G.O. Sars, 1883
Description
Eye cylindrical in appearance, with upper and lower lobes (almost) equal in width [S.elongatum eye & rostrum ]. The overall height of eye is 1.4-1.7 mm. The upper lobe has 13-16 only slightly enlarged crystalline cones in a distal transverse row.
Peduncle of first antenna: the second and third segments of the peduncle of the first antenna in the female are long and slender, but in the male are short and thicker. There are no lobes, lappets or spines on the peduncle of the first antenna that are species-specific in structure. [S.elongatum lappet lacking ]. 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.elongatum ped 1st ].
Rostrum extremely short and acute in both sexes. The frontal plate is produced as an acute triangle which narrows to a short spiniform process at the apex[S.elongatum dorsal head S.elongatum eye & rostrum ].
Carapace with elevated gastric area as a low dome with a low median [l][m]Glossary[/m][r]keel[/r]keel [S.elongatum ]. There are no lateral denticles.
Thoracic leg 3 greatly elongated, naked, with a false chela formed of distal bristles on the propodus and dactylus. This grasping organ is like the "false chela" found in other species of the "S. longicorne group"
Abdomen the sixth segment is extremely elongated, with length-height ratio being about 4.2 [S.elongatum S.elongatum-ph ]. The sixth segment is nearly three times as long as the fifth. There are no photophores on the abdominal segments 2-4.
Petasma: the terminal and proximal processes are slender, of similar length, and tapering to acute tips. The tip of the proximal process curves inward. The lateral process is minute (S.elongatum petasma).
Reproduction: nothing is known about the breeding of the species. The female carries 2-5 eggs, attached to the thoracic legs (Mauchline, 1984).
Size
Adults are 11.5-18 mm.
Larval stages
S.elongatum-A
S.elongatum-B
Remark
This species is characterized by the long slender body with an elongated sixth abdominal segment that is equal to or slightly longer than the combined lengths of the fourth and fifth segments.
Ecology
S. elongatum is consumed by fishes.
Depth range
Mesopelagic, 200-500 m depth; deeper than any other member of the "Stylocheiron longicorne group". There appears no diurnal vertical migration (Mauchline, 1984).
Distribution in the North Sea
S. elongatum might enter the North Sea from the Atlantic and might be found in the vicinity of the Orkney and Shetland Islands, as well as the Norwegian Deep. The species is recorded from north of the North Sea (S. Hay, pers. com.).
World distribution
S. elongatum is present in the Atlantic Ocean between 40°N and 40°S but has been found northward to Iceland, 65°N, in the Gulf Stream extension. In the Pacific Ocean it occurs between 40°N and 35°S but not in the 02-deficient parts of the Eastern Tropical Pacific. In the Indian Ocean it is between about 10°N and 40°S, being absent in the 02-deficient Bay of Bengal and Arabian Sea.
[After Brinton et al., 1999]