Stylocheiron suhmi G.O. Sars, 1883
Etymology: Stylocheiron - a column/hand; suhmi - after Willemos Suhm
Eye: The eye is narrow with the upper lobe bearing 3 enlarged crystalline cones in a transverse row (Stylocheiron table). The length of the eye is about 0.6 mm. The lower lobe is 3.9-4.3 times the width of the upper lobe measured at the base of the enlarged crystalline cones (S. suhmi eye & rostrum).
Peduncle of 1st Antenna: As in the generic diagnosis in Higher Taxa (S. suhmi lappet lacking).
Rostrum: It is slender and acute in females, extending to the anterior limit of the eyes. In males the frontal plate terminates in a short, acutely triangular plate (S. suhmi eye & rostrum).
Carapace: The gastric region is domed with a small-dorsal nub, scarcely keel-like (S. suhmi).
Thoracic legs: The 3rd endopodite bears a "false chela" (Stylocheiron-elongated legs) typical of the form found throughout the "S. longicorne group": distal spines and bristles on the propodus (penultimate) segment curve medially so as to oppose similar spines extending from the dactylus.
Abdomen: The 6th segment is about l.7 times as long as its maximum height and its ventral margin angles sharply upward at a point near the posterior limit of the segment (S. suhmi ). Thus the terminal height of the 6th segment is distinctly lessened.
Length: Adults are 6-7 mm.
Petasma: The terminal process is only slightly expanded distally with a smooth margin. The proximal process is about as long as the terminal, and slightly concave along the outer sub-distal margin. The lateral process is straight, simple, and shorter and more slender than the terminal and proximal processes (S. suhmi petasma).
Thelycum: Described by Costanzo and Guglielmo, 1976b and Costanzo and Guglielmo, 1991.
Comments: The adult is distinguished by (1) the three enlarged crystalline cones in a distal transverse row of the upper lobe of the eye, and (2) by the angular irregularity in the shape of the posterior part of the ventral margin of the 6th abdominal segment (also characteristic of S. insulare).
ECOLOGY
S. suhmi is consumed by fishes.
HORIZONTAL DISTRIBUTION
It occurs in Atlantic Ocean between about 40°N and 30°S. It occurs in the Indian Ocean between the equator and 30°S. In the North Pacific it occurs between 40°N and about 5°N and in the South Pacific 10 to 35°S (S. suhmi distribution).
VERTICAL DISTRIBUTION
S. suhmi is most common above 140 meters during day and night.
LARVAE
See the development summary (S. suhmi Table) for the stage descriptors and length in stage.
Metanauplius - undescribed
Calyptopis - (3 stages)
Eye: Rudiments of the eyes with few anterior facets, characteristic of the species, are visible in C3.
Carapace:
Shape - The carapace is smooth and close fitting.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent
Telson:
Terminal spines - There are 6 terminal telson spines in C1-C3.
Furcilia - (5 stages) (S. suhmi A), (S. suhmi B)
Eye: The eye is bilobed. There are generally 2 large facets in a distal transverse row on the upper portion as compared with 3 in the adult.
Carapace:
Shape - The carapace is short in relation to the abdomen length.
Frontal plate / rostrum - The rostrum is short, triangular, and pointed.
Marginal spines - absent
Postero-dorsal spines - 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.
Lateral spines - One pair is present situated relatively proximally.
Terminal spines - There are an even number of terminal spines during the reduction to 1 terminal spine, i.e. 6-4-1.
Comments: Lebour (1926b) notes that larvae of the genus Stylocheiron are adistinguished by no lateral denticles on the carapace in any stage, a relatively long first antennal peduncle with a short flagella, distinctive large eyes, and a common furcilia stage with 1"2' pleopods.
According to Lebour (1926c), in the calyptopis stages of S. suhmi the carapace is closely fitting and the abdomen joints are usually smooth and flattened so that when extended, the whole outline of the larva is smooth. The furcilia larvae are characterized by the number of large facets in the upper lobe of the eye and the even number of terminal telson spines. Lebour (1949) notes that in life the larvae of S. suhmi are colorless or slightly pinkish, only the photophores are red and those on the coxa of leg 7 appear first.
Casanova (1974) observed a photophore on leg 2 in a first juvenile of S. suhmi from the Mediterranean. This photophore was not developed in a sample of larvae from the Eastern Pacific. An eyestalk photophore was not observed by Lebour (1926c) in the Mediterranean but was found in larvae of S. suhmi in the Eastern Pacific.
Development
(S. suhmi Table), development summary for the stage descriptors and length in stage.
(S. suhmi Graph 1) , a histogram of developmental form frequency, from the Mediterranean.
(S. suhmi Graph 2), a histogram of length freqency by stage, from the Mediterranean.
Pictures
S. suhmi, selected stages
S. suhmi A [furcilia 2-4]
S. suhmi B [furcilia 5, juvenile]
key to larval illustrations