Table 1 Hallmarks of each developmental stage in O. vulgaris

Stage 0

Cleavage

Stage I

Morula (advanced cleavage)

Stage II

Blastula; disk flatter

Stage III

Onset of epiboly

Stage IV

Formation of the germinal disk, as wide as the yolk

Stage V

Epiboly reaches 1/4th of the yolk

Stage VI

Epiboly reaches 1/2nd of the yolk

Stage VII.1

Epiboly reaches 3/4th of the yolk

Visible thickening of placodes starts

Stage VII.2

Embryo completed first reversion

Primordia of eyes, mouth, mantle and arms clearly visible

Stage VIII

Mouth and eye invagination

Mantle elevated and embryo thicker

Funnel pouches visible in lateral view

Stage IX

First eye pigmentation (yellowish)

Primordia more prominent

Funnel tube rudiments are distinct

Contraction of yolk envelope evident

Stage X

Mantle flat, no depression in the middle

Eye vesicles sticking out with light orange retina (“saddle” shape)

Funnel tube rudiments fuse at the ventral margins

Stage XI

Mantle tilted

Funnel tube rudiments bended towards midline

Arm buds ‘elevated’ from yolk

Stage XII.1

Mantle thicker and covers 1/2nd of gills

Funnel tube rudiments start to form a tube ventrally

First suckers recognizable on the posterior side

Stage XII.2

Mantle bowl-shaped

Funnel formed siphon at ventral extremity

Stage XIII

Mantle is bigger

Formation of the funnel complete

Arms elongated and pointed with (3) prominent suckers

Stage XIV

Mantle as wide as long and cube-shaped, covers gills completely

Heartbeat starts

Embryo and yolk have equal size

Stage XV.1

Mantle completely covers ventral margin of funnel

Inner yolk strongly constricted (connection inner and outer yolk sac very thin)

Appearance of two chromatophores laterally from the funnel

Stage XV.2

Mouth encircled by anterior arms

First chromatophores on posterior mantle appear

Stage XVI

Mouth completely covered by arm crown

Yolk size 1/3rd of embryo + yolk

Few chromatophores on anterior mantle evident

Stage XVII

Chromatophores darker and more numerous

Stage XVIII.1

Yolk size 1/4th of embryo + yolk

Posterior chromatophores darker and chromatophores appear next to the eye

Embryo more active in egg (mantle contraction)

Stage XVIII.2

Anterior chromatophores darker. Chromatophores react to light stimulus

Stage XIX.1

Eyes tilted and covered with iridophores

Yolk size 1/6th of embryo + yolk

Embryos react to mechanical stimulus

Pigmentation of ink sac

Stage XIX.2

Embryos completed second reversion

Stage XX.1

Minimal outer yolk sack

Chromatophore expansion and contraction more widely distributed

Stage XX.2

Absence of outer yolk

Hatching