Species of squid
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Teuthowenia megalops, sometimes known as the Atlantic cranch squid, is a species of glass squid from the subarctic and temperate waters of the northern Atlantic Ocean. They are moderately sized squid with a maximum mantle length of . Their very large eyes are the source for the specific name "megalops" (Greek for "large eyes"). Like other members of the genus "Teuthowenia", they are easily recognizable by the presence of three bioluminescent organs (photophores) on their eyeballs.
Description.
"Teuthowenia megalops" are moderately sized glass squid. They are sexually dimorphic, reaching mantle lengths between for females, and for males. They have very large forward-facing eyes, each possessing three bioluminescent organs (photophores), a defining characteristic of the genus "Teuthowenia". One of the photophores is oval-shaped and small, located at the upper part near the lens of each eye. The other two are concentric and located at the sides and bottom of the eyeball. One of them is very narrow and curves halfway around the lens.; the other is very large and crescent-shaped, straddling half of the bottom surface of the eyeball.
The mantle is conical and elongated with a narrow rear tip. The mantle walls are thin and leathery. The funnel locking-apparatus are small and oval to spindle-shaped. A single cartilaginous tubercle is present at the mantle margins, though rarely it may be absent. The fins are long and narrow, starting at about 40 to 60% of the length of the mantle, far from where the body is the widest. They extend past the rear tip of the body, fuse with each other at the middle, and end with a pair of small lobes.
The arms are short to medium length, about a fourth to half of the mantle length. In subadults and females, the third pair of arms is the longest, followed by the second, first, and the fourth pairs (length formula: III ;II ;I ;IV). Sometimes the fourth and first pairs may be of equal or almost equal length (length formula: III ;II ;I=IV). Adult males may have the first and second pair of equal length (but shorter than the third pair), or the first pair may be the longest of all the arms (length formula: III ;I=II ;IV or I ;III ;II ;IV). They all possess two rows of cup-shaped to globular suckers that lack hooks. The suckers on the midportion of the third and second pair of arms are greatly enlarged, exceeding the width of the arms.
In subadults of both sexes and mature females, the basal and mid-arm suckers are usually smooth, though the small terminal suckers possess six to ten short triangular teeth on the distal and lateral rims. About 6 to 9% of the tips of all the arms of subadult and adult females are also modified into grasping end-organs. In adult males, the arms are all sexually modified in some way. The first pair becomes elongated and thicker, with swollen midportions and expanded protective membranes. The second pair has elongated and swollen tips, with reduced protective membranes. The third and fourth pairs all have slender whip-like tips. The suckers on the tips of the second pair have swollen collars and possess small pointed teeth around their entire margin, while the rest of the suckers on all the arms have small rounded or triangular teeth on the distal and lateral rims. The mid-arm and basal suckers of the third and fourth pairs also have swollen collars and greatly reduced openings.
The tentacles are short and muscular. They have four rows of carpal ("wrist") suckers, arranged into a zigzag pattern on ½ to ⅔ of the distal end of the tentacles leading towards the tentacular clubs. The suckers on the tentacular club are set on long protrusions and possess numerous short, sharp teeth on their entire margins. The largest of these possess 19 to 24 teeth.
Distribution and habitat.
"Teuthowenia megalops" are deep-water pelagic squid. They are found in the water column at depths of , and at water temperatures of , though adults may rarely be found at depths of less than from the surface. They exhibit diel vertical migration, moving closer to the surface in nighttime and diving deeper during daytime. Paralarvae and subadults are usually found in the upper of the surface, and gradually dive deeper as they grow larger.
The distribution of "Teuthowenia megalops" is restricted to the subarctic and temperate waters of the northern Atlantic Ocean. In the northwestern Atlantic, they can be found along the eastern coast of North America, from Labrador in Canada to Georgia in the United States; extending eastward towards the Bermuda islands and the New England Seamounts. In the northeastern Atlantic, they inhabit the waters between Greenland and Iceland and south towards the Azores (following the Mid-Atlantic Ridge); and the area around the Rockall Basin, Britain, Ireland, and (during winter) northern Portugal. A single specimen has also been recovered from the Mediterranean Sea, making it possible that their range extends further south than currently known. "Teuthowenia megalops" are known to be relatively abundant. They are frequently encountered in submersibles, and younger individuals are frequently caught in shallower waters.
The three species of "Teuthowenia" do not appear to have overlapping ranges, with "Teuthowenia pellucida" restricted to the southern hemisphere and "Teuthowenia maculata" to the eastern central part of the Atlantic Ocean off the coast of West Africa. Thus specimens can be reliably identified based on where they may have been caught. Several squid specimens have also been recovered and observed from the Pacific Ocean in 1969, 1974, 2000, and 2002. Their close resemblance to "Teuthowenia megalops" led them to be continually misidentified as belonging to "Teuthowenia megalops". However closer examinations reveal that the arrangement of photophores on the eyes is different, making it certain that they are not "Teuthowenia megalops". They are very likely to be individuals of "Galiteuthis phyllura".
Ecology and biology.
Like other members of the family Cranchiidae, "Teuthowenia megalops" possess a remarkable flotation system that enables them to remain in the water column at neutral buoyancy. The system uses enormous bilobed coelomic chambers filled with ammonium chloride derived as waste products from their nitrogen metabolism. The ammonium chloride has a lower density than water, which enables the squid to float. In experiments, puncturing the chambers and draining the ammonium chloride resulted in the squid sinking rapidly. The chamber is also lined with muscles that contract and expand regularly in peristaltic waves. The movement draws water into the mantle cavity and pushes it out again through the funnel, aerating the gills in the process.
