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Eastern Oyster (Crassostrea virginica)

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 [5]. Spawning of C. 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 [7].

Eastern oysters can reach sexual maturity at 4 months [4]. The eastern oyster has a complex reproductive cycle. It begins with the storage of glycogen energy reserves during late summer and autumn [11]. This glycogen is then used to support gametogenesis during the next winter and early spring when food intake is at a minimum [11]. The gametes begin to mature in late spring and then, from June to August they are spawned into the water column, where fertilization occurs [11]. Each female produces from 75 to 150 million eggs, but only 1 in 1000 survive [2]. 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 [5]. The trochophore larvae depend on its internal yolk supply for energy [9]. 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” [5]. During this time the shelled veliger larvae use their ciliated velum to capture food and swim [9]. 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 [5]. During this time the pediveliger larvae “settle to the bottom of the water column where they seek a hard substrate”[5]. 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’” [5][10]. 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 [9]. 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 [7]. Then, some females can even change back to males again [7].

Composition of the Larval Shell of the Eastern Oyster

In the article “Oysters: Composition of Larval Shell” written by H. B. Stenzel, it says that the prodissoconch, the shell of the free-swimming veliger larval stage of C. virginica is composed of aragonite, as opposed to the calcite composition of a post larval adult oyster shell [6]. The epithelium of the oyster’s mantle secretes both the prodissoconch and the post larval shells, but at different times [6]. Stenzel ran some tests to try to determine the reason why larval and adult shells have different compositions. At the Biological Laboratory of the U.S. Bureau of Commercial Fisheries in Milford, Connecticut, larvae from the eastern oyster C. virginica were reared in breeding tanks and were then, collected, washed with distilled water, and dried as they died [6]. The sample included a variety of larval stages, “from the straight-hinge veliger larva with its shell, the protostracum, to the very latest stage of the umbo larva with its shell, the prodissoconch. The shell of either larval stage is very thin, hyaline, and translucent” [6]. Stenzel says the specific gravity of aragonite is 2.95 and calcite is 2.72, so as far as weight is concerned, there really is not an advantage for a larval oyster to have a shell made of one composition over the other [6]. Stenzel then decided to compare oyster larval shells not with adult shells, but instead with other bivalvia larval shells [6]. He concluded that all, or almost all, bivalvia have aragonitic larval shells because the majority of them have aragonitic adult shells, and it can be assumed that C. virginica oyster larvae have an aragonitic shell simply to conform to the general pattern in the bivalvia [6]. Also, Stenzel says, “The larval oyster shells have aragonitic shells because their ancestors did, and there was and is no adaptive need for free-swimming larvae to have shells of a composition other than aragonite” [6]. Stenzel then posed the question, “Why do larval oysters suddenly begin depositing calcite after they have attached to a substratum and begun metamorphosis?” [6] Stenzel concluded it is because “adult oysters need a thick shell for defense against predators since they are permanently immobilized and live in a different environment than that of the larvae, so adaptations are required” [6].

Effects of Deepwater Horizon Oil Spill

Harvestable size of a C. virginica oyster is 75mm, which can take from 12 to 36 months, depending on temperature, salinity of the water, and food supply [7]. Salinity is a very important climatological variable that affects spatfall. Oysters do best where salinities are from 10 to 30 ppt; 15 to 18 ppt is considered optimal [7]. Puglisi reports that “larvae will not settle and metamorphose into spat when salinity is less than 6 ppt [8]. In 2010, 665 miles of coastline were affected by the Deepwater Horizon oil spill [3]. To keep the oil at bay and to spare the oystermen, the authorities of Louisiana made an unprecedented decision to maximize the fresh water flow through the region’s canals to three times usual levels [2]. At the mouth of the canals, salinity fell to almost zero which was probably why most of the oysters died [2]. Sujata Gupta ventured into the marshlands and Gulf of Mexico with Brad Robin, a man from a line of generations of oystermen in southeastern Louisiana. Robin and his crew threw a net over the side to haul in a catch [2]. There were dozens of palm-sized oysters, but 75% of them were “boxes,” or empty shells [2]. However, as they traveled further towards the Gulf of Mexico, where the water was less salinity stressed by the flush, only 20% of the haul came back as boxes, a promising sign the oysters are trying to come back [2]. Gupta reports, “Now since there are so many empty shells scattered on the sea floor, the larvae have more to latch onto, improving their odds [2].

1.Crisp, D.J. Journal of Animal Ecology. "Chemical Factors Inducing Settlement in Crassostrea virginica (Gmelin)"http://www.jstor.org/stable/2916

2. Gupta, Sujata. “Crunch Time Ahead for Gulf Oyster Fisheries.” http://web.ebscohost.com.libezp.lib.lsu.edu/ehost/detail?vid=4&hid=21&sid=70fcf611-e398-4e7a-9ec3-ae45c6d16daa%40sessionmgr4&bdata=JnNpdGU9ZWhvc3QtbGl2ZSZzY29wZT1zaXRl#db=a9h&AN=54605141

3.Repanich, Jeremy. “The Deepwater Horizon Spill by the Numbers.” http://www.popularmechanics.com/science/energy/coal-oil-gas/bp-oil-spill-statistics

4.Smithsonian Marine Station at Fort Pierce http://www.sms.si.edu/irlspec/Crassostrea_virginica.htm

5. South Carolina Oyster Restoration and Enhancement. “Oyster Biology & Ecology.” http://score.dnr.sc.gov/deep.php?subject=2&topic=15

6. Stenzel, H.B. "Oysters: Composition of the Larval Shell." http://www.jstor.org/stable/1714142

7. Wallace, Richard K. “Cultivating the Eastern Oyster, Crassostrea virginica.” https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/92/

8. Puglisi, Melaney P. "Crassostrea virginica." http://www.sms.si.edu/irlspec/Crassostrea_virginica.htm

9. Food and Agriculture Organization of the United Nations. “Cultured Aquatic Species Information Programme: Crassostrea virginica (Gmelin, 1791).” www.fao.org/fishery/culturedspecies/Crassostrea_virginica/en

10. Carriker, Melbourne Romaine. “Ecological Observations on the Distribution of Oyster Larvae in New Jersey Estuaries.” http://www.jstor.org/stable/1948644

11. Kimmel, David G., Newell, Roger I. E. “The Influence of Climate Variation on Easter Oyster (Crassostrea virginica) Juvenile Abundance in Chesapeake Bay.” http://www.jstor.org/stable/4499668