G.O. Sars, 1883, 1885
Generic diagnosis:
Eyes typically elongate and bilobed (eye shape), width of the upper lobe is usually narrower than the width of the lower (except in S. longicorne , S. elongatum and S. maximum , where the width of the upper lobe may be equal in width to the lower). In the small species, 6-12 mm in length, the upper lobe bears conspicuously enlarged crystalline cones distally.
For species in the "Stylocheiron longicorne group" the number of enlarged distal crystalline cones in a transverse row is a taxonomic character. This "transverse row" is not linear but, appears as such in a strictly lateral view of the eye, and consists of those fully developed cones visible from side to side at the top of the eye: as few as 2 are found in S. mirophthalma and as many as 19 in the largest specimens of S. longicorne.
The 2nd and 3rd segments of the peduncle of the 1st antenna in the female are long and slender, but in the male are short and thicker (Stylocheiron- ). There are no lobes, lappets or spines on the peduncle of the 1st antenna that are species-specific in structure (except in S. indicum). The upper flagellum is shorter than the lower. The peduncle of the flagellum-bearing endopod of the 2nd antenna is extremely elongate, with the penultimate segment reaching beyond the end of the scale (squama).
The 1st and 2nd thoracic legs are short and slender. The 3rd leg is extremely elongate and has short ischial, meral and carpal segments; strong spiniform bristles on the broadened propodal segment are opposed to elongate spines or a spine-like process on the terminal segment forming either a chela-like grasping structure or a true chela (Stylocheiron-elongated legs). The 4th-8th legs are diminished in length, the 8th being rudimentary.
The carapace is without lateral denticles.
The processes of the petasma, except for the spine-shaped process, are only slightly curved. The terminal and proximal processes in most species are blunted, blade-like and insufficiently specific to be reliable as primary taxonomic characters.
The abdomen bears only a single photophore, situated on the mid-ventral line of the 1st segment (Stylocheiron-photophores).
Body length is 6 mm (S. microphthalma) to 30 mm (S. maximum).
The genus consists of 12 species, falling into three groups. The "Stylocheiron longicorne group" includes 7 species characterized by the presence of a "false chela" on the 3rd thoracic leg formed by distal bristles on the propodus and distal bristles on the dactylus: S. longicorne, S. affine, S. elongatum, S suhmi, S. microphthalma, S. insulare and S. indicum. The "Stylocheiron maximum group" includes 3 species characterized by a true chela on the 3rd thoracic leg: S. maximum, S. robustum and S. abbreviatum. The "Stylocheiron carinatum group" includesS. carinatum and S. armatum and differs from the other two groups in that a false chela on the elongate 3rd thoracic leg is formed from lateral spines or bristles on the propodus and distal bristles on the dactylus.
Evidence of morphological variability in several Stylocheiron species, illustrated in part by the several presumed ecophenotypical "forms" in S. longicorne (S. longicorne Table ) and S. affine (S. affine Table ), and by the species pair S. carinatum-S. armatum , points to a need for more detailed study of this genus.
Eggs are carried externally by the female, attached to thoracic limbs by a glutinous adhesive.