a wider gape. Finally, the gills are exposed to the outside, allowing the fish to continue respiring while slowly swallowing large prey.[6]

However, contrary to its apparent morphological specialization, the diet of Malacosteus consists primarily of zooplankton, chiefly large calanoid copepods, with smaller numbers of krill, shrimps, and fishes. It is yet unclear how Malacosteus captures such small planktonic prey given the open structure of its mouth.[5] The unexpected diet of Malacosteus is theorized to be a result of the small volumes that it searches for food, in which large prey items are rare. The rapid attenuation of red light in sea water gives Malacosteus a shorter visual range than species that use blue light, and it does not migrate vertically into more productive waters like other stomiids. Therefore, its strategy may be one of "snacking" on copepods, which are three orders of magnitude more abundant than fishes at its native depths, in between larger meals.[5]

The red photophore of Malacosteus.

The other factor believed to be partly responsible for Malacosteus' diet is its unique visual system, which uses a derivative of chlorophyll as a photosensitizer that absorbs long-wave light (around 700 nm) and then indirectly stimulates the fish's two visual pigments, which have maximum absorbances at only 520 and 540 nm. No vertebrates are known to synthesize chlorophyll derivatives, and Malacosteus is believed to obtain these derivatives from the copepods it consumes.[7] The red photophore of Malacosteus consists of a pigmented sac with a reflective inner lining and an internal mass of gland cells. Inside the gland cells, blue-green light is produced via the same chemical reaction found in other stomiids, which is then absorbed by a protein that fluoresces in a broad red band. This light is then reflected out through the photophore aperture, where it passes through a brown filter, yielding a far-red light with a maximum absorbance at 708 nm (almost infrared). In live fish, the suborbital and postorbital photophores both flash vigorously, the suborbital at a slower rate.[8][9]

References

  1. ^ a b c d e Kenaley, C.P. (2007). "Revision of the Stoplight Loosejaw Genus Malacosteus (Teleostei: Stomiidae: Malacosteinae), with Description of a New Species from the Temperate Southern Hemisphere and Indian Ocean". Copeia. 2007 (4): 886–900. doi:10.1643/0045-8511(2007)7[886:ROTSLG]2.0.CO;2.
  2. ^ Hunter, R.; Williams, J.A. & Herrtage, S.J.H. (1897). The American Encyclopaedic Dictionary. R.S. Peale and J.A. Hill.
  3. ^ Ellis, R. (1996). Deep Atlantic: Life, Death, and Exploration in the Abyss. Alfred A. Knopf. ISBN 0-679-43324-4.
  4. ^ Froese, Rainer and Pauly, Daniel, eds. (2012). Species of Malacosteus in FishBase. February 2012 version.
  5. ^ a b c Sutton, T.T. (November 2005). "Trophic ecology of the deep-sea fish Malacosteus niger (Pisces: Stomiidae): An enigmatic feeding ecology to facilitate a unique visual system?". Deep-Sea Research Part I: Oceanographic Research Papers. 52 (11): 2065–2076. Bibcode:2005DSRI...52.2065S. doi:10.1016/j.dsr.2005.06.011.
  6. ^ Jones, A.M. (1997). Environmental Biology. Routledge. ISBN 0-415-13620-2.
  7. ^ Douglas, R.H.; Mullineaux, C.W. & Partridge, C.J. (September 29, 2000). "Long-Wave Sensitivity in Deep-Sea Stomiid Dragonfish with Far-Red Bioluminescence: Evidence for a Dietary Origin of the Chlorophyll-Derived Retinal Photosensitizer of Malacosteus niger". Philosophical Transactions: Biological Sciences. 355 (1401): 1269–1272. doi:10.1098/rstb.2000.0681. PMC 1692851. PMID 11079412.
  8. ^ Herring, P.J. (2002). The Biology of the Deep Ocean. Oxford University Press. ISBN 0-19-854956-3.
  9. ^ Herring, P.J. & Cope, A. (December 2005). "Red bioluminescence in fishes: on the suborbital photophores of Malacosteus, Pachystomias and Aristostomias". Marine Biology. 148 (2): 383–394. doi:10.1007/s00227-005-0085-3. S2CID 86463272.
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Stoplight loosejaw: Brief Summary

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Northern stoplight loosejaw, Malacosteus niger, caught off Newfoundland

The stoplight loosejaws are small, deep-sea dragonfishes of the genus Malacosteus, classified either within the subfamily Malacosteinae of the family Stomiidae, or in the separate family Malacosteidae. They are found worldwide, outside of the Arctic and Subantarctic, in the mesopelagic zone below a depth of 500 meters (1,600 feet). This genus once contained three nominal species: M. niger (the type), M. choristodactylus, and M. danae, with the validity of the latter two species being challenged by different authors at various times. In 2007, Kenaley examined over 450 stoplight loosejaw specimens and revised the genus to contain two species, M. niger and the new M. australis.

Malacosteus and the related genera Aristostomias, Chirostomias and Pachystomias are the only fishes that produce red bioluminescence. As most of their prey organisms are not capable of perceiving light at those wavelengths, this allows Malacosteus to hunt with an essentially invisible beam of light. Furthermore, Malacosteus is unique amongst animals in using a chlorophyll derivative to perceive red light. The name Malacosteus is derived from the Greek malakos meaning "soft" and osteon meaning "bone". Another common name for these fishes is "rat-trap fish", from the unusual open structure of their jaws.

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cc-by-sa-3.0
copyright
Wikipedia authors and editors
original
visit source
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wikipedia EN