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 40 cm (16 in). 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 37 and 40 cm (15 and 16 in) for females, and 18.2 to 24.4 cm (7.2 to 9.6 in) 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 1⁄2 to 2⁄3 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 405 to 4,515 m (1,329 to 14,813 ft), and at water temperatures of 21 to 3 °C (70 to 37 °F), though adults may rarely be found at depths of less than 25 m (82 ft) 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 300 m (980 ft) 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
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