Eastern Oyster - The Life Cycle

The Life Cycle

The life cycle of C. virginica is as follows: spawning, floating fertilized egg, trochophore, swimming straight-hinge veliger, swimming late veliger, swimming and crawling pediveliger, early spat, later spat, and adult oysters. Spawning ofC. virginica is controlled by water temperatures which varies from north to south, northern oysters spawn at temperatures between 60 and 68 °F (15.5 and 20 °C), while southern oysters spawn at temperatures above 68 °F (20 °C). Spawning can also occur throughout the warm months.

Eastern oysters can reach sexual maturity at 4 months old. The eastern oyster has a complex reproductive cycle. The cycle begins during late summer and autumn months with the storage of glycogen energy reserves. This glycogen is then used to support gametogenesis during the next winter and early spring when food intake is at a minimum. The gametes begin to mature in late spring and then, from June to August they are spawned into the water column, where fertilization occurs. Each female produces from 75 to 150 million eggs, but only 1 in 1000 survive. Then, fertilized eggs develop into planktonic, free-swimming, trochophore larvae, also known as the early umbo stage, which have cilia and a small shell, in about 6 hours. The trochophore larvae depend on its internal yolk supply for energy. They then develop into a fully shelled veliger larva, also known as the late umbo stage, which have a hinged side and a velum, that is formed within 12 to 24 hours”. During this time the shelled veliger larvae use their ciliated velum to capture food and swim. The larvae remain planktonic for about 2 or 3 weeks depending on food and temperature conditions, and towards the end of this period they develop into pediveliger larvae, also known as eyed larvae, which have an umbo, and eyespot and a foot. During this time the pediveliger larvae “settle to the bottom of the water column where they seek a hard substrate”. Ideally, the pediveliger larvae try to locate an adult oyster shell to cement themselves to, but other hard surfaces will suffice. Lastly, “after careful selection of the proper attachment site, in or during relatively still water, the pediveliger larvae cement themselves to a firm clean surface and metamorphoses to the adult form; these newly attached oysters are known as ‘spat’”. Upon being stimulated to settle, a larva cements its left valve to the substrate and metamorphoses into an oyster spat by discarding its velum, reabsorbing its foot, and enlarging its gills. During the first year of life, C. virginica oysters are protandric. Most spat are male, but once they reach sexual maturity within 4 months in southern waters, some males change to females after the first or second spawning. Then, some females can even change back to males again.