As a result of this adaptation, "Teuthowenia megalops" are sluggish swimmers. Unlike other squids, they do not need to contract their mantle muscles to breathe, but their modified flotation and respiratory systems restrict their ability to jet away from threats. They generally float passively in the "cockatoo" posture typical of glass squids (which are also known as cockatoo squids for this reason). They resemble a horizontal cockatoo, hence the name. In this posture, they rotate around their spindle-shaped digestive glands, the only internal organs of the squid clearly visible through their mostly transparent bodies. Regardless of the direction their heads or mantles are facing, the digestive glands are always kept oriented vertically.
Life cycle.
The life cycle of "Teuthowenia megalops" has not been extensively observed. From the recorded depths of captured adult specimens, it is assumed that mating and spawning happens in the deep midwaters, at depths greater than . At these depths, the waters are quite dark. The bioluminescent photophores as well as pheromones released into the water may help the squid find mates. Once the males find the females, they harpoon them with spermatophores directly on the mantles. The spermatophores penetrate into the mantle cavity of the female where they discharge sperm. The females spawn only once before dying. No eggs of the species have ever been collected from the wild, but examination of collected specimens of gravid females puts the estimated fecundity at 70,000 to 80,000 eggs per individual.
Upon hatching, the paralarvae gradually float or swim towards shallower waters. The paralarvae differ from adults in having stouter barrel-shaped bodies with a blunt posterior end. The fins are very small and unfused. The funnel is broad and large and lack tubercles on the funnel-mantle fusion, unlike in adults. The eyes are small with developing photophores visible. They are slightly flattened dorsoventrally and mounted on short thick stalks. The arms are mere stubs, with lengths of when they reach mantle lengths of . In contrast, the tentacles are very long and thick but without an expanded club. Four rows of suckers extend from the club to almost the base of the stalks. The mantle, head, arms, and tentacles are covered with relatively large widely spaced chromatophores.
The paralarvae develop the characteristic tubercle at the funnel-mantle fusion once they reach the mantle lengths between . They enter the subadult stage once the eyes lose their stalks, becoming sessile, at sizes between . The arms and arm suckers enlarge rapidly and males begin to develop their arm modifications (with the exception of the modifications of the first arm pair, which occurs near maturity). At this stage they may already have moved back to depths exceeding .
The entire lifespan of "Teuthowenia megalops" is estimated to be two to three years.
Predators.
"Teuthowenia megalops" are preyed upon by predatory fish like the blue shark ("Prionace glauca") and swordfish ("Xiphias gladius"). They also constitute a major part of the diet of cetaceans like the long-finned pilot whale ("Globicephala melaena"), bottlenose whale ("Hyperoodon ampullatus"), sperm whale ("Physeter macrocephalus"), and Cuvier's beaked whale ("Ziphius cavirostris").
Defense.
"Teuthowenia megalops" exhibits a bizarre defensive behavior when threatened. Upon initial disturbance, its response is similar to that of other cephalopods, releasing a cloud of ink into the water and jetting away. However, if the threat persists, it reacts by inflating itself into a ball. This happens in several steps, at any point in which it may cease inflating further if the disturbance stops.
The first step is to turn its fins and gladius inside out. This is followed by the head and arms being inverted into the mantle cavity. Then the mantle is filled with seawater, turning them perfectly spherical. The tentacles are the last to be retracted. At this point the chromatophores (normally mere pinpoints in its predominantly transparent body) expand to about four times their usual diameter. They are arranged into concentric circles on the bloated mantle, confusing predators as to the location of the eyes. If the threat still does not go away, its last resort is to again eject ink. However, the ink is not released, instead it is retained inside the now spherical mantle cavity. The squid thus becomes a completely opaque black ball.
The squid retains the shape for about half an hour then its starts to tentatively extend its head and tentacles out of the mantle, followed by the fins and gladius. When assured that the threat has gone, it then gradually releases the ink and seawater stored in its mantle. All this happens with no apparent injury to the squid, despite the considerable contortions its body just went through.
This behavior was first observed among captured live specimens in 1972 by the marine biologist Peter Noel Dilly. It was the first instance of such a behavior being observed among cephalopods. Since then, similar behavior has been found in other glass squids, like "Cranchia scabra" and "Teuthowenia pellucida".
Taxonomy and nomenclature.
"Teuthowenia megalops" is one of the three species currently classified under the genus "Teuthowenia". They are included under the subfamily Taoniinae of the family Cranchiidae. The species does not have widely used common names, but the Food and Agriculture Organization (FAO) of the United Nations gives them the names of Atlantic cranch squid in English, "encornet-outre Atlantique" in French, "cranquiluria Atlantica" in Spanish, and "totano tutt'occhi" in Italian. Like other genera belonging to Cranchiidae, "Teuthowenia megalops" has gone through numerous and convoluted taxonomic and nomenclatural revisions. It has been variously classified under the genera "Cranchia", "Leachia", "Megalocranchia", and "Taonius"; as well as the now unaccepted genera "Desmoteuthis", "Hensenioteuthis", "Loligopsis", and "Verrilliteuthis". Various combinations of which has been paired with the specific names of "Cranchia megalops", "Leachia hyperborea", "Taonidium pfefferi" (now nomen dubium), "Desmoteuthis tenera", and "Desmoteuthis thori".
Taxonomic history.
The species was first described by the Danish zoologist Ferdinand Victor Alphons Prosch in 1849 as "Cranchia megalops". The type specimens were paralarvae obtained