Stylocheiron longicorne G.O. Sars, 1883
Etymology: Stylocheiron - a column/hand; longicorne - long horned
Eye: The width of the upper lobe approaches the width of the lower lobe and sometimes exceeds it (S. longicorne eye photo ). The width-of-lower-lobe : width-of-upper-lobe ranges from about 0.9-1.4. In adults the upper lobe has 7-19 enlarged crystalline cones in a distal transverse row, the fewer cones being in the smaller specimens (S. longicorne Table ).
Peduncle of 1st Antenna: As in the generic diagnosis in Higher Taxa (S. longicorne lappet lacking).
Rostrum: The frontal plate is produced into a short acute rostrum which, in the Long Form (S. longicorne-Long) and Short Form (S. longicorne-Short), extends about to the anterior limit of the eye when the eye is vertical. In the North Indian Ocean Form (S.longicorne-North Indian Ocean)the rostrum extends forward of the eye.
Carapace: The gastric region is domed, with an indistinct short, low, median keel.
Thoracic legs: The 3rd elongated leg bears a false chela (S. longicorne chela photo), formed of bristles from the propodal and dactyl segments, as in other species of the "S. longicorne" group.
Abdomen: The 6th segment varies in length. The 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 "North Indian Ocean" form has the longest 6th segment with length : height about 2.35-2.6 (S. longicorne Table ). The ratio of the length of the 6th to the 5th abdominal segment is larger in the "North Indian Ocean" form (~2.0) than in the "Short" form (~1.35) or the "Long" form (~1.5).
Length: Adults are 7.1-11.3 mm, "Long Form". "Short Form", 6.2-10.3 mm. "North Indian Ocean Form", 7-12 mm.
Petasma: The terminal and proximal processes 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).
Thelycum: Described by Sebastain, 1966; Costanzo and Guglielmo, 1976b; Costanzo and Guglielmo, 1991.
Comments: The taxonomic significance of the forms is not understood. The morphology and separate distribution of the North Indian Ocean Form suggests a stronger genetic basis than may exist between the Long and Short Forms, whose distributions overlap. The eye of the "Short" form of S. longicorne may resemble the eye of the "Central" form of S. affine. The tables giving ranges of eye-size and proportions of 6th abdominal segment measurements for S. longicorne and S. affine forms may be consulted in such instances.
ECOLOGY
S. longicorne is consumed by fishes.
HORIZONTAL DISTRIBUTION
S. longicorne occurs in the Pacific, Indian and Atlantic Ocean basins. 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 North Indian Ocean Form occurs north of the equator in the region of 02-deficient subsurface waters (S. longicorne distribution).
VERTICAL DISTRIBUTION
Most of the populations occur below 140 m during both day and night.
LARVAE
See the development summary (S. longicorne Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis - (3 stages) (S. longicorne A)
Carapace:
Shape - The carapace is short in relation to the abdomen length by C3. The anterior margin is slightly pointed in C1-C2 and flat in C3.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Telson:
Postero-lateral spines - PL3 is relatively long in C3.
Furcilia - (5 stages) (S. longicorne B), (S. longicorne C)
Eye: There are 3-4 facets distally in a transverse row in the upper portion of the eye. By F3 the eye appears almost cylindrical.
Carapace:
Shape - The carapace is short in relation to the abdomen length.
Frontal plate / rostrum - The frontal plate is broadly triangular with a small rostral spine.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Thoracic legs: There is precocious development of leg 3, the elongate leg of the adult, in the furcilia phase.
Abdomen:
Mid-dorsal spines - absent
Pleopods: The common developmental pathway is 0 - 1' - 1"2' - 3"2'- 5".
Telson:
Postero-lateral spines - PL3 is longer than PL2. PL1 moves to a more proximal position in the furcilia phase.
Lateral spines - One pair is present with a relatively proximal position.
Comments: Lebour (1926b) notes that larvae of the genus Stylocheiron are distinguished by no lateral denticles on the carapace in any stage, a relatively long first antennal peduncle with short flagella, distinctive large eyes, and a common furcilia stage with 1"2' pleopods.
S. longicorne and S. affine may both have 3 large ommatidia in the upper lobe of the eye. They differ in the proportions of the eye; the upper lobe is narrower than the lower portion of the eye in S. affine but not in S. longicorne . (S. longicorne vs. S. affine)
The eye of S. longicorne is similar to that of N. flexipes (S. longicorne vs. N. flexipes). The eyes differ in the distribution of pigment. In N. flexipes there is pigment in the upper and lower portions with a small band near the lower pigmented area. In S. longicorne , the whole lower half of the eye is pigmented.
Development
(S. longicorne Table), development summary for the stage descriptors and length in stage.
(S. longicorne Graph 1) , a histogram of developmental form frequency, from the Mediterranean.
(S. longicorne Graph 2), a histogram of length freqency by stage, from the Mediterranean.
Pictures
S. longicorne, selected stages
S. longicorne A [calyptopis 1-3]
S. longicorne B [furcilia 1-3]
S. longicorne C [furcilia 4-5, juvenile]
S. longicorne vs. N. flexipes [furcilia eye photo]
S. longicorne vs. S. affine [furcilia eye photo]
key to larval illustrations