Development of Amphioctopus fangsiao (Mollusca: Cephalopoda) from eggs to hatchlings: indications for the embryonic developmental management

Amphioctopus fangsiao is an important fishery resource in northern coastal China, and there is an interest to develop its commercial cultivation under controlled conditions. In this study, we describe the embryonic development of A. fangsiao from eggs to hatchlings, focusing on the formation of the mantle, eyes and arms. Results indicate that this species has several favorable characteristics for aquaculture, such as a short incubation period and a large body size of hatchlings. During the organogenesis, the primordia of organs, including the optic sac and arm crown, first appeared at stage Ⅷ; then the mantle primordium, eye primordia and arm crown become visible at stage Ⅸ-Ⅹ, and the rudiment of embryo (mantle, eyes and arms) was completely outlined at stage Ⅺ. The retinal pigment appeared yellowish at stage Ⅹ-Ⅺ and gradually deepens in pigmentation in the subsequent stages, turning from yellowish to yellow, red, dark-red and eventually black. At stage ⅩⅤ, the primary lid fold, with only a circular opening remaining, covered the eye, and the dorsal ocular edge did not reach the ventral side until hatching (day 1-7). Afterwards, about 24 days after hatching, both the dorsal and ventral sides of eyelid could contract freely, with an oval and elongate slit remaining that was a miniature version of the adult morphology. The chromatophores and ink sac appeared at stage ⅩⅣ. These observations provide a foundation for the reproductive biology and aquaculture of A. fangsiao and also offer an important reference for the research of embryonic development in related species.