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Author: Cocco, 1829

Argyropelecus hemigymnus Cocco, 1829

Diagnosis: body laterally compressed, deep; eye tubular, directed dorsally; mouth vertical; gillrakers on first arch 19-23; branchiostegal rays 10, 3 on epihyal. Dorsal finrays 8, followed by a dorsal adipose fin; dorsal blade long, derived from 7 supraneurals, the exposed parts of posterior two fused together with hooks forming barbs; pectoral finrays 10-11; pelvic finrays 6; anal fin with two distinct groups of finrays (6 + 5) separated by central AC1 photophore. Upper preopercular spine long, extending beyond posterior margin of preopercle; lower preopercular spine long, ventrally directed; a single posteriorly directed post-abdominal (iliac) spine with serrate edges and bearing small dorso-posterior spine; abdominal keel well defined. Vertebrae 36-39. Swimbladder well developed, gas-filled, euphysoclistous. Scales deciduous. Photophores: ORB1, pigmented, directed towards eye lens; BR (6); OP 3, OP3 first to develop; IP (6); PV (12); VAV (4); ACI (6); ACz (4); OA (2) + 6; VAV, ACI and AC2 groups well separated, the photophores of each sharing a common photogenic mass; VAV, ACI and AC2 elevated relative to posterior 6 OA. Colour: back dark, flanks silver; midline pigment patches dorsal to VAV, ACI and AC2, the latter chromatophore distinct to about 9 mm SL. Size: male to 28 mm, female to 39 mm SL.

Habitat: oceanic, mesopelagic at 50-800 m, precise depth range dependent upon developmental stage, time, latitude, season; pre-metamorphic postlarvae 50-300 m (non-migratory); metamorphic post-larvae 300-600 m (non-migratory) with youngest stages deepest; sub-adults, adults 200-800 m (day) with smaller individuals shallower, adults all levels, and 100-600 m by night, adults making marked vertical migrations. Submersible observations: actively swimming, horizontally orientated, singly or in small groups. North of 40° N seasonal vertical displacement (about 200 m) of adults, sinking June, rising March. Food: opportunistic feeder at dusk on calanoid copepods, ostracods, small fishes, etc. Reproduction: dioecious; spawning season: north of 40° N, fairly restricted, early summermidsummer; south of 40° N, throughout year peaking early summermidsummer; Mediterranean, throughout the year, peaking winter early spring. Female first maturity at about 25 mm SL; ripe eggs 0.92-1.08 mm diameter; individuals spawn more than once per season with egg batches about 50-500 egg/ovary pair, the number increasing with increased animal size. Photophores develop over about 8-10.5 mm SL; post-larvae make substantial downward migration at onset of photophore development.

Distribution: widespread, circumglobal, common throughout eastern North Atlantic to about 60° N; throughout Mediterranean, abundant in western basin, less common in eastern Mediterranean.

Eggs, larvae and young stages. Brauer, 1906: 69, fig. 46 | Ehrenbaum, 1909: 357 | Holt & Byrne, 1913: 21 | Jespersen, 1915: 6 | Jespersen & Taning, 1919: 220 | Sanzo, 1928: 50, fig. | Borodin, 1930a: 44 | Joubin, 1934 fiche 15
Otoliths (sagitta). Kotthaus, 1967: 22, photo.

Argyropelecus hemigymnus