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Linné, 1758

Mantle in larvae and males semiovoid, in females egg-like, caudal end curved slightly upwards. Mantle fused with head and connected with funnel by well-developed connective apparatus conspicuous even in larvae. Body firm, muscular, mantle surface smooth. Head wide, eyes large. Mantle opening reaching level of anterior eye margin. Funnel free. Skin pores (water pores) lacking. Arms in larvae short, 1st longest, others subequal, arm ends tapering. In males arms 1st, 2nd and 4th subequal, 3rd shortest; in females arms long, 1st much longer than others, 4th and 2nd shorter, 3rd are the shortest. Fourth arms 10-20% longer than 2nd, 1.5-2 times the length of 3rd. In maturing and mature females distal section of 1st arms with very extensible wide membranous skin flap which secretes and holds the shell (secondary shell). Web vestigial. Three primary suckers in one row (two first larger than 3rd), then 2 rows of small suckers on all arms. Ink sac present, photophores absent. Twenty eight lamellae per demibranch in females. Female builds characteristic thin, fragile calcareous shell where it lives and incubates eggs. Female stays within shell with 1st arms extended along the sides of the shell and other arms retracted inside. Shell formation begins at ML approx. 5-7 mm; during subsequent life, beginning from approx. 8 mm, shell is enlarged, in large females reaching 20-30 cm in length. Shell elongated, flattened sideways, with very narrow (4-5%, rarely 6% of shell length) keel and thin, slightly bent, outside earlets; with many (>50 in large shells) smooth ribs, drawn close to one another, commonly bifurcated and ending in small, acute, triangular nodules on the keel. In young females the shell is small, ribs are few, set far apart, keel wide, up to 10% shell length. Eggs very small (0.6-0.8 mm), laid and incubated in rear part of shell between caudal end of mantle and posterior shell wall. Newborn larvae liberated by female probably every night. Female begins to reproduce shortly after building shell and continues to spawn eggs during entire life. Males dwarfed, their 3rd left arm totally modified and enclosed in sac on stalk (from ML approx. 1.5 mm), giving the impression that immature males (even newborns) have 7 arms. During maturation the hectocotylus everts and becomes much longer than male itself, detaches during mating. Part of the hectocotylus with sperm crawls into female's mantle cavity and remains there; male dies after mating and is probably devoured by female. Up to 3 hectocotyli may be found in one female. Color of females purple-blue to wine-red from above, light from below; extended 1st arm flap purple-red. Fixed females red-brown. Shell porcelain-white, early section and nodules on the keel dark brown. Males with many small chromatophores. Hatchlings ML 0.85-0.9 mm. Early larvae (up to 3 mm) covered by jelly coat with only arm ends protruding. Eyes directed somewhat antero-ventrally. Larvae clear, with some (23-24 in hatchlings) sparsely distributed large chromatophores. Flaps on 1st arms detectable at ML 3, developed at >4 mm. ML of females up to 30 cm, of males to 1 cm.

Dwells in coastal areas, rare in the open ocean. Juveniles and adult females rise to the surface during the night. Another Atlantic species, A. hians Solander, 1786, is also tropical-subtropical, but oceanic; with small shell (up to 10 cm), usually smooth, not flattened sideways, keel wide (10-15% of shell length), ribs not numerous (<35), set far apart, nodules brown. Fourth arms much shorter than 2nd-3rd, length in large females approx. 60% of 2nd. Egg size 0.7-0.8 mm. 20 lamellae per demibranch. A third species, A. nodosa Solander, 1786, dwells in the tropical Indo-West Pacific; it was often recorded off Namibia and South Africa (Sánchez, 1988; Laptikhovsky, 1989; Villanueva and Sánchez, 1993); it is a large species with wide keel (10-15% of shell length), and ribs (up to 30-40) beset by chains of separate tubercles or nodules.

Argonauta argo