Stylocheiron affine Hansen, 1910

Etymology: Stylocheiron - a column/hand; affine - related, adjacent

Eye: The eye is oblong. The width of the lower lobe is about 2x the width of the upper lobe measured at the base of the crystalline cones (S. affine eye & rostrum), the ratio being regionally variable, from 1.5-2.6 among geographical forms. The length of the eye varies from about 0.62-1.25 mm (S. affine Table ). The upper lobe has 4-8 enlarged crystalline cones in a distal transverse row (Stylocheiron table).

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

Rostrum: It is long and slender in the female reaching to about the anterior limit of the eye. It is short and acute in the male, not reaching to the anterior limit of the eye (S. affine eye & rostrum).

Carapace: The gastric region is domed, with a small poorly-defined mid-dorsal keel(S. affine- California Current).

Thoracic legs: The 3rd leg is greatly elongated, with a false chela formed of distal bristles on the propodus and dactylus (S. affine thoracic leg). This grasping organ is characteristic of the "S. longicorne group".

Abdomen: Length of the 6th segment differs among geographical forms of S. affine, with length : height of the 6th segment differing from 1.4-2.44 (S. affine Table ). The ventral margin of the 6th segment is smoothly curved; in lateral view it is not abruptly narrowed posteriorly (S. affine- California Current).

Length: Adults are 5.4-8.5 mm.

Petasma: The terminal process is generally of the same width throughout its length, it is distally blunted and serrated, with the inner margin concave near the mid-point, and the outer sub-distal surface somewhat hollowed, forming a groove-like indentation against which the tip of the proximal process appears to rest. The proximal process is nearly as long as the terminal, with the distal part broadened and the terminal margin serrated. The lateral process, is more slender and a little shorter than the terminal and proximal, and is distally curving outward and terminating sub-acutely (S. affine petasma). Differences among the petasmas of the different geographical forms are indistinct or inconsistent.

Thelycum: Described by Costanzo and Guglielmo, 1991.

Comments: Five distinct geographical forms, or presumed races, were recognized by (Brinton, 1962a, 1975) from the Pacific and Indian ocean basins: "California Current," "East Equatorial," "West Equatorial," "Central," and "Indo-Australian". Each is distinguished by a characteristic range of values for (1) width of upper lobe of eye : width of lower lobe, and (2) for length : maximum height of 6th abdominal segment ( in lateral view) (S. affine Table ).

ECOLOGY
S. affine is consumed by fishes.

HORIZONTAL DISTRIBUTION
The complex of S. affine forms occupies all of the tropical and subtropical seas beyond continental shelves ([l][][f][/f]# S. affine ). Specimens from the tropical and subtropical South Atlantic appear to agree with the "Central" Form, but have not been studied in detail.

VERTICAL DISTRIBUTION
S. affine appears not to be a vertical migrant and lives in and above the thermocline. Layers above the 02-deficient depths of the Indian and Eastern Tropical Pacific ocean basins are occupied by the Indo-Australian Form and the East Equatorial Form, respectively.

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

Metanauplius - undescribed

Calyptopis 3 (3 stages) (S. affine A)

Eye: The ommatidia in the upper portion of the eye are differentiated in C3.

Carapace:
Shape - The carapace is short in relation to the abdomen length.
Marginal spines - absent
Postero-dorsal spine - absent
Lateral denticles - absent

Furcilia - (5 stages) (S. affine A), (S. affine B)

Eye: The eye is bilobed with 3 large facets distally in a transverse row in the upper portion of the eye, as compared with 4 facets in the adult. (S. affine eyes & thorax photo)

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
Dorsal keel - A small keel is present.

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".

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.

According to Brinton (1975), as there are no more than 3 ommatidia in a transverse row throughout the furcilia phase of S. affine , the larvae may be confused with adult S. suhmi which also have 3 enlarged ommatidia in the upper lobe of the eye. The adults of S. shumi range from 6-7 mm while F5 of S. affine measures about 3.4 mm. Also, the eye pigment in preserved larvae tends to be darker in S. affine and light yellowish in S. shumi .

S. longicorne may also have 3 ommatidia in the upper lobe of the eye. The upper lobe is narrower than the lower lobe in S. affine and about equal in S. longicorne . (S. longicorne vs. S. affine)

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

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

Pictures
S. affine, selected stages
S. affine A [calyptopis 3, furcilia 1-2]
S. affine B [furcilia 3, 5]
S. affine eyes & thorax photo [furcilia 4]
S. longicorne vs. S. affine [furcilia eye photo]
key to larval illustrations

Kingdom Animalia
Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Subclass Eumalacostraca
Superorder Eucarida
Order Euphausiacea
Family Euphausiidae
Genus Stylocheiron
Species Stylocheiron affine

Status in World Register of Marine Species

Accepted name: Stylocheiron affine Hansen, 1910

Scientific synonyms and common names

Stylocheiron affine n. sp. Hansen, 1910

Baker, A. de C., B. P. Boden and E. Brinton, 1990. A practical guide to the euphausiids of the world. British Museum (Natural History), London. 96 pp.

Baker, A. de C., 1970. The vertical distribution of euphausiids near Fuertaventura, Canary Islands ("Discovery" SOND Cruise, 1965). Journal of the Marine Biological Association of the United Kingdom 50: 301-342.

Boden, B. P., M. W. Johnson and E. Brinton, 1955. The Euphausiacea (Crustacea) of the North Pacific. Bulletin of the Scripps Institution of Oceanography, University of California 6: 287-400.

Brinton, E. and J. G. Wyllie, 1976. Distributional atlas of euphausiid growth stages off southern California, 1953 through 1956. California Cooperative Oceanic Fisheries Investigations, Atlas 24: 1-289.

Brinton, E., 1962b. The distribution of Pacific euphausiids. Bulletin of the Scripps Institution of Oceanography, University of California 8: 51-270.

Brinton, E., 1967. Vertical migration and avoidance capability of euphausiids in the California Current. Limnology and Oceanography 12: 451-483.

Brinton, E., 1975. Euphausiids of Southeast Asian Waters. Naga Report. Scientific Results of Marine Investigations of the South China Sea and the Gulf of Thailand 4(5): 1-287.

Costanzo, G. and L. Guglielmo, 1991. Diagnostic value of the thelycum in euphausiids. II. Oceanic species. Genus Stylocheiron. Journal of Crustacean Biology 11: 437-450.

Hansen, H. J., 1910. The Schizopoda of the Siboga Expedition. Sibogae-Expedite, Leiden 37: 1-123.

Hirota, Y., 1987. Vertical distribution of euphausiids in the Western Pacific Ocean and the Eastern Indian Ocean. Bulletin of the Japan Sea Regional Fisheries Research Laboratory 37: 178-224.

Mathew, K. J., 1983. Studies on larval euphausiids from the southwest coast of India with notes on their developmental pathways and breeding seasons. Journal of the Marine Biological Association of India 25: 51-70.

Mauchline, J. and L. R. Fisher, 1969. The biology of euphausiids. Advances in Marine Biology 7: 1-454.

Mauchline, J., 1980. The biology of mysids and euphausiids. Advances in Marine Biology 18: 1-681.

Roger, C., 1971. Distribution verticale des euphausiaces (crustaces) dans les courants equatoriaux de l'Ocean Pacifique. Marine Biology 10: 134-144.

Silas, E. G. and K. J. Mathew, 1977. A critique to the study of larval development in Euphausiacea. In "Proceedings of the Symposium on Warm Water Zooplankton". Special Publication of National Institute of Oceanography, Goa, India: 571-582.

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