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G.O. Sars, 1883

Description:
Eye bilobed, lower lobe slightly wider than upper lobe, but sometimes smaller [S.longicorne-eye-ph ]. The ratio of the width of the lower lobe and the width of the upper lobe ranges from about 0.9 to 1.4. In adults the upper lobe has 7-19 enlarged crystalline cones in a distal transverse row, the fewer cones being in the smaller specimens.
Peduncle of first antenna with long and slender second and third segments in female are, but short and thicker in male. There are no lobes, lappets or spines on the peduncle of the first antenna that are species-specific in structure. The upper flagellum is shorter than the lower. The peduncle of the flagellum-bearing endopod of the second antenna is extremely elongate, with the penultimate segment reaching beyond the end of the scale (squama). [S.longicorne lappet lacking ]
Rostrum: the frontal plate is produced into a short acute rostrum [S.longicorne-m-long-drawing ], extends about to the anterior limit of the eye when the eye is vertical.
Carapace with domed gastric region, with an indistinct short, low, median keel. There are no lateral denticles.
Thoracic leg 3 elongated, bearing a false chela [S.longicorne-chela-ph ], formed of bristles from the propodal and dactyl segments, as in other species of the "S. longicorne" group.
Abdomen with sixth segment varying in length. The ratio length-height ranges from 1.6 to 2.6. The "Long" and "Short" forms of this species are distinguished on the basis of whether this proportion is greater or less than 2.0. The long form is the only in the area.
Petasma with terminal and proximal processes that are nearly equal in length. Both bend inward in the direction of the spine-shaped process, and are blunt distally. The terminal process is a little longer and is serrated or crenulated along the distal margin. The lateral process is 2/3 the length of the proximal process, and is slender and sub-acute. In the "North Indian Ocean Form" the distal end of the of the terminal process appears more deeply crenulated, appearing as a row of inward-curving finger-like projections [S.longicorne petasma ].
Reproduction: matures sexually at a length >6mm to breed, possible over an extended summer season in British waters, at an age of one year. The female carries 4 to 16 eggs (after Mauchline, 1984).

Size:
Adults of the "Long Form" are 7.1-11.3 mm.

Habitat:
S. longicorne is consumed by fishes.

Depth range:
Most of the populations occur below 140 m during both day and night.

Distribution in the North Sea:
Northern North Sea, intruded from the NE Atlantic.

World distribution:
Pacific, Indian and Atlantic Oceans. The Long Form is most widespread: 60°N-40°S in the Atlantic and to 40°S in the Indian and Pacific. The Short Form seems to relate to the subtropical Central Water masses and the tropical Indo-Pacific connection; its distribution in the Atlantic has not been critically examined. The N Indian Ocean Form occurs north of the equator in the region of 02-deficient subsurface waters.

Remark
The dajid isopod Branchiophryxus caulleryi has been found attached to this species (Mauchline, 1984).

Stylocheiron longicorne