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Stylocheiron armatum Colosi, 1917

Etymology: Stylocheiron - a column/hand; armatum - armed

Eye: The eye is bilobed with the lower lobe large and nearly spherical (S. armatum eye & rostrum). The upper lobe is short and narrow with 6-8 enlarged crystalline cones in a distal transverse row (Stylocheiron table).

Peduncle of 1st Antenna: As in the generic diagnosis in Higher Taxa (S. armatum lappet lacking ).

Rostrum: It is straight and acute, extending beyond the anterior limit of the eye in both sexes (S. armatum eye & rostrum).

Carapace: The gastric region is scarcely elevated, bearing a short, prominent, dorsal keel (S. armatum).

Thoracic legs: The elongated 3rd thoracic leg has the propodus (penultimate segment) bearing as many as 9 short setae on the upper anterior edge; the lower posterior edge bears several short setae proximally. Distally there are 3 long strong setae which, together with a cluster of opposing setae on the dactylus, form a false chela or grasping organ.
This grasping organ is like the "false chela" found in the other species of the "S. carinatum group" (Stylocheiron-elongated legs). At the distal underside of the carpus there is a heavy obliquely directed tubercle bearing a strong outward directed spine. The combined length of the tubercle and spine is half the length of the propodus. The propodus bears numerous short curved bristles in addition to 3 longer ones (S. armatum dactyl-thoracic).

Abdomen: The body appears generally slender. The 6th abdominal segment is long, length : height is about 2.2 (S. armatum).

Length: Adults are 6-12 mm.

Petasma: The terminal process has a serrated distal margin with a short serrated ridge on the inner end. The proximal process is broad, the distal margin bears minute crenulations. The lateral process is a little shorter and much more slender than the proximal process (S. armatum petasma ). Significant differences between the petasmas of S. carinatum and S. armatum have not yet been established with certainty.

Thelycum: Not yet described.

Comments: This species closely resembles S. carinatum, and its validity has been unclear. Mathew (1980b) distinguished S. armatum in the Arabian Sea from S. carinatum by the structure of the elongated 3rd thoracic leg, particularly the prominent distal tubercle on the carpus which is armed with a longer and stronger spine than in S. carinatum (S. armatum dactyl-thoracic leg). The only character used here to distinguish S. armatum from S. carinatum is the combined length of the carpal tubercle and its spine being greater than 1/2 the length of the propodal segment of the 3rd thoracic leg in S. armatum . Baker et. al. (1990) agreed that this is a quantifiable character, amenable to study of variability.

HORIZONTAL DISTRIBUTION
Colosi (1917) described S. armatum from the Red Sea. Mathew (1980), citing Colosi, states that it is distributed extensively in the tropical waters of the Indian, Pacific and Atlantic ocean basins (S. armatum distribution). However, there is uncertainty with respect to the Pacific and Atlantic because of variability there in the S. carinatum 3rd leg armature.

VERTICAL DISTRIBUTION
As with S. carinatum , occupancy of the layer above about 140 m permits it to inhabit tropical waters, including the Arabian Sea and Eastern Tropical Pacific which are 02-deficient below that depth.

LARVAE
See the development summary (S. armatum Table) for the stage descriptors and length in stage.

Metanauplius - undescribed

Calyptopis - (3 stages) (S. armatum A)

Carapace:
Shape - The carapace is smoothly rounded with large lateral lobes in C1.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

Telson:
Postero-lateral spines - PL3 is longer than PL2.
Terminal spines - There are 6 terminal spines in C1-C2.

Furcilia - (5 stages) (S. armatum B), (S. armatum C)

Eye: The facets are distinct and the pigmented area is somewhat pear-shaped in early stages. The eye becomes bilobed in F4-F5.

Carapace:
Frontal plate / rostrum - The rostrum tapers to a point.
Marginal spines - absent
Postero-dorsal spines - absent
Lateral denticles - absent
Dorsal keel - There is a small dorsal keel. It is situated in the posterior half of the carapace in F2 and moves near the midpoint of the carapace in later stages.

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 a little longer than PL2. PL1 moves to a more ventral, proximal position in F4-F5.
Lateral spines - One pair of lateral spines is present.

Comments: Lebour (1926b) notes that the larvae of the genus Stylocheiron are distinguished by no lateral carapace denticle in any stage, a relatively long first antennal peduncle with short flagella, distinctive large eyes, and a common furcilia stage with 1"2' pleopods.

Development
(S. armatum Table), development summary for the stage descriptors and length in stage.

(S. armatum Graph 1) , a histogram of developmental form frequency, from the southwest coast of India.

Pictures
S. armatum, selected stages
S. armatum A [calyptopis 1-3]
S. armatum B [furcilia 1-3]
S. armatum C [furcilia 4-5, juvenile]
key to larval illustrations

Stylocheiron armatum